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Example of muscle activation (EMG) in the leg muscles of a subject during Hold & Release from an EONT trial ( arrow : time of release). Differential recordings of surface EMG were made from two pairs of muscles: (a) tibialis anterior and gastrocnemius lateralis and (b) biceps femoris (long head) and rectus femoris. The first pair flexes and extends the ankle, and the second pair act on both knee and hip. The data were band-pass filtered at 10–500 Hz and sampled at 1300 Hz. The envelope of the rectified signals is presented with the signals offset to zero in the hold period. This allows for easier visualization of the resulting activity changes. After release, there is bursting of all four muscles with peaks at different latencies. These bursts are the initial responses that arrest the body’s forward motion. When the body is settled, the gastrocnemius and biceps femoris are more active than during the hold period while the tibialis and biceps femoris are less active. In the transient period between bursting and settling, there are multiple cycles of oscillation in the EMG patterns of all the muscles. This pattern indicates that the harmonic nature of the multilink postural model is correct
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We studied the kinematics and kinetics of human postural responses to "recoverable falls." To induce brief falling we used a Hold and Release (H&R) paradigm. Standing subjects actively resisted a force applied to their sternum. When this force was quickly released they were suddenly off balance. For a brief period, approximately 125 ms, until resto...
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Context 1
... 4 is the equation of an inverted pendulum (unstable) and is always applicable regardless of the multi-degree of freedom nature of the system if the motion of the system is analyzed as motion of the center of mass (see Goldstein et al. 2002). Equation 4 is also applicable to all remaining phases of the paradigm. In the general case the moment of inertia of the system becomes time varying, and eventually it must be differentiated together with the angular velocity during the calculation of the inertia forces. (This component is neglected here because the differential of the moment of inertia is an infinitesimal of higher order during H&R). During the release phase, the forcing component on the right hand side of upper Eq. 4 is a constant pushing- over torque since the misalignment of the center of foot pressure with respect to the center of mass remains unchanged for at least 90 ms due to the slower reaction time of the CNS (see Fig. 4C). Such a constant pushover torque is a function only of the offset cp x of the center of mass with respect to the center of pressure, which in turn is determined by the magnitude of the holding force H . The quantity cp x can be reliably measured by means of the force plate the subject stands on since it is the difference between the values of the center of foot pressure during the hold phase and the settled phase. During the 90 ms after release before the cp x changes position the system accelerates under gravity ( m k g k Á k cp x k is constant) and builds up speed even though it is moving very little. Figure 4C shows that the shear force S remains approximately constant as well, as predicted by Eq. 4. This means that when released, the center of mass is accelerating forward freely, i.e., “falling,” with an acceleration equivalent to the removed holding force H until much later when the center of pressure starts to react. Figure 4C shows that within 50– 60 ms from the onset of release the EMG activity in the leg muscles changes with the gastrocnemius (gastrocnemius is the first to react) and the biceps femoris increasing and the tibialis anterior and the rectus femoris decreasing in activation (see also Figure 8). Due to the muscle activation dynamics, at least another 50–70 ms elapses before the ankles and knees start to extend and the center of foot pressure begins to move (see Figure 4C). This synergy moves the center of foot pressure forward. To break the fall, the cp x needs to go ahead of the center of mass. A mirror symmetric synergy later moves the center of foot pressure in the opposite direction. Succinctly, we can say that after release the body’s joint torques are reversed from the ones of the holding phase and begins to shift the cp x forward, a process taking approximately 90 ms. In the interim the body is falling forward under the pull of gravity and the action of ground reaction forces. This reaction time is comparable across subjects, so we can estimate the peak velocity prior to the recoil as J _ 0 % m k g k Á k cp x k ml 2 (from the impulse theorem of mechanics), which yields J _ / cp l 2 The calculated velocity J 0 dictates the amount of energy that is acquired during the off-balance interim, “fall,” generated by the release. During the recoil phase, postural control must not only realign the center of mass with the center of foot pressure, but in the process it must dissipate this acquired velocity J _ 0 . This energy can only be dissipated by controlling the center of foot pressure and the center of mass misalignment in phase contrast to the sway velocity J _ to perform negative work. The CNS skill in doing this is revealed by the shape of the transient (see Figure 3) rather than by the magnitude of the sway response, i.e., fast vs. slowly decaying transient. The perturbation introduced by the H&R depends on the magnitude of center of masscenter of foot pressure misalignment and the J _ 0 acquired after the release. Both are determined by the holding force, H , and are quantified by the measure cp x . Therefore the data across trials and subjects can be normalized by using the parameter cp x (the linearity of the H&R dynamics is demonstrated below) and importantly the holding force need not be measured or controlled. The only requirement is that the maximum sway produced by the release is not so excessive as to elicit a step. In summary, the normalized peaks of the sway (see Fig. 6) and the rapidity of restoration of the upright posture (damping, see Fig. 3) are two of the parameters yielded by the H&R that provide quantification of CNS performance during sudden off- balance exposure. After about 5–10 s from release, the subject’s posture reaches a position of equilibrium. We adopt this as a measurable experimental zero. This phase is regular standing posture. In order not to overload the reader with technical details, further features of analysis are discussed in Appendices A and ...
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... We demonstrated experimentally and analytically that by using a H&R paradigm we are able to induce sudden loss of balance and a brief period of forward falling. The peak angular displacements at the shoulders, hips, and knees were small following release. This allowed us to model the resulting biomechanical behavior of the body as a multilink, inverted pendulum in a regime of overall small oscillations. We found that a three-link, inverted- pendulum model described the experimental data very well under all of our experimental conditions. Important- ly, a compound linear inverted-pendulum model ade- quately described the behavior of all our subjects. This means that the postural response to H&R could be characterized using manageable linear systems theory. The linearity of the paradigm dynamics is very convenient for data scaling, data modeling, and system analysis. However, the real significance lies in the implications for the nature of the neuromuscular mech- anisms responsible for balance recovery following release. Posture dynamics consists of biomechanics and control. Because the biomechanics is linearizable, the demonstrated linearity of the whole H&R response indicates that stiffness and damping (the linear PD controller we hypothesized above) is an excellent approximation of the neurophysiological characteristics of the posture controller at the joint level. In addition, we demonstrated that the stiffness and damping values are produced by reflexive mechanisms and not by passive muscle properties alone. Moreover, stiffness, and damping are with good approximation constant across the recoil. This implies that starting with the sudden release some mechanism determines the gains, set points, and phases of the sensory-motor feedback to be used across the whole recovery. The question is what mechanism produces this behavior. Figure 4 panels A–C show a representative case of H&R with touch cues from the fingertip. Note that the center of foot pressure and the ground shear remain unchanged for 125 ms after the onset of release. EMG activity increases 55 ms after onset of release in the gastrocnemius and biceps femoris muscles (see Fig. 4C). Before the EMG activations only acceleration, velocity, and position of the body segments (head included) show variations. A vestibular elicited response with 60–80 ms latencies is a possible basis for the EMG activity (Greenwood and Hopkins 1976, 1980); however, muscle spindle information is available sooner. Considering that muscle spindles have a low threshold and large afferent fibers, it is likely that within a few milliseconds after release spindle feedback signals could drive corrective responses. Another possibility is a predictive response. However, in such a case it would be difficult to explain why the response would take 55 ms to develop instead of occurring sooner after release in order to avoid exposing the body to falling. (In Appendix A, we discuss what happens to the center of foot pressure variation if the release is anticipated and how this can be used in the H&R paradigm to discard predicted and therefore unsuccessful trials.) Figure 9 presents a simplified schema of the neuromuscular mechanism underlying joint control. It shows how spinal circuitry and supraspinal modulation can perform postural joint control and how this could account for our findings. Descending supraspinal commands, a and b, drive a spinal servos. The common mode of the supraspinal drives (a+b) sets or modulates the stiffness and damping of the muscle pair around the joint. The differential drive (ab) is proportional to the net joint torque, and it can provide additional set gain feedback or modulated feedback proportional to velocity offsets (damping) or position (stiffness) or force (impedance, omitted here) through the mediation of higher centers (supraspinal mechanisms). Spindle afferents and other sensory signals are mediated and integrated at a supraspinal level and relayed to the spinal servos by means of the a and b drives. This schema provides desired modulation of mechanical properties otherwise not achievable solely by the a spinal circuitry. Our findings suggest (1) that the common (a+b) drive and reflexive spindle pathways are probably set to provide constant stiffness since we find no significant frequency p ffiffiffiffiffiffiffiffiffi changes across conditions ( w 1⁄4 K = m , where K is stiffness and m is mass), and (2) that the g drives and/or the differential (ab) drive are modulated by higher centers to provide joint velocity feedback (torque) appropriately timed to produce effective damping of body sway during sudden off-balance conditions. This sensory-motor integration scheme possibly could be tuned before release; however, its precise configuration and parametrical settings might also be adjusted within a few tens of milliseconds following release. In summary, we have demonstrated that postural recovery following H&R has a reflexive viscoelastic behavior. Figure 8 shows the muscle synergies responsible for this finding. Patterned activation of the gastrone- mius-biceps femoris pair vs. the tibialis anterior-rectus femoris pair (and other muscles) allows for the control of the fore-aft motion of the center of foot pressure. If reflexive and descending control mechanisms were not involved, the motion of the perturbed posture would resemble that of a rocking chair and would extinguish very slowly. The stiffening and relaxing of the muscles must actually be timed in phase contrast to their stretching and shortening to produce dissipation of sway (kinetic energy). From an external point of view, this means that the center of foot pressure is adjusted in phase contrast to the sway velocity so that external forces create negative work on the body. The light touch force on the finger in touch trials is not responsible for the triggering of the EMG responses because it shows variations at the same time as the earlier EMG responses. However, light touch makes a difference in how rapidly the muscle synergies succeed in restoring balance. This suggests that there ae two separate mechanisms, one ruling the early part of the response and another the later phase (see also Denier and Dichgans 1986). Alternatively, there might be only one mechanism which utilizes all available information in a sensory fusion manner. Further analysis and experimen- tation are needed to clarify this issue. The touch cues also enabled the subjects to have smaller peak deflections of the torso than in the absence of touch. As discussed above, linear behavior carries the con- venience that the magnitude of the postural response is proportional to the magnitude of the initiating perturbation. Consequently, the shift, cp x , of the center of foot pressure position between the hold phase and the settled posture (see Fig. 4A) can be used as a scaling factor to normalize the postural data across trials. This simplifies the H&R paradigm by not requiring identical perturbations in every trial and it makes the comparison and statistical treatment of data across conditions much simpler. An additional advantage is that the H&R paradigm can be used in patient populations using low hold force levels or in strong and healthy subjects using high force levels without affecting the validity of the later analyses. The H&R paradigm gives insights into what happens during unintended falls and potential clues with regard to possible prevention strategies. Even more importantly H&R allows for quantification of recovery performance from falling onset. Existing perturbation paradigms in which the support surface is tilted and/or translated yield important insights into postural control but are not specific models of natural off-balance events and falling onset. Posture platforms translate or rotate to induce a misalignment between the center of foot pressure and the center of mass, however, trials start with the two aligned. Friction between the platform and the subject’s feet during platform translation provides the force that constitutes the perturbation. The soles of the feet are therefore stimulated in the process and contribute somatosensory cues during the course of the perturbation that are not specific to an off-balance condition, but only to the posture platform. Moreover, a relatively long period of time is commonly used to carry out the perturbation, about 450 ms (see Horak and McPherson 1996). Such a period is long enough to stimulate and engage feedback loops from short to long, as well as volitional responses (see Brooks 1986). With stance surface perturbations the disturbance is also transmitted from the bottom up, link- by-link, through the entire chain of the body making the analysis of the overall process difficult. The H&R paradigm bypasses this first phase of joint flexion/ extension and starts with the center of foot pressure and center of mass already misaligned. Paradigms involving push and release from a hold force applied laterally or frontally at the waist (see Wing et al. 1993, 1995) by means of electromechanical devices involve much longer time courses (200 ms) for force removal than the simple manual releases in the H&R (35– 40 ms). A perturbation applied to the hip yields generally greater angular displacement at the hip than at the shoulder (Wing et al. 1995), which is the consequence of nonuniform and nonsimultaneous stimulation of the whole mechanical body chain. Wing et al. (1995) have implemented a releasing from push paradigm in which a mechanical apparatus applies a force laterally or frontally to the subject’s waist which can then be diminished to zero over an ffi 200 ms period. This procedure is not analogous to H&R because the body is partially mechan- ically constrained during the 200 ms in which the push force is eliminated. In H&R, the release takes approximately 35 ms and the EMG synergies are activated within approximately 55 ms of the onset of the release (see Fig. ...
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... very close agreement between the postural sway data and the fit of the presented model corroborate the thesis that posture control behaves as a linear controller. In physiological terms this means that during exposure of the body to sudden off balance conditions reflexive mechanisms provide specific stiffness and damping characteristics to the joints of the body that are not simply the result of passive muscles mechanics. Figure 4 panels B and C show rectified EMG recordings for one subject in a forward touch condition, in which clear bursts are present, which indicates reflexive control of posture. Figure 8 shows the EMG recordings taken in an eyes closed without touch trial from another subject. The recordings from the leg muscles are aligned to show the strong pattern of muscle synergies that is responsible for the viscoelastic (stiffness and damping) response predicted by our modeling and numerical analysis. First principles dictate that postural control can only resort to synchronization of muscle contractions in phase contrast with muscle stretching in order to dissipate energy. The results of this investigation indicate that with the aid of haptic cues the postural controller is more effective in dissipat- ing the energy of the perturbation. These two facts combined mandate that the muscle synergies (patterned muscle contractions) shown in Fig. 8 are finely timed with respect to each other in a context specific manner. Great interest lies in understanding the nature of postural restabilization following sudden loss of balance. Our approach was to study postural responses to recoverable falls created experimentally to mimic key features of tripping or loss of footing and at the same time to allow easy mathematical treatment. We wanted to assess whether recovery from being suddenly off-balance was handled by a posture controller regulating stiffness and damping properties of the body’s joints. In addition, we wanted to gauge whether haptic and visual cues affected the performance of such a controller. Finally, the conjunction of a technique for inducing recoverable falls and a model for analyzing them provides the potential for easy and portable clinical use. We demonstrated experimentally and analytically that by using a H&R paradigm we are able to induce sudden loss of balance and a brief period of forward falling. The peak angular displacements at the shoulders, hips, and knees were small following release. This allowed us to model the resulting biomechanical behavior of the body as a multilink, inverted pendulum in a regime of overall small oscillations. We found that a three-link, inverted- pendulum model described the experimental data very well under all of our experimental conditions. Important- ly, a compound linear inverted-pendulum model ade- quately described the behavior of all our subjects. This means that the postural response to H&R could be characterized using manageable linear systems theory. The linearity of the paradigm dynamics is very convenient for data scaling, data modeling, and system analysis. However, the real significance lies in the implications for the nature of the neuromuscular mech- anisms responsible for balance recovery following release. Posture dynamics consists of biomechanics and control. Because the biomechanics is linearizable, the demonstrated linearity of the whole H&R response indicates that stiffness and damping (the linear PD controller we hypothesized above) is an excellent approximation of the neurophysiological characteristics of the posture controller at the joint level. In addition, we demonstrated that the stiffness and damping values are produced by reflexive mechanisms and not by passive muscle properties alone. Moreover, stiffness, and damping are with good approximation constant across the recoil. This implies that starting with the sudden release some mechanism determines the gains, set points, and phases of the sensory-motor feedback to be used across the whole recovery. The question is what mechanism produces this behavior. Figure 4 panels A–C show a representative case of H&R with touch cues from the fingertip. Note that the center of foot pressure and the ground shear remain unchanged for 125 ms after the onset of release. EMG activity increases 55 ms after onset of release in the gastrocnemius and biceps femoris muscles (see Fig. 4C). Before the EMG activations only acceleration, velocity, and position of the body segments (head included) show variations. A vestibular elicited response with 60–80 ms latencies is a possible basis for the EMG activity (Greenwood and Hopkins 1976, 1980); however, muscle spindle information is available sooner. Considering that muscle spindles have a low threshold and large afferent fibers, it is likely that within a few milliseconds after release spindle feedback signals could drive corrective responses. Another possibility is a predictive response. However, in such a case it would be difficult to explain why the response would take 55 ms to develop instead of occurring sooner after release in order to avoid exposing the body to falling. (In Appendix A, we discuss what happens to the center of foot pressure variation if the release is anticipated and how this can be used in the H&R paradigm to discard predicted and therefore unsuccessful trials.) Figure 9 presents a simplified schema of the neuromuscular mechanism underlying joint control. It shows how spinal circuitry and supraspinal modulation can perform postural joint control and how this could account for our findings. Descending supraspinal ...
Context 4
... very close agreement between the postural sway data and the fit of the presented model corroborate the thesis that posture control behaves as a linear controller. In physiological terms this means that during exposure of the body to sudden off balance conditions reflexive mechanisms provide specific stiffness and damping characteristics to the joints of the body that are not simply the result of passive muscles mechanics. Figure 4 panels B and C show rectified EMG recordings for one subject in a forward touch condition, in which clear bursts are present, which indicates reflexive control of posture. Figure 8 shows the EMG recordings taken in an eyes closed without touch trial from another subject. The recordings from the leg muscles are aligned to show the strong pattern of muscle synergies that is responsible for the viscoelastic (stiffness and damping) response predicted by our modeling and numerical analysis. First principles dictate that postural control can only resort to synchronization of muscle contractions in phase contrast with muscle stretching in order to dissipate energy. The results of this investigation indicate that with the aid of haptic cues the postural controller is more effective in dissipat- ing the energy of the perturbation. These two facts combined mandate that the muscle synergies (patterned muscle contractions) shown in Fig. 8 are finely timed with respect to each other in a context specific manner. Great interest lies in understanding the nature of postural restabilization following sudden loss of balance. Our approach was to study postural responses to recoverable falls created experimentally to mimic key features of tripping or loss of footing and at the same time to allow easy mathematical treatment. We wanted to assess whether recovery from being suddenly off-balance was handled by a posture controller regulating stiffness and damping properties of the body’s joints. In addition, we wanted to gauge whether haptic and visual cues affected the performance of such a controller. Finally, the conjunction of a technique for inducing recoverable falls and a model for analyzing them provides the potential for easy and portable clinical use. We demonstrated experimentally and analytically that by using a H&R paradigm we are able to induce sudden loss of balance and a brief period of forward falling. The peak angular displacements at the shoulders, hips, and knees were small following release. This allowed us to model the resulting biomechanical behavior of the body as a multilink, inverted pendulum in a regime of overall small oscillations. We found that a three-link, inverted- pendulum model described the experimental data very well under all of our experimental conditions. Important- ly, a compound linear inverted-pendulum model ade- quately described the behavior of all our subjects. This means that the postural response to H&R could be characterized using manageable linear systems theory. The linearity of the paradigm dynamics is very convenient for data scaling, data modeling, and system analysis. However, the real significance lies in the implications for the nature of the neuromuscular mech- anisms responsible for balance recovery following release. Posture dynamics consists of biomechanics and control. Because the biomechanics is linearizable, the demonstrated linearity of the whole H&R response indicates that stiffness and damping (the linear PD controller we hypothesized above) is an excellent approximation of the neurophysiological characteristics of the posture controller at the joint level. In addition, we demonstrated that the stiffness and damping values are produced by reflexive mechanisms and not by passive muscle properties alone. Moreover, stiffness, and damping are with good approximation constant across the recoil. This implies that starting with the sudden release some mechanism determines the gains, set points, and phases of the sensory-motor feedback to be used across the whole recovery. The question is what mechanism produces this behavior. Figure 4 panels A–C show a representative case of H&R with touch cues from the fingertip. Note that the center of foot pressure and the ground shear remain unchanged for 125 ms after the onset of release. EMG activity increases 55 ms after onset of release in the gastrocnemius and biceps femoris muscles (see Fig. 4C). Before the EMG activations only acceleration, velocity, and position of the body segments (head included) show variations. A vestibular elicited response with 60–80 ms latencies is a possible basis for the EMG activity (Greenwood and Hopkins 1976, 1980); however, muscle spindle information is available sooner. Considering that muscle spindles have a low threshold and large afferent fibers, it is likely that within a few milliseconds after release spindle feedback signals could drive corrective responses. Another possibility is a predictive response. However, in such a case it would be difficult to explain why the response would take 55 ms to develop instead of occurring sooner after release in order to avoid exposing the body to falling. (In Appendix A, we discuss what happens to the center of foot pressure variation if the release is anticipated and how this can be used in the H&R paradigm to discard predicted and therefore unsuccessful trials.) Figure 9 presents a simplified schema of the neuromuscular mechanism underlying joint control. It shows how spinal circuitry and supraspinal modulation can perform postural joint control and how this could account for our findings. Descending supraspinal commands, a and b, drive a spinal servos. The common mode of the supraspinal drives (a+b) sets or modulates the stiffness and damping of the muscle pair around the joint. The differential drive (ab) is proportional to the net joint torque, and it can provide additional set gain feedback or modulated feedback proportional to velocity offsets (damping) or position (stiffness) or force (impedance, omitted here) through the mediation of higher centers (supraspinal mechanisms). Spindle afferents and other sensory signals are mediated and integrated at a supraspinal level and relayed to the spinal servos by means of the a and b drives. This schema provides desired modulation of mechanical properties otherwise not achievable solely by the a spinal circuitry. ...
Citations
... Including plyometric lower jumps in the present study had a significant effect on improving the balance test results in the EG, which confirms previous studies which state that adding plyometric training improves the position sense of the ankle and enhances balance ability (Bortolami et al., 2003;Lin et al., 2021;Martín Nogueras, 2004;Nashner & Cordo, 1981;Winter et al., 1996). Plyometric training has been found to improve soccer (Clemente et al., 2022) and basketball players' (Bouteraa et al., 2020;Cherni et al., 2019) balance abilities. ...
... Because our intervention had a large component of hip musculature voluntary contractions, one explanation for these results is that movements with an ML direction are controlled by the hip musculature (Winter et al., 1996), and that this can help to improve gymnasts' core motor control, endurance, and strength. These findings agree with previous RG studies Esteban-García et al., 2021) and with studies that have reported that the hip musculature tends to act more when disturbances are faster and larger (Bortolami et al., 2003;Martín Nogueras, 2004;Nashner & Cordo, 1981;Winter et al., 1996). Moreover, maintaining a stable upright posture requires constant muscular corrective contraction that involves knee and hip joints when the oscillations are of significant amplitude, as was the case in our CPT (Pau et al., 2021). ...
It has been suggested that core stability and plyometric training (CPT) can enhance athletes’ postural control. Nevertheless, the effects of an integrated core and plyometric training program on rhythmic gymnastics (RG) performance are unclear. This study aimed to evaluate the effects of an integrated functional CPT program on young rhythmics gymnasts’ postural performance. A sample of 44 young female rhythmic gymnasts from a competitive team (age = 10.5 ± 1.8 years) participated in the study. The subjects were randomly divided into a control group and an experimental group. Pre- and posttest design was used. Postural control was assessed using single-leg stance tests and RG-specific balances over a force platform and evaluated by expert RG judges. The experimental group ( n = 23) completed an 8-week functional CPT program based on RG technical requirements. Meanwhile, the control group ( n = 21) received their usual training sessions. A mixed model of analysis of variance was applied to evaluate the effects of an intrasubject factor and an intersubject factor on each of the dependent variables. After 8 weeks, the experimental group obtained significant better results in some variables of the right support leg with eyes open and left support leg with eyes open single-leg support ( p < .01), improvements were also found in some specific RG balances: Arabesque measured on the force platform ( p < .01) and the side leg with help balance scored by the judges ( p < .01). In conclusion, an integrated functional CPT program improved postural control in young rhythmic gymnasts. Coaches should consider using this CPT to improve RG performance.
... imparting the perturbation (i) to the base of support by sliding or tilting the platform (Schmidt et al. 2015;Grassi et al. 2017;Robbins et al. 2017) or (ii) directly to the upper body. These two perturbation modes elicit fundamentally different PR (Bortolami et al. 2003;Colebatch et al. 2016;Chen et al. 2017) and thus are both worth to be pursued. However, while the moving platform is easily described and standardized in terms of extent and speed of displacement and rotation, description and quantification of upper body perturbation are more difficult. ...
PurposeMany studies have investigated postural reactions (PR) to body-delivered perturbations. However, attention has been focused on the descriptive variables of the PR rather than on the characterization of the perturbation. This study aimed to test the hypothesis that the impulse rather than the force magnitude of the perturbation mostly affects the PR in terms of displacement of the center of foot pressure (ΔCoP).Methods
Fourteen healthy young adults (7 males and 7 females) received 2 series of 20 perturbations, delivered to the back in the anterior direction, at mid-scapular level, while standing on a force platform. In one series, the perturbations had the same force magnitude (40 N) but different impulse (range: 2–10 Ns). In the other series, the perturbations had the same impulse (5 Ns) but different force magnitude (20–100 N). A simple model of postural control restricted to the sagittal plane was also developed.ResultsThe results showed that ΔCoP and impulse were highly correlated (on average: r = 0.96), while the correlation ΔCoP–force magnitude was poor (r = 0.48) and not statistically significant in most subjects. The normalized response, ΔCoPn = ΔCoP/I, was independent of the perturbation magnitude in a wide range of force amplitude and impulse and exhibited good repeatability across different sets of stimuli (on average: ICC = 0.88). These results were confirmed by simulations.Conclusion
The present findings support the concept that the magnitude of the applied force alone is a poor descriptor of trunk-delivered perturbations and suggest that the impulse should be considered instead.
... In the present work, we have used the hold and release paradigm proposed by Bortolami et al. [11], combined with a time-based system-identification technique to determine the time-varying stiffness at the hip and ankle that allows individuals to recover from a fall. ...
... The lower frequency modes have larger amplitudes and describe a large amount of variation in the position of the center of mass. The first mode of vibration often accounts for 80% of the variance of the center of mass displacement of the whole system [11]. Thus, a simple model of a single inverted pendulum can be a representation of the first mode of vibration of a more complex system. ...
... Thus, as more segments are added to the model, the mechanical power required to elicit an observable change in joint kinematics increases dramatically. Remarkably, the hold and release paradigm [11] is able to use the internal dynamics of the body as a perturbation allowing for enough power to elicit measurable changes in multiple joints. The technique used to estimate the neuro-mechanical parameters, in this case, is based on the estimation of a time series. ...
Background:
Balance control deteriorates with age and nearly 30% of the elderly population in the United States reports stability problems. Postural stability is an integral task to daily living reliant upon the control of the ankle and hip. To this end, the estimation of joint parameters can be a useful tool when analyzing compensatory actions aimed at maintaining postural stability.
Methods:
Using an analytical approach, this study expands on previous work and analyzes a two degrees of freedom human model. The first two modes of vibration of the system are represented by the neuro-mechanical parameters of a second-order, time-varying Kelvin-Voigt model actuated at the ankle and hip. The model is tested using a custom double inverted pendulum and healthy volunteers who were subjected to a positional step-like perturbation during quiet standing. An in silico sensitivity analysis of the influence of inertial parameters was also performed.
Results:
The proposed method is able to correctly identify the time-varying visco-elastic parameters of of a double inverted pendulum. We show that that the parameter estimation method can be applied to standing humans. These results appear to identify a subject-independent strategy to control quiet standing that combines both the modulation of stiffness, and the use of an intermittent control.
Conclusions:
This paper presents the analysis of the non-linear system of differential equations representing the control of lumped muscle-tendon units. It utilizes motion capture measurements to obtain the estimates of the system's control parameters by constructing a simple time-dependent regressor for estimating the time-varying parameters of the control with a single perturbation. This work is a step forward into the understanding of the neuro-mechanical control parameters of human recovering from a fall. In previous literature, the analysis is either restricted to the first vibrational mode of an inverted-pendulum model or assumed to be time-invariant. The proposed method allows for the analysis of hip related movement for stability control and highlights the importance of core training.
... Human quiet standing is often modeled as an inverted pendulum. The complexity of this model depends mainly on the number of segments included in the open kinematic chain representing said pendulum [9] and upon the constitutive law of the elastic elements used to model the mechanical properties of each joint. Given the structural configuration of the model, it is a common practice to model only the pendulum's first oscillatory mode as it accounts for more than 90% of the body's oscillation. ...
... A single perturbation used to evoke a response could eliminate these difficulties. The hold and release paradigm (H&R) is a technique designed to perturb quiet standing without the need of a motorized platform, with the intent of exciting an instinctive neuromechanical response against falling [2,7,9]. ...
... The RSS error of the state estimation step is used to calculate the fitness value of each individual using Eq. (9). In GA-EKF-2, the individual whose covariances matrices yield the best fitness value in the EKF is selected as an optimal value for the tuning. ...
The estimation of the human ankle's mechanical impedance is an important tool for modeling human balance. This work presents the implementation of a parameter-estimation approach based on the State-Augmented Extended Kalman Filter (AEKF) to infer the human ankle's mechanical impedance during quiet standing. However, the AEKF Filter is sensitive to the initialization of the noise covariance matrices. In order to avoid a time consuming trial-and-error method and to obtain a better estimation performance, an algorithm based on Genetic Algorithms (GA) is proposed for tuning the measurement noise Rk and process noise covariances Q of the Extended Kalman filter (EKF). Results using simulated data show the ef?cacy of the proposed algorithm for parameter-estimation of a third-order biomechanical model. An experimental test with real data on human subjects is also presented. The results suggest that age is a factor that influences human balance capability.
... Another aspect of this study concerns the possible role of anticipatory postural adjustments (APA) in modifying PR, an issue that has been explored in numerous studies (Latash and Hadders-Algra 2008). Although some general mechanisms have been elucidated, the diversity of testing protocols (Bortolami et al. 2003;Le Mouel et al. 2019) as well as the nature of the perturbation (Piscitelli et al. 2017) generally do not allow direct comparisons between findings. However, to assess the results, it was necessary to find out whether APAs have actually been present during the test and, if they did, to explore their effect. ...
... Whether or not the PR to forwardly imparted perturbations are substantially linear, or non-linear, may to a large extent depend on the magnitude of the perturbation. In a study where a posteriorly directing constant force of 10-40 N against the sternum of a standing subject was unexpectedly withdrawn, leading to a compensatory forward body movement, the authors demonstrated that under such a 'regime of small oscillation' the PR could be described in terms of a linear three-link inverted pendulum (Bortolami et al. 2003), while stronger perturbations could result in additional strategies and violation of linearity. Based on that model, the authors have also postulated that the magnitude of the PR was proportional to the magnitude of the stimulus, although that was not experimentally verified. ...
... In the present study, the intra-individual examination of postural responses evidenced a moderate correlation of the PR with PF of the perturbation but a much stronger correlation with the associated I. This proportionality between stimulus magnitude and postural response confirms and supports the concept of linear behavior of postural control (Bortolami et al. 2003;Kim et al. 2009). Moreover, the apparent superiority of I over PF in driving the CoP is a new finding that, to the best of our knowledge, has never before been reported. ...
PurposePostural reactions (PR) of standing subjects have been mostly investigated in response to platform displacements or body perturbations of fixed magnitude. The objective of this study was to investigate the relationship between PR and the peak force and impulse of the perturbation.Methods
In ten healthy young men, standing balance was challenged by anteriorly directed perturbations (peak force: 20–60 N) delivered to the back, at the lumbar (L) or inter-scapular (IS) level, by means of a manual perturbator equipped with a force sensor. Postural reactions as expressed by the displacement of the center of pressure (CoP) were recorded using a force platform. Two sets of 20 randomly ordered perturbations (10 to each site) were delivered in two separate testing sessions.ResultsThe magnitude of CoP response (∆CoP) was better correlated with the impulse (I) than with the peak force of the perturbation. The normalized response, ∆CoPn = ∆CoP/I, exhibited good reliability (ICCs of 0.93 for IS and 0.82 for L), was higher with IS than with L perturbations (p < 0.01), and was significantly correlated with the latency of CoP response: r = 0.69 and 0.71 for IS and L, respectively.Conclusion
These preliminary findings support the concept that manually delivered perturbations can be used to reliably assess individual PR and that ∆CoPn may effectively express a relevant aspect of postural control.
... The present work uses the hold-and-release paradigm [13], combined with a time-based system identification technique [14] to identify the parameters of a multiple DOFs third-order skeletal-muscle model actuated at the hip and ankle. We modeled the non-linear equations of motion of a double inverted pendulum and used both a linear regression and a Kalman Filter to identify the stiffness associated with both the tendons and the muscle, considered as separate lumped parameters, as well as the damping of the muscles. ...
... Human movement along the sagittal plane can be modeled by means of a double inverted pendulum such as the one shown in Figure 1a. in the figure, the bottom segment is used to represent the legs while the top segment stands for the head, arms and trunk. This model does not account for the flexing of the knee but is enough to study the hip control strategy as reported in [13]. The two segments are connected by muscles and tendons of different visco-elastic properties [19,11]. ...
... The experiment followed the hold & release paradigm [13]. ...
Postural stability is important in everyday life as falls can cause severe injuries. Risk of injuries is higher in the elderly whose balance is often impaired. Modeling postural stability and the parameters that govern it is important to understand the balance mechanism and allow for the development of fall prevention strategies. Several mathematical models have been proposed to represent postural stability of bipeds. These models differ on the number of degrees-of-freedom (DOF) of the skeletal structure, force generation function for the muscle models, and capability to change their behavior as a function of the task. This work proposes a nonlinear model that captures fall recovery using a hip–ankle strategy. The muscle actuation is modeled as a third-order Poynting–Thomson's (PT) mechanical system where muscles and tendons are represented as lumped parameters actuating the aforementioned joints. Both a regression technique and a Kalman Filter (KF) are used to estimate the muscle–tendon parameters of the model. With a good model, the direct estimation of these parameters would allow clinicians to improve postural stability in the elderly, monitor the deterioration of the physical condition in individuals affected by neuro-degenerative diseases, and develop rehabilitation appropriate processes.
... The groups were compared using the chi-squared or Fisher test for categorical data (age, weight, height, direction, velocity). A general linear model repeated-measures ANOVA with the Bonferroni correction for multiple comparisons was used to compare the differences between falls and recoveries for each recorded dependent variable (38). ...
... The first two phases (T1 and T2) were considered to be mostly passive, dictated primarily by the inherent inertia and tone of a poly-articulated body translated at its basis, as strongly suggested by the simulated response of a mechanical model. Properties such as stiffness and damping intrinsic to the joints and muscles appeared to play a major role at the beginning of a fall, as previously suggested for the regulation of quiet upright stance (22,25,(38)(39)(40)(41)(42)(43). In contrast, it is suggested that the third phase (T3) is concomitant to the moment when active adjustments can be made, a point that is of major importance in the context of applying martial arts (or other) techniques for safe falling (see introduction). ...
... Such a passive phase has been observed or described by others as well (27,30). In their modeling study, Bortolami et al. (38) showed that a period of 125 ms after perturbation onset is needed before forces are generated for the CoP to go past the CoM, with the body performing a forward falling motion. Altogether then, it appears that one can start applying motor strategies aimed at preventing a fall at around 300 ms after the onset of perturbation (with a fall lasting typically about 700 ms). ...
In the present experiments, multiple balance perturbations were provided by unpredictable support-surface translations in various directions and velocities. The aim of this study was to distinguish the passive and the active phases during the pre-impact period of a fall. It was hypothesized that it should be feasible if one uses a specific quantitative kinematic analysis to evaluate the dispersion of the body segments trajectories across trials. Moreover, a multi-joint kinematical model was created for each subject, based on a new 3-D minimally invasive stereoradiographic X-ray images to assess subject-specific geometry and inertial parameters. The simulations allowed discriminating between the contributions of the passive (inertia-induced properties) and the active (neuromuscular response) components during falls. Our data show that there is limited time to adjust the way one fall from a standing position. We showed that the pre-impact period is truncated of 200 ms. During the initial part of a fall, the observed trajectory results from the interaction between the destabilizing external force and the body: inertial properties intrinsic to joints, ligaments and musculotendinous system have then a major contribution, as suggested for the regulation of static upright stance. This passive phase is later followed by an active phase, which consists of a corrective response to the postural perturbation. We believe that during a fall from standing height, it takes about 300 ms for postural responses to start correcting the body trajectory, while the impact is expected to occur around 700 ms. It has been argued that this time is sufficient to change the way one falls and that this makes it possible to apply safer ways of falling, for example by using martial arts fall techniques. Also, our results imply visual and vestibular information are not congruent with the beginning of the on-going fall. This consequence is to be noted as subjects prepare to the impact on the basis of sensory information, which would be uniquely mainly of proprioceptive origin at the fall onset. One limitation of the present analysis is that no EMG was included so far but these data are the subject of a future study.
... Žmogaus organizmui senstant bet kurio pusiausvyros komponento veiklos nusilpimas mažina stabilumą ir trikdo eiseną (Gauchard, 2003). Todėl ankstyva pusiausvyros sutrikimų diagnozė ir nustatymas yra svarbūs vyresnių žmonių funkcinio mobilumo mažėjimo sulėtinimui ir kritimų prevencijai (Bortolami et al., 2003). ...
... Gauti pusiausvyros tyrimo rezultatai leidžia manyti, kad vien tik aktyvi kasdienė fi zinė veikla nepagerina vyresnio amžiaus žmonių pusiausvyros. Norint pastebimai pagerinti vyresnio amžiaus žmonių pusiausvyrą, reikia taikyti ilgalaikes fi zinio aktyvumo programas (Bortolami et al., 2003), kuriose būtų numatytas visapusiškas kompleksinės pusiausvyros sistemos komponentų lavinimas. ...
Tyrimo tikslas ― įvertinti amžiaus ir fi zinio aktyvumo poveikį kojų raumenų funkcinei būklei ir pusiausvyrai. Ti-
riamosios ― skirtingo amžiaus moterys. I grupė — amžiaus vidurkis 24,5 ± 5,5 m. (n = 15), kūno masės indeksas
(KMI) — 20,4 ± 2,4; II grupė — amžiaus vidurkis 44,5 ± 3,5 m. (n = 15); KMI — 25,4 ± 3,9; III grupė — amžiaus
vidurkis 72,5 ± 7,5 m. (n = 15); KMI — 25,5 ± 2,9. Buvo atlikti trys funkcinės būklės vertinimo testai: atsistojimų
nuo kėdės testas (Rikli, Jones, 1999), „Stotis ir eiti“ testas (Rikli, Jones, 1999), blauzdos raumenų ištvermės testas
(Markon et al., 1992; Gaigalienė, 1999). Pusiausvyros tyrimo metu buvo taikyta statinė posturografi ja. Naudota se-
rijinės gamybos jėgos platforma ir kompiuterinė įranga signalams registruoti (KISTLER, Šveicarija, Slimline System
9286). III grupės tiriamųjų pasirengimas atlikti fi zinį krūvį vertintas pagal PAR-Q klausimyną, o jų fi zinis aktyvumas
nustatytas pagal RAPA anketą (Topolski et al., 2006). Tyrimo rezultatų analizei atlikti taikyta dispersinė analizė.
Tiriamųjų požymių vidurkio reikšmingumas tarp grupių buvo tikrinamas Tukey Post Hoc testu.
Ištyrus III grupės tiriamųjų funkcinę būklę paaiškėjo, kad fi zinis aktyvumas nepaveikė vyresnio amžiaus moterų „Stotis
ir eiti“ testo atlikimo greičio ir kojų raumenų jėgos. Fiziškai aktyvių tirtųjų moterų blauzdų raumenų ištvermė buvo
statistiškai patikimai (p < 0,01) didesnė nei fi ziškai neaktyvių tiriamųjų.
Skirtingo amžiaus tiriamųjų funkcinės būklės testų rezultatai skyrėsi statistiškai patikimai. Vyresnio amžiaus tiriamųjų
„Stotis ir eiti“ testo atlikimo greitis buvo statistiškai patikimai didesnis (p < 0,01) nei jaunesnio amžiaus ir viduti-
nio amžiaus tiriamųjų. Jaunesnio amžiaus tiriamųjų kojų raumenų jėga statistiškai patikimai didesnė už vidutinio
(p < 0,05) ir vyresnio amžiaus (p < 0,01) tiriamųjų kojų raumenų jėgą. Jaunesnio amžiaus tiriamųjų blauzdos rau-
menų ištvermė statistiškai patikimai didesnė už vidutinio (p < 0,05) ir vyresnio amžiaus (p < 0,01) tiriamųjų.
Atlikus statinės posturografi jos rezultatų analizę nustatytas neigiamas amžiaus poveikis tiriamųjų pusiausvyrai. Tre-
čios grupės tiriamųjų kūno slėgio centro (SC) svyravimai į šonus (dx) statistiškai reikšmingai didesni (p < 0,05) nei
pirmos grupės tiriamųjų. Pirmos grupės tiriamųjų SC svyravimai pirmyn — atgal (dy) statistiškai patikimai mažesni
nei trečios (p < 0,01) ir antros grupės tiriamųjų (p < 0,05).
Tyrimo rezultatai parodė aiškias amžiaus, tiriamųjų kojų raumenų funkcinės būklės ir pusiausvyros sąsajas. Fiziškai
aktyvių vyresnio amžiaus tiriamųjų blauzdų raumenų ištvermė buvo didesnė nei fi ziškai neaktyvių tiriamųjų, tačiau
pusiausvyros ir kojų raumenų jėgos fi zinis aktyvumas nepaveikė. Didėjant amžiui, tiriamųjų kojų raumenų funkcinė
būklė ir pusiausvyra blogėjo.
Raktažodžiai: raumenų funkcinė būklė, pusiausvyra, fizinis aktyvumas.
... For example, when deprived of vision, humans have problems maintaining a stable body position. When allowed to touch an object, this supports balance, helps to control body sway [132][133][134][135], and prevents recovery falls [136]. Other examples of how tactile input influences action are haptic exploration [137,138] or precision grips [126,139]. ...
The study of somatosensory plasticity offers unique insights into the neuronal mechanisms that underlie human adaptive and maladaptive plasticity. So far, little attention has been paid on the specific influence of visual body perception on somatosensory plasticity and learning in humans. Here, we review evidence on how visual body perception induces changes in the functional architecture of the somatosensory system and discuss the specific influence the social environment has on tactile plasticity and learning. We focus on studies that have been published in the areas of human cognitive and clinical neuroscience and refer to animal studies when appropriate. We discuss the therapeutic potential of socially mediated modulations of somatosensory plasticity and introduce specific paradigms to induce plastic changes under controlled conditions. This review offers a contribution to understanding the complex interactions between social perception and somatosensory learning by focusing on a novel research field: socially mediated sensory plasticity.
... It serves as a tool to determine strategies employed by the CNS to recover balance. In H&R a person stands with feet side-by-side and arms crossed and actively resists a hold force applied against the sternum (Bortolami, DiZio, Rabin, & Lackner, 2003). Without warning, the force is rapidly withdrawn (released). ...
Gravity is such a constant environmental influence on spatial orientation and movement that the active CNS adaptations to it are easily underestimated, and they are best revealed when the background force conditions are altered. This chapter reviews illusions and movement errors that occur in hypo‐ and hypergravity environments as well as in a rotating artificial gravity environment where additional novel dynamic Coriolis forces are present. The phenomena reviewed reveal the importance of tactile cues from contact forces on the body surface and proprioceptive cues from muscle spindle and tendon receptor loading that are usually perceptually inaccessible, in addition to the vestibular cues that traditionally receive consideration. The role of these touch, pressure, and kinesthetic cues in visual and auditory localization, body schema, motor coordination, and learning of simple reaching movements to complex postural balancing are discussed.
