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A graphic depiction of the findings of Fling et al. (2011). Panel (A) represents fractional anisotropy (FA) of transcallosal fibers connecting homologous motor regions. Thicker tubes indicate higher FA values. SMA fibers exhibit greater FA values than pre-SMA, M1 and S1. Transcallosal PMd fiber FA values are greater than those connecting pre-SMA, M1 and S1. Fibers connecting homologous M1 and homologous pre-SMA exhibit higher FA values than S1 fibers. Panel (B) represents the quantity of interhemispheric fiber tracts connecting homologous motor regions. There are more fiber tracts connecting homologous SMA regions than M1, PMd and pre-SMA. There are more fibers connecting pre-SMA than M1, S1 or PMd. Transcallosal M1 fibers are more numerous than those connecting S1 or PMd. (See text for details).
Source publication
Cross education is the process whereby training of one limb gives rise to enhancements in the performance of the opposite, untrained limb. Despite interest in this phenomenon having been sustained for more than a century, a comprehensive explanation of the mediating neural mechanisms remains elusive. With new evidence emerging that cross education...
Citations
... As the contralateral limb remains able to perform high-intensity muscle contractions in this situation, clinicians and practitioners may prescribe unilateral training to counter the adverse effects of detraining in the exerciserestricted limb (Hendy et al. 2012;Farthing, Krentz et al. 2009). However, the cross-education of strength may be relatively ineffective in trained individuals as the proposed cortical adaptations (Ruddy and Carson 2013;Lee et al. 2010) underlying the response may already be induced. In support of this theory, a recent study has reported that unilateral leg extensor training prescribed at increasing weekly loads (70-85% of one-repetition maximum [1-RM] for 4 weeks) was unable to maintain muscle strength and power in the contralateral leg extensors of recreationally engaged older women (> 60 years) (de Souza Teixeira et al. 2023). ...
... While unilateral strength training improves activation in homologous, contralateral muscles (Green and Gabriel 2018;Lee, Gandevia, Carroll 2009), alterations in motor strategy (learning) might at least partly explain the effect (Ruddy and Carson 2013); the 'bilateral access' hypothesis is one theoretical model of this phenomenon (Ruddy and Carson 2013;Lee et al. 2010). According to this hypothesis, the repeated performance of specific motor skills generates motor engrams or stored memories. ...
... While unilateral strength training improves activation in homologous, contralateral muscles (Green and Gabriel 2018;Lee, Gandevia, Carroll 2009), alterations in motor strategy (learning) might at least partly explain the effect (Ruddy and Carson 2013); the 'bilateral access' hypothesis is one theoretical model of this phenomenon (Ruddy and Carson 2013;Lee et al. 2010). According to this hypothesis, the repeated performance of specific motor skills generates motor engrams or stored memories. ...
Purpose
Unilateral strength training may attenuate the decline in muscle strength and size in homologous, contralateral muscles. This study aimed to determine whether the cross-education of strength could specifically attenuate the effects of detraining immediately after a short (prehabilitation-type) period of strength training.
Methods
Twenty-six strength-trained participants were assigned to either four weeks of unilateral strength training of the stronger arm (UNI) or detraining (Detrain). Motor evoked potential (MEP) and cortical silent period (cSP) responses, muscle cross-sectional area (CSAFlexor; peripheral quantitative computed tomography) and maximal strength, rate of force development (RFD) and muscle activation (EMG) were examined in both elbow flexors before and after the intervention period.
Results
In UNI, one-repetition maximum (1-RM) strength improved in both the trained (∆ = 2.0 ± 0.9 kg) and non-trained (∆ = 0.8 ± 0.9 kg) arms despite cessation of training of the weaker arm, whereas 1-RM strength was unchanged in Detrain. Maximal voluntary isometric contraction, isokinetic peak torque, and RFD did not change in either group. No neural changes were detected in UNI, but cSP increased in Detrain (∆ = 0.010 ± 0.015 s). CSAFlexor increased in the trained arm (∆ = 51 ± 43 mm²) but decreased in the non-trained arm (∆ = -53 ± 50 mm²) in UNI. CSAFlexor decreased in both arms in Detrain and at a similar rate to the non-trained arm in UNI.
Conclusion
UNI attenuated the effects of detraining in the weaker arm as shown by the improvement in 1-RM strength. However, the cross-education of strength did not attenuate the decline in muscle size in the contralateral arm.
... Cross-education of relative muscular endurance likely cannot be explained by increased strength in the contralateral limb as relative muscular endurance is scaled to current maximal strength. Two main theoretical models, which may not be mutually exclusive, have been proposed to explain the cross-education of strength and skills: "bilateral access" and "cross-activation" models [50]. Although speculative, these two models may also explain the cross-education of muscular endurance. ...
... Although speculative, these two models may also explain the cross-education of muscular endurance. The "bilateral access" model involves the development of a motor engram during unilateral resistance training, which can be accessed not only by the trained limb, but also by the untrained limb for the control and execution of movements [50,51]. A widely used example is the "callosal access" hypothesis, in which the motor engrams developed in the trained hemisphere may be accessed by the opposite untrained hemisphere via the corpus callosum during motor tasks in the untrained limb [50,51]. ...
... The "bilateral access" model involves the development of a motor engram during unilateral resistance training, which can be accessed not only by the trained limb, but also by the untrained limb for the control and execution of movements [50,51]. A widely used example is the "callosal access" hypothesis, in which the motor engrams developed in the trained hemisphere may be accessed by the opposite untrained hemisphere via the corpus callosum during motor tasks in the untrained limb [50,51]. In this model, it has been hypothesized that performing unilateral resistance training may develop an effective muscle recruitment pattern for maximum force production (i.e., muscle strength), such as coordination of synergists and inhibition of antagonists, which can be stored in neural circuits and accessed by the untrained hemisphere [4]. ...
Background
It is well established that performing unilateral resistance training can increase muscle strength not only in the trained limb but also in the contralateral untrained limb, which is widely known as the cross-education of strength. However, less attention has been paid to the question of whether performing unilateral resistance training can induce cross-education of muscular endurance, despite its significant role in both athletic performance and activities of daily living.
Objectives
The objectives of this scoping review were to provide an overview of the existing literature on cross-education of muscular endurance, as well as discuss its potential underlying mechanisms and offer considerations for future research.
Methods
A scoping review was conducted on the effects of unilateral resistance training on changes in muscular endurance in the contralateral untrained limb. This scoping review was conducted in PubMed, SPORTDiscus, and Scopus.
Results
A total of 2000 articles were screened and 21 articles met the inclusion criteria. Among the 21 included studies, eight studies examined the cross-education of endurance via absolute (n = 6) or relative (n = 2) muscular endurance test, while five studies did not clearly indicate whether they examined absolute or relative muscular endurance. The remaining eight studies examined different types of muscular endurance measurements (e.g., time to task failure, total work, and fatigue index).
Conclusion
The current body of the literature does not provide sufficient evidence to draw clear conclusions on whether the cross-education of muscular endurance is present. The cross-education of muscular endurance (if it exists) may be potentially driven by neural adaptations (via bilateral access and/or cross-activation models that lead to cross-education of strength) and increased tolerance to exercise-induced discomfort. However, the limited number of available randomized controlled trials and the lack of understanding of underlying mechanisms provide a rationale for future research.
... As there is a homunculi motoneuron organization, the homologous muscles may be spatially closer and thus better able to access this "spillover effect" than the further afield heterologous muscles. With the cross education, cross activation hypothesis, activation of homologous motor networks leads to bilateral activation and adaptations that facilitate subsequent performance (Ruddy & Carson, 2013). In the present context, learning to increase stretch or pain tolerance in one muscle group would be bilaterally taught or transferred preferentially to the contralateral homologous muscle group with less transfer to heterologous muscle groups. ...
Both an acute bout, as well as chronic static stretching (SS), can increase the joint range of motion (ROM). However, ROM increases of a non‐stretched muscle (non‐local) are reported following an acute SS session, and these effects have not been studied for long‐term SS training. Therefore, this study aimed to investigate the effects of a comprehensive 7‐week SS training program of the pectoralis muscles on ankle dorsiflexion ROM. Thirty‐three healthy, physically active participants (20 male and 13 female) were assigned to either the SS (n = 18) or the control (n = 15) group. The SS group performed a 7‐week SS intervention that comprised three sessions a week, including three exercises of the pectoralis muscles for 5‐min each. Before and after the intervention period, the ankle dorsiflexion ROM was tested with a dynamometer. There was no significant time (p = 0.93, F1,31 = 0.008; η2 = 0.000) or time x group effect (p = 0. 56, F1,31 = 0.342; η2 = 0.011) in ankle dorsiflexion ROM, indicating no changes in ROM in the intervention as well as the control group. Although previous studies on the acute effects of stretching reported non‐local increases in ROM, our study showed no such changes after 7 weeks of SS training. Consequently, if the goal is to chronically increase the ROM of a specific joint, it is recommended to directly stretch the muscles of interest.
... Drawing inspiration from previous studies that underscore the importance of prolonged observational periods (Carey et al., 2005;Park et al., 2022), our approach is tailored to study right-handed individuals as they perform tasks using their nondominant hand. This specific design choice not only harmonizes with the principles of cerebral lateralization but also aims to enhance the visibility of neural changes during the motor learning process (Liebert et al., 2015;Ruddy & Carson, 2013). Guided by prior studies that have highlighted variable brain region activations during motor learning (Ghilardi et al., 2000), we hypothesize that distinct stages-rapid learning, consolidation, and stable performance-will be characterized by unique cortical activation patterns. ...
... Factoring in challenges and insights from recent studies, our taskspecific design seeks to enhance the homogeneity of participants' motor learning baseline and accentuate the observation amplitude of neural changes (Ruddy & Carson, 2013). Drawing from recent research insights, we understand that cortical activation patterns' evolution is influenced by task complexity and the learning phase. ...
Background
Motor learning is essential for performing specific tasks and progresses through distinct stages, including the rapid learning phase (initial skill acquisition), the consolidation phase (skill refinement), and the stable performance phase (skill mastery and maintenance). Understanding the cortical activation dynamics during these stages can guide targeted rehabilitation interventions.
Methods
In this longitudinal randomized controlled trial, functional near‐infrared spectroscopy was used to explore the temporal dynamics of cortical activation in hand‐related motor learning. Thirty‐one healthy right‐handed individuals were randomly assigned to perform either easy or intricate motor tasks with their non‐dominant hand over 10 days. We conducted 10 monitoring sessions to track cortical activation in the right hemisphere (according to lateralization principles, the primary hemisphere for motor control) and evaluated motor proficiency concurrently.
Results
The study delineated three stages of nondominant hand motor learning: rapid learning (days 1 and 2), consolidation (days 3–7), and stable performance (days 8–10). There was a power‐law enhancement of motor skills correlated with learning progression. Sustained activation was observed in the supplementary motor area (SMA) and parietal lobe (PL), whereas activation in the right primary motor cortex (M1R) and dorsolateral prefrontal cortex (PFCR) decreased. These cortical activation patterns exhibited a high correlation with the augmentation of motor proficiency.
Conclusions
The findings suggest that early rehabilitation interventions, such as transcranial magnetic stimulation and transcranial direct current stimulation (tDCS), could be optimally directed at M1 and PFC in the initial stages. In contrast, SMA and PL can be targeted throughout the motor learning process. This research illuminates the path for developing tailored motor rehabilitation interventions based on specific stages of motor learning.
NEW and NOTEWORTHY
In an innovative approach, our study uniquely combines a longitudinal design with the robustness of generalized estimating equations (GEEs). With the synergy of functional near‐infrared spectroscopy (fNIRS) and the Minnesota Manual Dexterity Test (MMDT) paradigm, we precisely trace the evolution of neural resources during complex, real‐world fine‐motor task learning. Centering on right‐handed participants using their nondominant hand magnifies the intricacies of right hemisphere spatial motor processing. We unravel the brain's dynamic response throughout motor learning stages and its potent link to motor skill enhancement. Significantly, our data point toward the early‐phase rehabilitation potential of TMS and transcranial direct current stimulation on the M1 and PFC regions. Concurrently, SMA and PL appear poised to benefit from ongoing interventions during the entire learning curve. Our findings carve a path for refined motor rehabilitation strategies, underscoring the importance of timely noninvasive brain stimulation treatments.
... The lack of growth in the untrained limb is in alignment with the existing body of literature (Dankel et al. 2020;Bell et al. 2023). Prior work on cross-education indicates that adaptations within the nervous system primarily influence the cross-education of strength (Ruddy and Carson 2013). Cross-education mechanisms have been attributed to bilateral coactivation via corticospinal pathways (Carr et al. 1994), impulse diffusion between cerebral hemispheres (Cernaek 1961), coordination learning or postural stabilization (Carolan and Cafarelli 1992), and modulation of afferents (Hortobágyi et al. 1999) have all been suggested as possible mechanisms. ...
Introduction
The application of blood flow restriction (BFR) to low-intensity exercise may be able to increase strength not only in the trained limb but also in the homologous untrained limb. Whether this effect is repeatable and how that change compares to that observed with higher intensity exercise is unknown.
Purpose
Examine whether low-intensity training with BFR enhances the cross-education of strength compared to exercise without BFR and maximal efforts.
Methods
A total of 179 participants completed the 6-week study, with 135 individuals performing isometric handgrip training over 18 sessions. Participants were randomly assigned to one of four groups: 1) low-intensity (4 × 2 min of 30% MVC; LI, n = 47), 2) low-intensity with blood flow restriction (LI + 50% arterial occlusion pressure; LI-BFR, n = 41), 3) maximal effort (4 × 5 s of 100% MVC; MAX, n = 47), and 4) non-exercise control (CON, n = 44).
Results
LI-BFR was the only group that observed a cross-education in strength (CON: 0.64 SD 2.9 kg, LI: 0.95 SD 3.6 kg, BFR-LI: 2.7 SD 3.3 kg, MAX: 0.80 SD 3.1 kg). In the trained hand, MAX observed the greatest change in strength (4.8 SD 3.3 kg) followed by LI-BFR (2.8 SD 4.0 kg). LI was not different from CON. Muscle thickness did not change in the untrained arm, but ulna muscle thickness was increased within the trained arm of the LI-BFR group (0.06 SD 0.11 cm).
Conclusion
Incorporating BFR into low-intensity isometric training led to a cross-education effect on strength that was greater than all other groups (including high-intensity training).
... Our findings showed that CE+PT had led to a better functional motor recovery compared to conventional PT. The improvement of BBT, FMA-UE, and grip strength scores was significantly more pronounced in patients treated with CE than those treated with conventional PT [44,45]. It was hypothesized that spinal circuitry is adjusted by CE. ...
Background
Stroke is one of the causes of long-term morbidity. Despite rehabilitation strategies, most survivors live with motor deficits in the upper limbs.
Objectives
The aim of the study was to compare the effect of contralateral cross education (CE) and high-frequency repetitive magnetic stimulation (HF-rTMS) on the function of upper extremity in subacute phase of stroke.
Methods
Forty patients were randomly assigned into 4 groups. Group “A” received physical therapy (PT) for 10 sessions, 3 times per week. Group “B” received PT and HF-rTMS as follows: stimulation of 20 Hz for 5 s, intertrain interval for 50 s, 20 trains, 2000 pulses at 90% resting motor threshold, and conventional PT. Group “C” was treated with CE and PT. In group “D,” HF-rTMS, CE, and PT were administered.
Results
Significant differences were found in the Fugl-Meyer scale between “A” and “C” (P = 0.01), “A” and “D” (P = 0.02), and “B” and “C” groups (P = 0.01). In the box-block test, there were significant differences between “A” and “B” (P = 0.01), “A” and “C” (P < 0.001), “B” and “D” (P = 0.001), and “B” and “C” groups (P = 0.01). Statistical differences were observed in grip strength between “A” and “B” (P = 0.01) and “A” and “C” groups (P = 0.02).
Conclusions
It is suggested that clinicians select the therapeutic methods in line with their expected goal. When the goal is to improve upper extremity function, CE+PT could be more effective than HF-rTMS+PT. Also, CE+PT and HF-rTMS+PT were more effective than CE+HF-rTMS+PT at improving grip strength. Therefore, combining several methods would not always lead to better results.
... 3 The mechanisms behind the cross education of strength includes cortical and spinal adaptations, which alter the neural drive to the contralateral, untrained limb. 4 Neural plasticity in the cortical regions of the brain have been proposed to explain the cross education phenomenon. 5 The two main ideas used to explain the concept of cross training are "cross-activation" and "bilateral access." ...
... A suggested strategy to prevent decline in muscle force and power during a short-term detraining is cross-education. Cross-education, or more commonly called cross-transfer, described primarily by Scripture Smith, and Brown (1894) (Scripture et al., 1894), can be defined as the increase in maximal strength of the untrained contralateral limb after a period of resistance training (RT) in the homologous limb (Hendy and Lamon, 2017;Pelet and Orsatti, 2021;Ruddy and Carson, 2013). The effects of cross-education are thought to involve changes at different levels of the nervous system, including cortical, subcortical, and spinal reflex pathways (Ruddy and Carson, 2013). ...
... Cross-education, or more commonly called cross-transfer, described primarily by Scripture Smith, and Brown (1894) (Scripture et al., 1894), can be defined as the increase in maximal strength of the untrained contralateral limb after a period of resistance training (RT) in the homologous limb (Hendy and Lamon, 2017;Pelet and Orsatti, 2021;Ruddy and Carson, 2013). The effects of cross-education are thought to involve changes at different levels of the nervous system, including cortical, subcortical, and spinal reflex pathways (Ruddy and Carson, 2013). Although skeletal muscle-local-mechanism as contributor to the observed increase in strength are largely ruled out (Lee et al., 2010;Manca et al., 2018), the increase provoked by RT on muscle force seems not be dependent of muscle mass gain (Loenneke et al., 2019). ...
... Although skeletal muscle-local-mechanism as contributor to the observed increase in strength are largely ruled out (Lee et al., 2010;Manca et al., 2018), the increase provoked by RT on muscle force seems not be dependent of muscle mass gain (Loenneke et al., 2019). Finally, theories of crosseducation that focus on the interaction between the two cerebral hemispheres, particularly cross-activation and bilateral access, provide the most robust evidence (Lee et al., 2010;Manca et al., 2018;Ruddy and Carson, 2013). ...
... In principle, three models for the bilateral transfer effect are postulated on central levels but mostly tested for motor skills of the hands. Ruddy and Carson, (2013) postulated that neural adaptations induced during unilateral exercise would spread to the opposite side of the body (crossactivation model; (Parlow and Kinsbourne, 1989;Ruddy and Carson, 2013). At sub-cortical and cortical levels, previous work confirms the presence of a neural interaction between the two hemispheres (Carroll et al., 2006;Farthing et al., 2007), supporting the cross-activation/spill over model proposed by (Carroll et al., 2006;Ruddy and Carson, 2013). ...
... In principle, three models for the bilateral transfer effect are postulated on central levels but mostly tested for motor skills of the hands. Ruddy and Carson, (2013) postulated that neural adaptations induced during unilateral exercise would spread to the opposite side of the body (crossactivation model; (Parlow and Kinsbourne, 1989;Ruddy and Carson, 2013). At sub-cortical and cortical levels, previous work confirms the presence of a neural interaction between the two hemispheres (Carroll et al., 2006;Farthing et al., 2007), supporting the cross-activation/spill over model proposed by (Carroll et al., 2006;Ruddy and Carson, 2013). ...
... Ruddy and Carson, (2013) postulated that neural adaptations induced during unilateral exercise would spread to the opposite side of the body (crossactivation model; (Parlow and Kinsbourne, 1989;Ruddy and Carson, 2013). At sub-cortical and cortical levels, previous work confirms the presence of a neural interaction between the two hemispheres (Carroll et al., 2006;Farthing et al., 2007), supporting the cross-activation/spill over model proposed by (Carroll et al., 2006;Ruddy and Carson, 2013). Secondly, that the trained motor plan of a unilateral task is accessible by an attempt of reproducing the same task in the opposite side of the body and would therefore facilitate motor activation in the untrained limb (access model/ callosal model; (Sainburg and Wang, 2002)). ...
Motor imagery training could be an important treatment of reduced muscle
function in patients and injured athletes. In this study, we investigated the
efficacy of imagery training on maximal force production in a larger muscle
group (hip abductors) and potential bilateral transfer effects. Healthy
participants (n = 77) took part in two experimental studies using two imagery
protocols (~30 min/day, 5 days/week for 2 weeks) compared either with no
practice (study 1), or with isometric exercise training (study 2). Maximal hip
abduction isometric torque, electromyography amplitudes (trained and
untrained limbs), handgrip strength, right shoulder abduction (strength and
electromyography), and imagery capability were measured before and after the
intervention. Post intervention, motor imagery groups of both studies exhibited
significant increase in hip abductors strength (~8%, trained side) and improved
imagery capability. Further results showed that imagery training induced bilateral
transfer effects on muscle strength and electromyography amplitude of hip
abductors. Motor imagery training was effective in creating functional
improvements in limb muscles of trained and untrained sides
... 5,6 The transfer of strength to the untrained homologous muscle on the contralateral side of the body is known as the cross-education effect. [7][8][9] This improvement in strength within the untrained muscles is generally considered to be neural in origin, 10,11 with previous studies presenting increased strength in the arms, 12 legs, 13 and muscles of the hand. 10 This transfer of strength is estimated to be 52 % of the ipsilateral training effect, 8 can occur in as little as a few weeks of training, 14 and generally does not coincide with any phenotypical changes (e.g. ...
