Step Frequency and Lower Extremity Loading During Running

Abstract

The purpose of the present study was to ascertain whether increase in step frequency at a given velocity during running reduces the lower extremity loading variables, which is associated with tibial stress fracture in runner. We hypothesized that the lower extremity loading variables at a given speed would be minimized at around +15% f step. 10 male subjects were asked to run at 2.5 m/s on a treadmill-mounted force platform. 5 step frequencies were controlled using a metronome: the preferred, below preferred (-15 and -30%) and above preferred (+15 and +30%). From the vertical ground reaction force, we measured following lower extremity loading variables; vertical impact peak (VIP), vertical instantaneous loading rate (VILR) and vertical average loading rate (VALR). We found that there were significant differences in lower extremity loading variables among 5 step frequency conditions. Furthermore, quadratic regression analyses revealed that the minimum loading variable frequencies were 17.25, 17.55, and 18.07% of preferred step frequency for VIP, VILR and VIAR, respectively. Thus, adopting a step frequency greater than one's preferred may be practical in reducing the risk of developing a tibial stress fracture by decreasing lower extremity loading variables.

Full text

310 Orthopedics & Biomechanics
Hobara H et al. Step Frequency and Lower … Int J Sports Med 2012; 33: 310–313
accepted after revision
September 14 , 2011
Bibliography
DOI http://dx.doi.org/
10.1055/s-0031-1291232
Published online:
March 1, 2012
Int J Sports Med 2012; 33:
310–313 © Georg Thieme
Verlag KG Stuttgart · New York
ISSN 0172-4622
Correspondence
Dr. Hiroaki Hobara
Research Institute
National Rehabilitation Center
for Persons with Disabilities
Department of Rehabilitation
for the Movement Functions
4-1
Namiki
T o k o r o z a w a
Japan
359-8555
Tel.: + 81/4/2995 3100
(ext.: 7274)
Fax: + 81/4/2995 3132
hobara-hiroaki@rehab.go.jp
Key words
●
▶
tibial stress fracture
●
▶
running injury
●
▶
step frequency
Step Frequency and Lower Extremity Loading During
Running
the risk of developing a running-related injury or
facilitating recovery from an existing injury
[ 5 , 6 , 10 , 12 ] . However, it is still unknown how the
f step manipulations aff ect overall lower extremity
loading during running. The purpose of the
present study was to ascertain whether increase
in f step at a given velocity during running reduces
the lower extremity loading variables. We
hypothesized that the lower extremity loading
variables at a given speed would be minimized at
around + 15 % of preferred f step .
Methods
▼
Participants
10 male participants with no neuromuscular dis-
orders or functional limitations in their lower
extremity participated in the study. Their physi-
cal characteristics were: age 28.8 ± 3.0 years,
body mass 71.5 ± 9.3 kg, and height 1.75 ± 0.04 m
(mean ± SD). They were all moderately active and
recreationally trained but not engaged in system-
atic running training. Our study has been per-
formed in accordance with the ethical standards
Introduction
▼
Tibial stress fractures are one of the most com-
mon and potentially serious overuse injuries in
runners [ 13 ] . The stress fractures are thought to
be related, in part, to abnormal lower extremity
loading variables, such as vertical impact peak
(VIP), vertical instantaneous loading rate (VILR)
and vertical average loading rate (VALR). In order
to understand associations between loading and
the history of tibial stress fracture, several
researchers have compared the characteristics of
the ground reaction force (GRF) between control
and stress fracture group [ 26 ] . For example, some
studies reported that runners who developed
tibial stress fractures had higher VILR and VALR
than a group of age and mileage matched control
subjects [ 3 , 8 , 22 , 27 ] . Furthermore, there was a
trend towards higher VIP in the group that had a
stress fracture compared to the control group
[ 8 , 17 ] . Thus, modifying these loading mechanics
may decrease a runner’s risk for stress fractures.
Some studies suggest that adopting a 10–20 %
increase in step frequency ( f step ) greater than
one’s preferred may prove benefi cial in reducing
Authors H. Hobara
1 , T. Sato
2 , M. Sakaguchi
2 , T. Sato
3 , K. Nakazawa
4
Affi liations
1 Research Institute , National Rehabilitation Center for Persons with Disabilities , Department of Rehabilitation for the
Movement Functions , Tokorozawa , Japan
2 Graduate School of Sport Sciences , Waseda University , Tokorozawa , Japan
3 Faculty of Human Life Sciences , Jissen Women’s University , Hino , Japan
4 Graduate School of Arts and Sciences , The University of Tokyo , Japan
Abstract
▼
The purpose of the present study was to ascer-
tain whether increase in step frequency at a
given velocity during running reduces the lower
extremity loading variables, which is associ-
ated with tibial stress fracture in runner. We
hypothesized that the lower extremity loading
variables at a given speed would be minimized
at around + 15 % f step . 10 male subjects were
asked to run at 2.5 m/s on a treadmill-mounted
force platform. 5 step frequencies were control-
led using a metronome: the preferred, below
preferred ( − 15 and − 30 %) and above preferred
( + 15 and + 30 %). From the vertical ground reac-
tion force, we measured following lower extrem-
ity loading variables; vertical impact peak (VIP),
vertical instantaneous loading rate (VILR) and
vertical average loading rate (VALR). We found
that there were signifi cant diff erences in lower
extremity loading variables among 5 step fre-
quency conditions. Furthermore, quadratic
regression analyses revealed that the minimum
loading variable frequencies were 17.25, 17.55,
and 18.07 % of preferred step frequency for VIP,
VILR and VIAR, respectively. Thus, adopting a
step frequency greater than one’s preferred may
be practical in reducing the risk of developing a
tibial stress fracture by decreasing lower extrem-
ity loading variables.
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311Orthopedics & Biomechanics
Hobara H et al. Step Frequency and Lower … Int J Sports Med 2012; 33: 310–313
of the International Journal of Sports Medicine [ 11 ] . The experi-
mental protocol was approved by the local ethical committee
and is in accordance with guidelines set out in the Declaration of
Helsinki (1964).
Task and procedure
The subjects were asked to run on a treadmill-mounted force
platform (ITR3017, Bertec Corporation, Columbus, OH), from
which the vertical ground reaction force (GRF) was recorded at
1 000 Hz. The natural frequency of vertical vibration of the force
platform mounted in the treadmill was 240 Hz. The GRF was then
low-pass fi ltered at 100 Hz. Running speed was set at 2.5 m/s,
because this was low enough to allow a larger range of step fre-
quencies above and below the preferred f step of normal subjects.
Each subject ran on the treadmill and directed the experimenter
to increase or decrease the f step until a comfortable f step was found.
On average, the preferred f step was 2.73 ± 0.14 Hz (mean ± standard
deviation). Subjects ran with a digital audio metronome to facili-
tate the appropriate f step . Since a past fi nding reported that a
10–20 % increase in step rate substantially reduces tibial accelera-
tion and energy absorption [ 5 , 10 ] , we controlled 5 f step : the pre-
ferred, below preferred ( − 15 and − 30 %) and above preferred
( + 15 and + 30 %). Before data collection, all participants were
instructed to practice for as long as they needed until they felt
comfortable with the task (it ranged from 3 to 4 min). According
to their subjective impression, this practice session was enough to
get used to the task. Also, none reported feeling fatigue. Then, they
performed running for 30 s at each of the 5 f step in a random order
with 5 min rest periods in between.
Data collection and analyses
10 consecutive steps from both legs were used for the analysis.
Foot-ground contact was determined at a vertical GRF threshold
of 20 N. From the measurement of GRF, the VIP, VILR and VALR
were determined (
●
▶
Fig. 1 ). VILR was the peak sample-to sam-
ple loading rate occurring during 20–80 % of VIP for this period
[ 1 , 18 ] . VALR was calculated as the total change in force divided
by the total change in time over this period. When no distinct
impact transient was present, the same parameters were meas-
ured using the average percentage of stance as determined for
each condition in trials [ 15 ] .
Statistics
One-way repeated measures ANOVA and Bonferroni post-hoc
multiple comparison test were performed to compare the load-
ing variables among 5 frequencies. Further, we performed quad-
ratic regression analyses by the least square method to each
loading variable for obtaining the minimum value frequency.
Statistical signifi cance was set at P < 0.05. SPSS for Windows
software (Version 13.0, SPSS Inc.) was used for all statistical
analyses. All data are presented as the mean ± the standard devi-
ation (SD).
Results
▼
In all conditions, actual performed f step was within 5 % of the
designated metronome frequency. Statistical analyses revealed
that there were signifi cant diff erences in VIP, VILR and VALR
among 5 f step conditions ( ●
▶
Fig. 2 ). The results of the regression
analyses were as follows: VIP, y = 0.0002 x
2 − 0.0069x + 1.7033,
R 2 = 0.982; VILR, y = 0.0277 x 2 − 0.9725x + 99.623, R 2 = 0.997; VALR,
y = 0.0239 x 2 − 0.8642x + 77.121, R 2 = 0.997, where x is step fre-
quency ( %) and y is the loading variables. According to these equa-
tions, the minimum loading variable frequencies were 17.25,
17.55, and 18.07 % of f step for VIP, VILR and VALR, respectively.
Discussion
▼
Since a 10–20 % increase in step rate substantially reduces joint
loading, adopting an f step greater than one’s preferred may prove
Fig. 1 Vertical ground reaction force (vGRF) during the stance, recorded
from a single subject in preferred f step conditions. Lower extremity loading
variables (VIP, VILR and VALR) were determined at early stance phase.
3.0
2.5
VIP
80%
20%
VILR
VALR
2.0
vGRF (BW)
1.5
1.0
0.5
0.0 020 40
% STANCE
60 80 100
Fig. 2 Comparison of VIP, VILR and VALR among 5 f step conditions.
A dagger and asterisk indicates signifi cant diff erences from value for
the − 30 % f step ; P < 0.01, P < 0.05, respectively.
3.0
*
††
†
†
†
†
**
200
150
Loading rate (BW/s)
100
50
0
2.5
2.0
VIP (BW)
1.5
1.0 –30% –15% 0%
Step frequency (%)
+15% +30%
VIP VILR VALR
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312 Orthopedics & Biomechanics
Hobara H et al. Step Frequency and Lower … Int J Sports Med 2012; 33: 310–313
benefi cial in reducing the risk of developing a running-related
injury [ 5 , 6 , 10 , 12 ] . We found that there were signifi cant diff er-
ences in lower extremity loading variables among 5 f step condi-
tions (
●
▶
Fig. 2 ). Furthermore, the minimum loading variable
frequencies were 17.25, 17.55, and 18.07 % of f step for VIP, VILR
and VALR, respectively. Thus, the results of the present study
support our null hypothesis that the lower extremity loading
variables at a given speed would be minimized at around + 15 %
of preferred f step .
One possible explanation for reduced lower extremity loading
variables in + 15 % f step may be a change in the foot strike pattern.
Indeed, Laughton et al. [ 14 ] and Lieberman et al. [ 15 ] demon-
strated that as compared to rearfoot strike, forefoot landing pat-
tern diminishes impact transient in early stance phase during
running. A second explanation for the reduced lower extremity
loading variables may be knee angle at foot strike. Derrick et al.
[ 4 ] showed that increased knee fl exion at contact could reduce
the forces experienced by the body and therefore decrease injury
potential. Furthermore, the changes in knee contact angle could
increase perpendicular distance from the line of action of the
resultant GRF to the knee joint center, which played a role in
increased energy absorption [ 4 ] . Thus, reduced lower extremity
loading variables in + 15 % f step may be attributed to foot strike
pattern and knee angle at foot strike, or combinations of these
changes. The mechanisms of reduced lower extremity loading
variables should be the subject of future investigation.
Stress fractures are thought to be related, in part, to abnormal
lower extremity loading in runners [ 3 , 8 , 17 , 26 , 27 ] . Recently,
some studies demonstrated reduced impact loading during run-
ning by using a real-time visual feedback gait retraining system
[ 1 , 2 ] . However, the methodologies of these studies required
very specialized materials or apparatus (e. g. treadmill, acceler-
ometer, computer, and monitor). Adopting a f step greater than
one’s preferred is practical not only in reducing the risk of devel-
oping a running-related injury, but also no materials or appara-
tus are needed.
Since increased f step at a given speed results in an increase in
loading cycle, the running technique could lead to another prob-
lem. First, if increase in the f step were to increase the metabolic
cost of running, the muscle fatigue is likely to onset sooner in
comparison to a preferred one. Indeed, Milgrom et al. [ 16 ] sug-
gested that the fatigue state increases bone strains well above
those recorded in rested individuals and may be a major factor in
the stress fracture etiology. However, a previous study suggested
that a 10 % reduction in stride length does not signifi cantly
change oxygen consumption and heart rate from a preferred
stride frequency [ 10 ] . Consequently, Edwards et al. [ 6 ] con-
cluded that the change in metabolic cost from a 10 % reduction in
stride length is negligible and that this type of kinematic adjust-
ment would not accelerate the bone microdamage process
through fatiguing muscles. Second, an increase in loading cycle
will increase running mileage for the probability of tibial stress
fracture at the tibia. However, Edwards et al. [ 6 ] suggested that
strain magnitude plays a more important role in stress fracture
development than the total number of loading cycles. Thus,
adopting a f step greater than one’s preferred is unlikely to induce
another running-related injury.
In the present study, a treadmill-mounted force platform was
used for our experiment. It is relatively well-established that the
biomechanical characteristics of running on a treadmill are
quite diff erent from the characteristics of actual overground
running [ 7 , 9 , 19 , 20 ] . This might be attributed to the diff erences
in air resistance, visual and auditory surroundings, and running
surfaces [ 23 ] . On the other hand, some studies reported overall
similarities between treadmill and overground running
[ 21 , 24 , 25 ] . Thus, future studies should attempt to confi rm
whether our results are applicable to actual overground running.
In summary, the purpose of the present study was to ascertain
whether an increase in f step at a given velocity during running
reduces the lower extremity loading variables. The results of the
present study suggest that a 15 % increase in f step minimizes the
lower extremity loading, such as VIP, VILR and VALR. Thus,
adopting an f step greater than one’s preferred may be practical in
reducing the risk of developing a tibial stress fracture without
additional use of materials or apparatus.
Acknowledgements
▼
The authors thank Ms. Ayako Mitsui, Department of Environ-
mental Sciences, Faculty of Human Life Sciences, Jissen Women’s
University, for data collection. Finally, the authors thank
Dr. Andrew Lavender for his careful review of earlier drafts.
Confl ict of Interest: None of the authors have any confl icts of
interest associated with this study.
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