F-32 Free Communication/Poster - Sports Biomechanics II: MAY 29, 2009 1:00 PM - 6:00 PM ROOM: Hall 4F
Stress fractures are one of the most serious overuse injuries in runners. High tibial shock, or peak positive tibial acceleration (PPA), is correlated to high loading rates and tibial stress fracture incidence. Runners are able to reduce their PPA by altering their footstrike pattern. This change may be associated with earlier onset times of the ankle musculature.
PURPOSE: To compare the onset times of the ankle dorsiflexors and plantarflexors, before and after a gait retraining protocol.
METHODS: 9 subjects with high PPA have been studied to date. Following a baseline instrumented gait analysis, all subjects underwent a gait retraining protocol aimed at reducing their PPA. Along with kinematics, electromyographic (EMG) data were collected from the medial and lateral gastrocnemius, soleus, and tibialis anterior muscles. Data were sampled at 1200 Hz. Onset was determined when the linear envelope exceeded 2.5 standard deviations above the resting mean. Onsets were extracted from the period from 125 ms before footstrike through the end of stance. These times were compared between baseline and post-gait retraining.
RESULTS: Subjects reduced their PPA by 39%. 6/10 increased, and 2/10 decreased their dorsiflexion at footstrike. Mean EMG onset times were similar before and after the retraining (Figure 1). However, the between-subject variability for these times was extremely high for all four muscles tested. The individual differences were largest in the soleus and tibialis anterior. For example, one subject activated the soleus as much as 22 ms later while another 27 ms prior to the pre-retraining onset time.
CONCLUSION: While all subjects reduced their tibial shock significantly, there were no consistent changes in ankle muscle onset times.