Adaptation of the muscle activation patterns to transitory increase in stride length during treadmill locomotion in humans

Abstract

How humans modulate the muscle activation patterns to increase the stride length is examined in this study. Subjects were running or walking on a treadmill at self-selected speeds. They were instructed to increase their ipsilateral stride length when a buzzer, triggered by the ipsilateral heel switch, came on unexpectedly at different times in the cycle. Four ipsi- and four contralateral muscle (soleus-SO, biceps femoris-BF, vastus lateralis-VL, and rectus femoris-RF) responses were quantified by taking average EMG (AEMG) values over specific phases in the cycle. The results showed that subjects were capable of increasing their stride length in the same stride only if the buzzer came on at ipsilateral heel contact (IHC) and contralateral toe-off (CTO) in walking and at IHC in running. This suggests that there are minimum functional determinants of stride length such that if the buzzer comes on after the completion of one function for example pushoff, subjects are unable to change the stride length. The AEMG changes demonstrate that subjects during ipsilateral stride length increase in walking altered: pushoff action during late stance (↑SO, ↑BF), pulloff action during late stance and early swing (↑RF), and braking action during late swing (↓BF) on the ipsilateral side; while modifying the pushoff action on the contralateral side (↑SO, ↑BF, ↑RF, ↑VL) during its stance phase. The changes during running were similar in function, but shifted in time due to the temporal structure of the cycle. In summary, to increase stride length, subjects showed muscle and phase-specific modulation in the ipsi- and contralateral muscles. How these complex changes may be initiated during locomotion is discussed.