The Effects Of Grade And Speed On Leg Muscle Activations During Walking

Abstract

The ability to walk up and downhill over a range of speeds is fundamental to everyday life. Humans modulate leg extensor muscle activations to raise and the lower the center of mass during uphill and downhill walking, respectively. However, the extent to which grade and walking speed interact to influence leg muscle activations has yet to be explored. We hypothesized that, compared to level walking, (1) hip, knee, and ankle extensor muscle activations would increase with steeper uphill grade (even during slow walking), but that (2) only knee extensor muscle activations would increase with steeper downhill grade (even during fast walking). PURPOSE: To investigate the effects of grade and speed on leg muscle activations during walking. METHODS: Ten healthy young adults walked on a treadmill at seven grades (0, ±3, ±6, ±9°) and three speeds (0.75, 1.25, 1.75 m/s) while we recorded electromyographic (EMG) signals from six leg muscles: medial gastrocnemius (MG), soleus (SOL), vastus medialis (VM), rectus femoris (RF), biceps femoris (BF), and gluteus maximus (GM). EMG signals were full-wave rectified and normalized to the mean amplitudes during level walking at 1.25 m/s. We computed average EMG amplitudes over eight consecutive strides from heelstrike to midstance for VM, RF, BF, and GM and from midstance to toe-off for MG and SOL, corresponding to the primary EMG bursts during stance. RESULTS: As hypothesized, hip (64-474%), knee (108-307%), and ankle (53-261%) extensor muscle activations increased with steeper uphill grade, but only knee extensor muscle activations increased (34-249%) with steeper downhill grade across the range of speeds (p<0.03). Muscle activations generally increased with faster walking speeds for all grades (p<0.03). CONCLUSIONS: Across a range of walking speeds, extensor muscles of the hip, knee, and ankle all contribute to raising the center of mass during uphill walking. However, even at a fast walking speed, only knee extensor muscles contribute to lowering the center of mass during downhill walking. Supported by NIH Grant 5T32AG000279-09 and a student Grant-in-Aid Award from the American Society of Biomechanics.