Swing- and support-related muscle actions differentially trigger human walk-run and run-walk transitions

There has been no consistent explanation as to why humans prefer changing t heir gait from walking to running and from running to walking at increasing and decreasing speeds, respectively. This study examined muscle activation as a possible determinant of these gait transitions. Seven subjects walked and ran on a motor-driven treadmill for 40s at speeds of 55, 70, 85, 100, 115, 130 and 145% of the preferred transition speed. The movements of subje cts were videotaped, and surface electromyographic activity was recorded fr om seven major leg muscles. Resultant moments at the leg joints during the swing phase were calculated. During the swing phase of locomotion at prefer red running speeds (115, 130, 145%), swing-related activation of the ankle, knee and hip flexors and peaks of flexion moments were typically lower (P <0.05) during running than during walking. At preferred walking speeds; (55 , 70, 85%), support-related activation of the ankle and knee extensors was typically lower during stance of walking than during stance of running (P < 0.05), These results support the hypothesis that the preferred walk-run tra nsition might be triggered by the increased sense of effort due to the exag gerated swing-related activation of the tibialis anterior, rectus femoris a nd hamstrings; this increased activation is necessary to meet the higher jo int moment demands to move the swing leg during fast walking. The preferred run-walk transition might be similarly triggered by the sense of effort du e to the higher support-related activation of the soleus, gastrocnemius and vastii that must generate higher forces during slow running than during wa lking at the same speed.