LOWER-EXTREMITY MUSCLE ACTIVATION DURING HORIZONTAL AND UPHILL RUNNING

To provide more comprehensive information on the extent and pattern of muscle activation during running, we determined lower extremity muscl e activation by using exercise-induced contrast shifts in magnetic res onance (MR) images during horizontal and uphill high-intensity (115% o f peak oxygen uptake) running to exhaustion (2.0-3.9 min) in 12 young women. The mean percentage of muscle volume activated in the right low er extremity was significantly (P < 0.05) greater during uphill (73 +/ - 7%) than during horizontal (67 +/- 8%) running. The percentage of 18 individual muscles or groups activated varied from 41 to 90% during h orizontal running and from 44 to 83% during uphill running. During hor izontal running, the muscles or groups most activated were the adducto rs (90 +/- 5%), semitendinosus (86 +/- 13%), gracilis (76 +/- 20%), bi ceps femoris (76 +/- 12%), and semimembranosus (75 +/- 12%). During up hill running, the muscles most activated were the adductors (83 +/- 8% ), biceps femoris (79 +/- 7%), gluteal group (79 +/- 11%), gastrocnemi us (76 +/- 15%), and vastus group (75 +/- 13%). Compared with horizont al running, uphill running required considerably greater activation of the vastus group (23%) and soleus (14%) and less activation of the re ctus femoris (29%), gracilis (18%), and semitendinosus (17%). We concl ude that during high-intensity horizontal and uphill running to exhaus tion, lasting 2-3 min, muscles of the lower extremity are not maximall y activated, suggesting there is a limit to the extent to which additi onal muscle mass recruitment can be utilized to meet the demand for fo rce and energy. Greater total muscle activation during exhaustive uphi ll than during horizontal running is achieved through an altered patte rn of muscle activation that involves increased use of some muscles an d less use of others.