Title: Neural compensation within the human triceps surae during prolonged walking

25 26 During human walking, muscle activation strategies are approximately constant across 27 consecutive steps over a short time, but it is unknown whether they are maintained over 28 a longer duration. Prolonged walking may increase tendinous tissue compliance, which 29 can influence neural activation, but the neural responses of individual muscles have not 30 been investigated. This study investigated the hypothesis that muscle activity is upor 31 down-regulated in individual triceps surae muscles during prolonged walking. Thirteen 32 healthy subjects walked on a treadmill for 60 minutes at 4.5 km/h while triceps surae 33 muscle activity, maximal muscle compound action potentials and kinematics were 34 recorded every 5 minutes, and fascicle lengths were estimated at the beginning and end 35 of the protocol using ultrasound. After 1h of walking, soleus activity increased by 36 9.3±0.2% (P<0.05) and medial gastrocnemius activity decreased by 9.3±0.3% (P<0.01). 37 Gastrocnemius fascicle length at ground contact shortened by 4.45 ± 0.99% (P < 0.001), 38 whereas soleus fascicle length was unchanged (P = 0.988). Throughout the stance 39 phase, medial gastrocnemius fascicle lengthening decreased by 44 ± 13 % (P<0.001), 40 whereas soleus fascicle lengthening amplitude was unchanged (P = 0.650). The data 41 suggest that a compensatory neural strategy exists between triceps surae muscles, and 42 that changes in muscle activation are generally mirrored by changes in muscle fascicle 43 length. These findings also support the notion of muscle-specific changes in tendinous 44 tissue compliance after prolonged walking, and highlight the ability of the central 45 nervous system to maintain relatively constant movement patterns in spite of 46 neuromechanical changes in individual muscles. 47 48