Absence of lateral gastrocnemius activity and differential motor unit behavior in

Abstract 25 In standing, the vertical projection of the center of mass passes in front of the ankle, which requires active 26 plantar-flexor torque from the triceps surae to maintain balance. We recorded motor unit (MU) activity in the 27 medial (MG) and lateral (LG) gastrocnemius muscle and the soleus (SOL) in standing balance and voluntary 28 isometric contractions to understand the effect of functional requirements and descending drive from 29 different neural sources on motoneuron behavior. Single MU activity was recorded in seven subjects with 30 wire electrodes in the triceps surae. Two 3-minute standing balance trials and several ramp-and-hold 31 contractions were performed. Lateral gastrocnemius MU activity was rarely observed in standing. The lowest 32 thresholds for LG MUs in ramp contractions were 20-35 times higher than SOL and MG MUs (P<0.001). 33 Compared to MUs from the SOL, MG MUs were intermittently active (P<0.001), had higher recruitment 34 thresholds (P=0.022), and greater firing rate variability (P<0.001); this difference in firing rate variability 35 was present in standing balance and isometric contractions. In SOL and MG MUs, both recruitment of new 36 MUs (R