Effect of knee joint position on triceps surae motor unit recruitment and firing rates.

Gastrocnemii muscle fibers shorten when the knee joint is in a flexed compared to an extended position. This leads to inhibition of medial gastrocnemius (MG) motor units, however, it is unclear whether this affects motor unit properties of the lateral gastrocnemius (LG) or soleus (SOL). We recorded 171 motor units from the MG (61), LG (39) and SOL (71) at an extended (160°) and flexed (100°) knee joint position with the ankle and hip joints at 90°. Subjects performed isometric ramp plantar flexion contractions at 25, 50 and 100% of the maximal voluntary contraction. MG (p = 0.0002) and LG (p = 0.02) motor unit recruitment thresholds (RT) were higher, whereas only MG motor unit firing rates (FR) were lower (p = 0.008) in the flexed compared to the extended knee joint position. SOL motor unit RT (p = 0.66) and FR (p = 0.08) were not statistically different between positions. When comparing properties of the same motor unit followed during contractions at both knee joint positions, RT of ten gastrocnemii motor units were higher (p = 0.0008) and FR were lower (p = 0.01) when the knee was flexed. Additionally, in six SOL motor units, RT (p = 0.42) and FR (p = 0.96) were not different between the two positions. Thus, MG and LG activation is similarly inhibited during plantar flexion contractions in a flexed compared to an extended knee joint position. Furthermore, our findings indicate that knee joint position changes have no effect on SOL excitability.