Human calf muscle responses during repeated isometric plantarflexions

In the present study, we measured the contraction-induced shortening (dL) of individual synergistic human muscles in a repeated motor task to assess their contractile behaviour. Ultrasonography was used to obtain dL measurements in the gastrocnemius (GS) and soleus (SOL) muscles of six men performing 1 I consecutive isometric plantarflexions. Contractions 1 and 11 were performed with maximal effort, and contractions 2-4, 5-7 and 8-10 were performed with efforts generating 50, 70 and 90%, respectively, of the plantarflexion moment produced in contraction 1. In contractions 5-10, the SOL muscle dL was similar (p > 0.05) to that produced in contraction 1 (similar to 6mm), indicating that the SOL muscle became fully activated at 70% of the maximum plantarflexion moment. The GS muscle dL in contractions 10 and 1 1 exceeded by similar to 0.5 mm (p < 0.05) and 1.3 mm (p < 0.01), respectively, that generated in contraction 1 (similar to 10mm), despite evidence obtained by superimposed stimulation that contraction I was produced with full motor unit activation. The consequent paradox that the GS muscle would produce in contractions 10 and 11 a greater activation and therefore more force than its actual potential is resolved when considering the interaction between the time-dependent tensile response of tendon and the performance of muscle as dictated by the sliding filament mechanism of contraction. (c) 2005 Elsevier Ltd. All rights reserved.