Motor Unit Firing Behaviour of Soleus Muscle in Isometric and Dynamic Contractions

Introduction: Understanding the detailed control of human locomotion and balance can be improved, when individual motor units can be isolated and their firing rates followed in natural movement of large, fuctionally important muscles. For this reason the present study investigated the motor unit discharge rate (MUDR) in isometric and dynamic contractions of the soleus muscle. Methods: Eleven males performed isometric (10-100% MVC) and dynamic (10-40% MVC) plantar flexions. Intramuscular EMG was measured from Soleus with bipolar wire-electrodes and decomposed with custom built Daisy software. Results: The Soleus MUDR was significantly higher in concentric compared to isometric or eccentric contractions at all submaximal force levels (P<0.05). In isometric contractions MUDR increased up to 100% MVC. Conclusion: Motor unit discharge properties of a large plantarflexor can be measured in dynamic and maximal contractions. For a given torque output, MUDR is dependent upon contraction type, as set by the major mechanical differences between concentric and eccentric actions.