The relationship between soleus and gastrocnemius muscle activity in conscious cats--a model for motor unit recruitment?

Force and electromyogram (e.m.g.) data were recorded from medial gastrocnemius and soleus muscles of conscious cats using chronically implanted devices. A digital computer was used to take simultaneous samples of the data from both muscles and construct two-dimensional frequency distributions relating the activities in the two muscles. The results show that posture is the only activity where soleus may be active without corresponding activity in the medial gastrocnemius muscle. In locomotion the ratio between soleus and medial gastrocnemius muscle activities changed with treadmill speed, although peak soleus force remained constant at approximately 80% of the isometric tetanic tension measured in terminal experiments. A hypothesis is put forward, associating these findings with the activities of slow and fast motor units and emphasizing the influence of neural activity in the determination of motor unit recruitment.