Simple lower extremity two-joint synergy.

The purpose of this study was to describe the existence of a simple synergy in the lower extremity. Subjects performed discrete knee flexion or extension movements or ankle plantar or dorsiflexion movements in a sagittal plane, moving one of the joints "as fast as possible." Joint angles and electromyographic (EMG) signals from the biceps femoris, rectus femoris, soleus, and tibialis anterior were recorded. Typically, EMG patterns in both muscle pairs acting at the joints demonstrated the "triphasic" pattern. The knee flexor (biceps femoris) and ankle plantar flexor (soleus) tended to show simultaneous EMG bursts, while the knee extensor (rectus femoris) and ankle dorsiflexor (tibialis anterior) had similar patterns of activation. A two-joint simple synergy previously established for upper extremities seems pertinent for lower extremities as well. Such a synergy is used by the central nervous system to simplify control of the postural component of a motor task.