Muscle synergies and joint linkages in sit-to-stand under two initial foot positions

The purpose of this study was to investigate the activation pattern of six lower limb muscles during standing up from two initial foot positions and to examine relationships between the onsets of muscle activity and the dynamics of the action. In contrast to earlier studies, an algorithm was used to measure onsets and duration of muscle activity.

The activity of biceps femoris, rectus femoris, vastus lateralis, tibialis anterior, lateral gastrocnemius and medial soleus was studied as nine male subjects between the ages of 20 and 35 stood up from two initial foot positions (foot forward, foot back). Subjects sat on a height-adjustable seat with their right foot on a force platform and were videotaped as they stood up.

When the feet were forward there were significant increases in movement duration, displacement and velocity of trunk segment flexion at the hips reflecting the increased distance the body mass must be moved forward. The sequence of onsets and the duration of muscle activity, however, were consistent across the two conditions. Onsets of extension at hip, knee and ankle, however, correlated with different muscle activity depending on the foot position.

The results suggest that muscles working over different joints are temporally linked and that different muscle linkages may be utilised in the lower limb to coordinate the sit-to-stand action under different functional demands.