Figure 9. Soleus interlinked behaviour during normal standing.

For each 3 s interval of a whole trial the correlation between soleus and body position was calculated. The intervals were classified into negatively (white bars) and positively correlated intervals (black bars) and whole trial (grey bars). For these groups the following quantities are shown: A, correlation between the soleus change in muscle length and CoG; B, correlation between the gastrocnemius change in muscle length and CoG, based on the trial partition obtained from soleus correlation; C, correlation between the deep compartment of tibialis anterior change in muscle length and CoG, based on the trial partition obtained from soleus correlation; D, correlation between the superficial compartment of tibialis anterior change in muscle length and CoG, based on the trial partition obtained from soleus correlation. The average values from the nine subjects are presented and error bars show the standard error of the mean. On average soleus is negatively correlated with body position (A) and when soleus changes from a negative to positive correlation (A), gastrocnemius’ correlation with CoG becomes less negative (or more positive) (B), and the superficial compartment of tibialis anterior shows a more negative correlation (D). The deep compartment of the same muscle does not show any difference in behaviour related to soleus (C). The association between soleus and the superficial compartment of tibialis anterior is evidence that the counter-intuitive behaviour in the superficial compartment of tibialis anterior has an anatomical or physiological explanation.