Similar to a pencil balanced on a finger tip, the standing human body is inherently unstable (Loram & Lakie, 2002). The vertical projection of the body’s center of mass passes in front of the ankle when we stand, which means active plantar flexion of the ankle is required to maintain balance (Loram et al., 2005). In humans, plantar flexion is primarily accomplished by the triceps surae group: the soleus muscle and the medial and lateral heads of the gastrocnemius muscle. The muscles of the human triceps surae have numerous anatomical, neurophysiological, and functional differences (e.g., Nardone et al., 1990), and it has been proposed that despite sharing a common distal tendon, the muscles of the human triceps surae may have distinct functional roles.
In collaboration with researchers from the University of British Columbia in Vancouver, Canada, two Neuroscience Research Australia researchers, Dr. Martin Héroux and Dr. Billy Luu, investigated the behaviour of motor units from the muscles of the triceps in standing human subjects to determine whether the soleus muscle and medial and lateral gastrocnemius muscles have distinct roles in the maintenance of balance. Motor units consist of an alpha motor neuron, with its cell body in the spinal cord and its axon in a peripheral nerve, and the group of muscle fibers innervated by the motor neuron. We used indwelling fine-wire electrodes inserted into the soleus muscle and the medial and lateral gastrocnemius muscles to record motor unit activity as study participants stood quietly or performed several ramped contractions when they were secured in an upright position.
WHAT DID WE FIND?
Similar to what has previously been reported (Moris, 1973; Vieira et al., 2012), the activity of motor units in the soleus muscle was quite consistent, whereas the activity of medial gastrocnemius motor units was much more intermittent. To our surprise, lateral gastrocnemius motor units showed little to no activity in standing balance. This was associated with motor unit recruitment thresholds being 20 –35 times higher in the lateral gastrocnemius compared to the soleus and medial gastrocnemius muscles. We also noted that the rate of motor unit discharge was much more variable in the medial gastrocnemius in standing compared to activity of soleus motor units. This difference in motor neuron discharge variability was the same when participants were supported, which indicates that this difference in the properties of soleus and medial gastrocnemius motor units is not simply due to the type of neural drive (e.g., voluntary vs. vestibular) to the motor neuron pool.
SIGNIFICANCE AND IMPLICATIONS
A two-fold difference in motor unit discharge variability between two agonists is not typical in humans and may reflect an adaptation to the type of contraction most often accomplished by each muscle. The differences we noted between the soleus and medial gastrocnemius muscles were present in low to moderate contractions in both standing balance and isometric contractions, which point to local (i.e., spinal) differences in the motor neuron pools that innervate the triceps surae rather than differences in descending neural drive.
The 20-fold difference in motor units recruitment threshold between two heads of a same muscle was unexpected and provides evidence of an important difference between the medial and lateral gastrocnemius muscles. We believe these results reflect an anatomical difference between the medial and lateral gastrocnemius muscles rather than a purely neurophysiological difference. The anatomical arrangement may reflect an optimization via which these muscles are preferentially activated within relatively distinct knee and ankle configurations, which would increase the functional capabilities of the triceps surae. The greater muscle volume of the medial gastrocnemius muscle may reflect that as bipeds, humans generate plantar flexor torques within a somewhat limited range of motion that is optimal for this muscle.
Héroux ME, Dakin CJ, Luu BL, Inglis JT, Blouin JS (2014). Absence of lateral gastrocnemius activity and differential motor unit behavior in soleus and medial gastrocnemius during standing balance. J Appl Physiol 116, 140-148.
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