High-density surface EMG to investigate muscle activity during standing: implications for the training of postural control with EMG biofeedback in the elderly

By recording surface EMG using standard bipolar EMG, previous studies have demonstrated that elderlies tend to activate their postural muscles during standing with a higher degree of activity and for a prolonged duration compared with young adults. The EMG biofeedback technique has been widely used to reduce the excessive level of muscle activity in different fields, e.g. the prevention and reduction of low back pain. In this view, EMG biofeedback could be a potential tool to assist aged subjects in reducing the excessive muscle activity during standing balance. However, whether the greater, prolonged activation observed locally in the muscles of aged subjects reflects the activation of the entire muscle is still an open question. It is possible that differences in the activation of postural muscles with aging are more or less expressive than previously appreciated. This thesis aimed at obtaining new insights into the rationale and the effects of the use of EMG biofeedback for the improvement of muscle efficiency during standing in the elderly. It was evaluated whether muscle activation during standing differ between young and aged subjects through a sophisticated detection system for the acquisition of surface EMGs from multiple regions of a single muscle (i.e., high-density surface EMG). Before to test this hypothesis, a methodological issue was addressed to verify whether high-density surface EMG is selective enough to detect during standing: (a) different activation between ankle muscles, as observed with other techniques (intramuscular electromyography); (b) variations in the activity within ankle muscles (i.e., soleus muscles). The results of this methodological study revealed that the medial portion of soleus muscle was activated continuously compared to the lateral portion of soleus and medial gastrocnemius, which were activated intermittently. These results suggest high-density surface EMGs can be used to discriminate the activity between ankle muscles (i.e., medial gastrocnemius and soleus) and muscle activity sampled from different regions of a single muscle (i.e., soleus) can provide estimates more representative of muscle activity during standing. High-density surface EMG was therefore used to assess muscle activity between young and aged subjects during standing. Key results indicate that during standing: (a) tibialis anterior and medial gastrocnemius muscles were active for a longer duration in aged than young subjects; (b) a greater proportion of medial gastrocnemius volume was active in aged individuals. Collectively, these results corroborate previous evidence that elderlies tend to stand with a greater muscle effort than young subjects. Thus, the well-documented attenuation effect of EMG biofeedback on muscle activity may extend to the control of human standing posture with aging. This thesis addressed additional issues which could be relevant to provide more representative EMGs of muscle activity to the subject through EMG biofeedback and to prove the attenuation effect of EMG biofeedback on the activity of lower limb muscles during standing. The following two questions were addressed: i) should EMGs be sampled from both lower limbs to provide more representative information about calf muscles activity? It was observed differences in muscle activity between left and right ankle muscle while young subjects stood at ease. These results indicate muscle activity should be sampled from the ankle muscles of both legs to avoid a biased recording and feedback of muscle activity during standing. ii) Is the attenuation effect of EMG biofeedback on the ankle muscles activity generalized to – or compensated by – other muscles during standing? These findings revealed the attenuation effect of EMG-audio feedback on ankle muscles is not compensated by other lower limbs muscles not included for the feedback. Therefore, the EMG biofeedback may be a promising technique to assist individuals in more efficiently controlling lower limbs muscles during standing. If the short-term, attenuation, effect of EMG-audio feedback on ankle muscles’ activity in young individuals observed here is generalized to other populations (e.g., the elderly) and retained after training, then, improvement in postural muscle efficiency may contribute significantly to an ability to maintain standing balance, to respond to unexpected perturbation, standing on narrow stances and walking; with potential implication for the prevention of falls.