During standing, age-related differences in the activation of ankle muscles have been reported from surface electromyograms (EMGs) sampled locally. Given though activity seems to distribute unevenly within ankle muscles, the local sampling of surface EMGs may provide a biased view on how often and how much elderly and young individuals activate these muscles during standing. This study aimed therefore at sampling EMGs from multiple regions of individual ankle muscles to evaluate whether the distribution of muscle activity differs between aged and young subjects during standing. Thirteen young and eleven aged, healthy subjects were tested. Surface EMGs were sampled at multiple skin locations from tibialis anterior, soleus and medial and lateral gastrocnemius muscles while subjects stood at ease. The root mean square amplitude of EMGs was considered to estimate the duration, the degree of activity and the size of the region where muscle activity was detected. Our main findings revealed the medial gastrocnemius was active for longer periods in aged (interquartile interval; 74.1–98.2%) than young (44.9–81.9%) individuals (P = 0.02). Similarly, while tibialis anterior was rarely active in young (0.7–4.4%), in elderly subjects (2.6–82.5%) it was often recruited (P = 0.01). Moreover, EMGs with relatively higher amplitude were detected over a significantly wider proximo-distal region of medial gastrocnemius in aged (29.4–45.6%) than young (20.1–31.3%) subjects (P = 0.04). These results indicate the duration and the size of active muscle volume, as quantified from the spatial distribution of surface EMGs, may discriminate aged from young individuals during standing; elderlies seem to rely more heavily on the active loading of ankle muscles to control their standing posture than young individuals. Most importantly, current results suggest different conclusions on the active control of standing posture may be drawn depending on the skin location from where EMGs are collected, in particular for the medial gastrocnemius.
These results showed that the global postural sway and the postural adjustments at ankle during standing balance change depending on the type of visual feedback information.
One of the most important muscular groups which contribute to maintain standing balance is triceps surae. However, it is unclear whether the postural controllers of triceps surae, medial gastrocnemius (MG) and soleus (SOL), have different temporal patterns of activation during upright stance. This paper aimed at evaluating whether the global temporal activation in triceps surae differ among young subjects during standing balance. Nine male volunteers performed two tasks: standing quietly and with voluntary back and forward sways over their ankle. Electromyograms (EMGs) from soleus medial (MSOL) and lateral (LSOL) regions and from MG were sampled with linear arrays of surface electrodes. The percentage of muscle activation in time (i.e. temporal index) was computed for each muscle during upright standing. The results revealed that the medial portion of soleus muscle (MSOL) was activated continuously compared to the lateral portion of soleus (LSOL) and MG, which were activated intermittently. Therefore, the global temporal activation differed among the postural muscles of triceps surae during standing balance.
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