Decomposition of surface electromyograms reveals changes in motor control after 14-day bed rest

We demonstrate the advantages of motor unit (MU) identification from noninvasively recorded high-density surface electromyograms (EMG) of gastrocnemius medialis, gastrocnemius lateralis and soleus muscles for detailed analysis... [ view full abstract ]

We demonstrate the advantages of motor unit (MU) identification from noninvasively recorded high-density surface electromyograms (EMG) of gastrocnemius medialis, gastrocnemius lateralis and soleus muscles for detailed analysis of motor control changes after two weeks of bed rest. Five young (18-28 years) and five older (53-65 years) healthy subjects participated in the study. High-density surface electromyograms of both gastrocnemii and soleus muscles were recorded in the non-dominant (left) leg before (pre) and immediately after (post) the bed rest. The signals were decomposed by Convolution Kernel Compensation method into contributions of individual MUs and the MU discharge rates in both recording sessions were mutually compared. On average, 22.6 ± 12.4 and 8.3 ± 6.9 motor units have been accurately identified in each gastrocnemius muscle of older and young subjects, respectively. In the soleus muscle these numbers were lower, i.e. 15.1 ± 7.8 and 4.3 ± 2.4 motor units for older and young subjects, respectively. In all the investigated muscles the recruitment thresholds of identified MUs were uniformly distributed over the investigated force ranges from 0 % to 60 % of maximum voluntary contraction (MVC). The only exceptions were gastrocnemii muscles of older subjects, where after the bed rest more than 70 % of identified MUs were recruited below 20 % of MVC. Principal component analysis of smoothed MU discharge rates showed statistically significant decrease of MU discharge rates and decrease of their common modulation after the bed rest in all investigated muscles of young subjects and in soleus muscle of older adults. No significant changes were observed in gastrocnemii muscles of older subjects. It was concluded that high-density surface EMG offers a detailed insight into the alternations of motor control strategies and is sensitive enough to detect and characterize the changes caused by a relatively short bed rest protocol.