Electromyographic Characteristics of the Aftercontraction Effect of Skeletal Muscles in Young, Middle-Aged, and Elderly People

The aftercontraction effect, or postactivation effect (PAE) is a special type of involuntary normal muscle tone. It is assumed that aging can specifically modify such characteristics of PAE as duration, latency, prevalence in the population, the presence of an “oscillatory” pattern, and the parameters of the surface electromyogram (sEMG). PAE was studied in the groups of junior adults (JA, 18–23 years, n = 46), young adults (YA, 24–44 years, n = 23), and middle-aged and elderly people (ME, 45–75 years, n = 24). In the deltoid muscles, PAE was triggered in 89% of individuals in the JA group, 87% in YA, and in 75% of individuals in the ME group. In the JA group, the “oscillatory” pattern (repeated periods of PAE) was present in 32% of trials, in the YA group, in 15%, and was completely absent in the ME group. The amplitude of sEMG during PAE was reduced in the ME group compared to JA by approximately two times (p < 0.05). The duration of PAE did not differ among the groups of different ages, and the latency of PAE was the lowest in the JA group (p < 0.05). The mean frequency of the sEMG spectrum during PAE compared to voluntary isometric contraction was higher in the JA and ME groups. Thus, PAE is generally characteristic of neurotypical persons under 75 years of age, but in middle and older age, PAE is characterized by the absence of repeated periods of PAE (“oscillatory pattern”), lower amplitude of sEMG and longer latency. It is assumed that with increasing age from 18 to 75 years, the tonic automatisms of skeletal muscles are inhibited and have a lesser ability to oscillate.