Age-related changes in the activation timing of postural muscles to the prime mover muscle for bilateral arm flexion during standing.

Background: We aimed to obtain the standard values of age-related changes in the activation timing of postural muscles to the prime mover muscle (anterior deltoid [AD]) for bilateral arm flexion during standing.

Methods: The study participants were 276 children (aged 3-14 years) and 32 adults (aged 20-26 years). In response to a visual stimulus, participants raised both arms from a fully extended position as quickly as possible, stopped their arms voluntarily at a horizontal level at the shoulder, and maintained that position for 2 s. Ten test trials were performed. By using surface electromyography, the duration from the burst onset of the postural muscles to that of AD was measured as the starting time of the postural muscles (rectus abdominis [RA], erector spinae [ES], rectus femoris [RF], biceps femoris [BF], tibialis anterior [TA], gastrocnemius medialis [GcM], and soleus [SOL]). The starting time was presented as a negative value when the burst onset of the postural muscles preceded that of AD, which was defined as the preceding activation. A positive value for the starting time was defined as delayed activation.

Results: In adults, the burst onsets of ES and BF significantly preceded that of AD. In ES, the starting time preceded the onset of AD in those aged ≥ 5-6 years; no difference with adults was found at age 13-14 years. On the other hand, in BF, significant delayed activation was found at ages 3-4 to 11-12 years. While the starting time decreased with age, no significant preceding activation similar to adults was found, even at age 13-14 years. In TA, no significant difference with the onset of AD was found at age 3-6 years, and significant delayed activation was found at age ≥ 7-8 years. Significant delayed activation in GcM, SOL, RA, and RF was observed in all age groups, and no age-related changes were observed in children.

Conclusion: These findings could provide standard values from childhood to adolescence for age-related changes in anticipatory postural muscle activity during voluntary movement while standing and contribute to applications in the fields of sports and rehabilitation.