Age-related Differences in Motor Recruitment Patterns of the Shoulder in Dynamic and Isometric Contractions. A Cross-sectional Study

Aging processes in the musculoskeletal system lead to functional impairments that restrict participation. Purpose: To assess differences in force and motor recruitment patterns of shoulder muscles between age groups to understand functional disorders. A cross-sectional study comparing thirty adults (20-64) and 30 older adults (>65). Surface-electromyography (sEMG) of the middle deltoid, upper and lower trapezius, infraspinatus and serratus anterior muscles was recorded. Maximum isometric voluntary contraction (MIVC) was determined at 45° glenohumeral abduction. For the sEMG signal registration, concentric and excentric contraction with and without 1 kg and isometric contraction were requested. Participants abducted the arm from 0° up to an abduction angle of 135° for concentric and excentric contraction; and from 0º to 45º and remained there at 80% of the MIVC level while isometrically pushing against a handheld dynamometer. Differences in sEMG amplitudes (root mean square, RMS) of all contractions, but also onset latencies during concentric contraction of each muscle between age groups were analyzed. Statistical differences in strength (Adults>Older adults;0.05) existed between groups. No significant differences in RMS values of dynamic contractions were detected, except for serratus anterior, but there were for isometric contraction of all muscles analyzed (Adults>Older adults;0.05). The recruitment order varied between age groups, showing a general tendency towards delayed onset times in Older Adults, except for the upper trapezius muscle. Age differences in muscle recruitment patterns were found, which underscores the importance of developing musculoskeletal data to prevent and guide geriatric shoulder pathologies.