Co-contraction of shoulder and upper extremity muscles in individuals with muscle dystrophy compared to healthy persons during reaching-to-target tasks

Facioscapulohumeral dystrophy (FSHD) causes progressive muscle weakness, limits function and may lead to altered muscle co-contraction patterns to accomplish task performance and joint stability. In this study we examined time-changes in co-contraction of scapular and humeral muscles in persons with FSHD vs. healthy controls during reaching. Participants with FSHD (N = 12, 56.0 ± 14.5 yrs) and age-matched healthy controls (N = 12, 55.8 ± 13.1 yrs) performed ipsilateral and contralateral reaching to target. Surface electromyograms, kinematics and maximum voluntary force data were recorded. The co-contraction index CI₁ was calculated as mean over the whole movement and further compared over time using statistical parametric mapping. During contralateral reaching, mean co-contraction of the trapezius ascendens-serratus anterior (TA-SA) muscle pair was higher in persons with FSHD vs. healthy controls (P = 0.007, Delta CI₁: +16, Hg: 1.15) and increased in the last phase of reaching (P < 0.008, Delta CIⁱ₁: +30 to + 48, normalized time: 75-100%), consistent with a pattern of high excitation of the TA muscle in FSHD participants. The increase in TA-SA co-contraction in FSHD during contralateral reaching is required to accomplish scapular stability and mobility, in the presence of muscle weakness when approaching the target. A higher co-contraction has implications for higher energy expenditure and affects joint loads.