Influence of fatigue and velocity on the latency and recruitment order of scapular muscles.

Objectives: To determine the influence of velocity and fatigue on scapular muscle activation latency and recruitment order during a voluntary arm raise task, in healthy individuals.

Design: Cross-sectional study.

Setting: University laboratory.

Participants: Twenty three male adults per group (high-velocity and low-velocity).

Main Outcome Measures: Onset latency of scapular muscles [Anterior deltoid (AD), lower trapezius (LT), middle trapezius (MT), upper trapezius (UT), and serratus anterior (SA)] was assessed by surface electromyography. The participants were assigned to one of two groups: low-velocity or high-velocity. Both groups performed a voluntary arm raise task in the scapular plane under two conditions: no-fatigue and fatigue.

Results: The UT showed early activation (p < 0.01) in the fatigue condition when performing the arm raise task at a high velocity. At a low velocity and with no muscular fatigue, the recruitment order was MT, LT, SA, AD, and UT. However, the recruitment order changed in the high-velocity with muscular fatigue condition, since the recruitment order was UT, AD, SA, LT, and MT.

Conclusions: The simultaneous presence of fatigue and high-velocity in an arm raise task is associated with a decrease in the UT activation latency and a modification of the recruitment order of scapular muscles.