Abstract
The aim of this study was to investigate if trunk muscle activation patterns during rapid bilateral shoulder flexions are affected by movement amplitude. Eleven healthy males performed shoulder flexion movements starting from a position with arms along sides (0°) to either 45°, 90° or 180°. EMG was measured bilaterally from transversus abdominis (TrA), obliquus internus (OI) with intra-muscular electrodes, and from rectus abdominis (RA), erector spinae (ES) and deltoideus with surface electrodes. 3D kinematics was recorded and inverse dynamics was used to calculate the reactive linear forces and torque about the shoulders and the linear and angular impulses. The sequencing of trunk muscle onsets at the initiation of arm movements was the same across movement amplitudes with ES as the first muscle activated, followed by TrA, RA and OI. All arm movements induced a flexion angular impulse about the shoulders during acceleration that was reversed during deceleration. Increased movement amplitude led to shortened onset latencies of the abdominal muscles and increased level of activation in TrA and ES. The activation magnitude of TrA was similar in acceleration and deceleration where the other muscles were specific to acceleration or deceleration. The findings show that arm movements need to be standardized when used as a method to evaluate trunk muscle activation patterns and that inclusion of the deceleration of the arms in the analysis allow the study of the relationship between trunk muscle activation and direction of perturbing torque during one and the same arm movement.
Highlights
Rapid voluntary arm movements at the shoulder perturb postural equilibrium and challenge the stabilization of the spine via reactive forces and torques
It has been reported that in healthy persons, the first trunk muscle to be activated at the initiation of a rapid shoulder flexion is the innermost abdominal muscle, the transversus abdominis (TrA), and that this early activation was absent in a group of patients with low back problems [4]
The present results show that movement amplitude needs to be standardized when performing rapid shoulder flexions to evaluate trunk muscle activation
Summary
Rapid voluntary arm movements at the shoulder perturb postural equilibrium and challenge the stabilization of the spine via reactive forces and torques. Trunk Muscle Activation during Bilateral Shoulder Flexion Movements supposedly aimed to counteract and minimize the impact of the disturbance from the arm movement [1,2,3]. Increased movement amplitude increases the duration of acceleration of the arm and results in higher velocity, a larger perturbation. This means that arm movements with different endpoints might induce perturbations of different magnitude to the trunk. Detailed mechanical description of symmetrical (bilateral) shoulder flexion movements with maximal speed but of different amplitudes, and the impact on trunk muscle activation patterns, such as onset and amplitude of activation, is still lacking
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