RESUMEN
The execution of activities of daily living requires the generation of three-dimensional shoulder torques through the coordinated activations of 20 shoulder muscles. Changes in shoulder posture can influence the contributions of individual shoulder muscles to shoulder torque generation, but it is unclear how the coordinated activity of shoulder muscles changes with shoulder plane of elevation and elevation angle. The purpose of this study was to characterize how neuromuscular coordination underlying three-dimensional shoulder function varies with arm posture. Muscle activations were obtained using surface electromyography from 16 shoulder muscles, as 12 healthy participants repeated eight three-dimensional isometric shoulder torques in 20 arm postures. Nonnegative matrix factorization revealed the muscle synergies underlying shoulder torque generation across each of the experimental postures, while the normalized similarity index assessed changes in overall synergy structure and linear mixed-effects models assessed changes in the weighted contributions of individual muscles to each synergy. Our analysis revealed that three distinct muscle synergies underlie healthy three-dimensional shoulder function. The overall structure of these synergies remained more similar than is expected by chance, despite changes in shoulder posture. However, the weighted contributions of five muscles composing the first synergy were influenced by changes in shoulder plane of elevation angle, and six muscles composing the second synergy were influenced by elevation angle. The weighted contributions of individual muscles composing the third synergy were unaffected by posture. These findings suggest that the neuromuscular control plan for healthy shoulder function consists of three distinct synergies, whose overall structure is fixed across shoulder posture.NEW AND NOTEWORTHY This study is the first to identify the muscle synergies underlying three-dimensional isometric shoulder torque generation. Although the overall structure of these synergies was unaffected by arm posture, the weighted contributions of several muscles composing two synergy patterns changed as a function of the elevation or plane of elevation of the shoulder. Our findings provide valuable insight for the development of targeted interventions for the restoration of shoulder function after neuromuscular or orthopedic pathologies.
Asunto(s)
Brazo/fisiología , Fenómenos Biomecánicos/fisiología , Contracción Isométrica/fisiología , Músculo Esquelético/fisiología , Postura/fisiología , Hombro/fisiología , Adulto , Femenino , Humanos , Masculino , Torque , Adulto JovenRESUMEN
Neck muscles are preferentially activated in specific force directions, but the constraints that the central nervous system considers when programming these preferred directions of muscle activity are unknown. The current study used ultrasound shear wave elastography (SWE) to investigate whether the material properties of the sternocleidomastoid (SCM) muscles exhibit preferred directions similar to their preferred direction of muscle activity during an isometric task. Twenty-four healthy participants matched isometric forces in 16 axial directions. All force targets were scaled to 20% of a maximum voluntary contraction. Muscle activity was recorded with surface electromyography (EMG) from six muscles (the bilateral SCMs, upper trapezius, and splenius capitis muscles), and shear wave velocities (SWVs) were recorded with SWE from both SCM muscles. We observed statistically significant differences between the preferred directions of muscle activity and SWVs for both the left SCM ( P = 0.002) and the right SCM ( P < 0.001), with the SWE data exhibiting a more lateral preferred direction. Significant differences in the spatial focus ( P < 0.001) were also observed, with the dispersion of SWV data covering a greater angular range than the EMG data during isometric tasks. The preferred directions of muscle activity and material properties for the SCM muscles were closer than previous comparisons of muscle activity and moment arms, suggesting muscle mechanics could play a more important role than anatomy in how the central nervous system spatially tunes muscle activation. NEW & NOTEWORTHY Our study used a novel combination of surface electromyography and ultrasound shear wave elastography to investigate the neuromuscular control of the neck. Our work highlights differences in how the activation and material properties of the sternocleidomastoid muscles are modulated as the central nervous system stabilizes the neck during isometric force production. These findings provide normative data for future studies to investigate pathologic changes to both the activation and material properties of the sternocleidomastoid muscles.