Medial-lateral position of hand changes shoulder muscle synergy during the push-up exercises

The aim of this study was to compare muscle synergy among different medial-lateral positions of the hand during the push-up exercises: narrow, normal, and wide. Fifteen fitness athletes participated (weight:
$68.35\pm 7.18$
kg, height:
$175\pm 3.40$
cm, age:
$24.50\pm 7.5$
years, and experience:
$12\pm 3$
years). The kinematics data were recorded via ten motion captures. The EMG activities were collected for selective muscles: the anterior deltoid, middle deltoid, posterior deltoid, infraspinatus, upper pectoralis major, middle pectoralis major, lower pectoralis major, latissimus dorsi, triceps lateral, triceps medial, biceps brachii, upper trapezius, middle trapezius, and lower trapezius muscles. An NMF (Non-negative matrix factorization) algorithm was applied to extract muscle synergy from EMG data for three different medial-lateral hand positions of the push-up exercises. The scalar product (SP) is a similarity index that was used to quantify the similarity of muscle synergies among different conditions. Based on the results, two synergies were extracted from the EMG of 14 muscles. Synergy 1 activated at the lowest position. While synergy 2 contains the before and after the lowest position. Therefore, the most active muscles correspond to shoulder and elbow extension/flexion. Each synergy was controlled groups of muscles with similar biomechanical action and associated with the push-up phases. The minimum similarity of synergy coefficient was observed between normal and wide positions for both synergy 1 (
$\text{SP}=0.88$
) and synergy 2 (
$\text{SP}=0.79$
). The minimum similarity of synergy weight was observed between normal and wide positions for both synergy 1 (
$\text{SP}=0.91$
) and synergy 2 (
$\text{SP}=0.84$
). We have demonstrated the presence of muscle coordination in the push-up and examined their relationship with biomechanical variables. These results indicate how athletes and coaches can improve their performances by understanding the muscles synergies.