Push-ups with hands or feet on unstable surface: Does it affects muscle activation and ground reaction forces?

Unstable surfaces (US) are often used to introduce instability into conventional push-up (PU) exercise. The goal of this study was to investigate the effects of an unstable surface (BOSUO), applied into different locations (under the hands or feet), on the muscle activation patterns and ground reaction forces (GRF) production. Twelve healthy male subjects (22 +/- 3.9 years), were selected and performed 3 PU conditions: standard PU, PU with US under hands and PU with US under feet. Surface EMG signals were collected from agonist, antagonist and stabilizer muscles. GRF was collect by using force plates embedded on the floor and kinematic variables was evaluated with a Motion Analysis 3D system. Vertical GRF (Fz) was significantly higher in the PU feet than in the other conditions (P > 0.05). However the highest values in the anteroposterior (Fx) and mediolateral (Fy) GRF were found in the PU hands (P > 0.001, in all comparisons). Muscle activation of the triceps and biceps significantly increased with US addition, while no changes were observed in the pectoralis major. Increased external oblique activation were only observed in the PU feet. On the other hand, PU hand, induced a substantial decrease of the serratus activation. The results showed that the addition of unstable surfaces produced changes on the activation pattern of agonist, antagonist muscles, and shoulder and trunk stabilizers muscles. Such changes were differently affected, whether instability was applied to the hands or to feet.