Effects of Various Lower Limb Ground States on Activation of the Shoulder and Trunk Muscles during Push-up Exercises

[Purpose] The purpose of this study was to determine the effect of the moment arm length on the muscle activation of the upper limbs and the trunk. Subjects performed push-up exercises on a stable surface and on an unstable surface placing their feet at a higher level than the hands. [Subjects] The subjects of this study were 33 normal adults in their 20s who had normal range of motion and who were without disorders of the shoulder complex, musculoskeletal disease in the upper limbs or low back pain. [Methods] The experiment was performed using the following four positions: on an unstable surface created by placing a 65 cm diameter exercise ball under the ankle or knee joints of the subjects, and on a stable surface created by placing a bench with a height of a 65 cm under the ankle or knee joints of the subjects. To prevent the effect of muscle fatigue, all exercises were randomly performed. To measure muscle activation in the trunk, electrodes were attached to the erector spinae, rectus abdominis, and external oblique abdominal muscle. The serratus anterior, deltoid middle fiber, pectoralis major; and triceps brachii muscle were chosen as scapular stabilizers. [Results] The muscle activations of the four different positions were compared and the results show that there were significant difference among the erector spinae, rectus abdominis, external oblique abdominal, serratus anterior, deltoid middle fiber and pectoralis major. [Conclusion] The push-up exercise with the lower limbs on unstable ground increased trunk and shoulder muscle activation more than those on stable ground. We assume that muscle activation of the distal parts might have affected the muscle activation of the shoulder stabilizers that are proximal part muscles.