Investigation of forces and stresses acting on a shoulder-hand system considering strains in muscles

A model is proposed to investigate the forces and stresses acting on bones in a shoulder-hand system due to straining of muscles during daily physical activities. This model is proposed as a truss system consisting of beams, or rods, corresponding to the considered bones and muscles, connected by frictionless joints. One of the hard loading positions, which occurs when one does pull-ups while holding onto a bar (a common fitness activity), is considered in the proposed model as an example. The stress is found to increase from the elbow end of the humerus bone toward the shoulder end. The stress in the example study reached a maximum value of approximately 8000 N/cm² near the shoulder end during the pull-up motion, which is about 20% less than the maximum stress calculated when the straining of muscles was neglected. The difference between these 2 maximum values of stress indicates the effect of strained muscles, which was first taken into consideration in this study. This stress value is considerably high when the strength of bones is taken into account. It is also necessary to take into account the stress fluctuations in the human body during various daily activities; fluctuating stresses acting on an implant may create fatigue cracks at the shoulder end of the humerus in addition to a high stress concentration factor due to shape and size changes in the humerus bone.