A novel approach to evaluate abdominal coactivities for optimal spinal stability and compression force in lifting.

A novel optimisation algorithm is developed to predict coactivity of abdominal muscles while accounting for both trunk stability via the lowest buckling load (P(cr)) and tissue loading via the axial compression (F(c)). A nonlinear multi-joint kinematics-driven model of the spine along with the response surface methodology are used to establish empirical expressions for P(cr) and F(c) as functions of abdominal muscle coactivities and external load magnitude during lifting in upright standing posture. A two-component objective function involving F(c) and P(cr) is defined. Due to opposite demands, abdominal coactivities that simultaneously maximise P(cr) and minimise F(c) cannot exist. Optimal solutions are thus identified while striking a compromise between requirements on trunk stability and risk of injury. The oblique muscles are found most efficient as compared with the rectus abdominus. Results indicate that higher abdominal coactivities should be avoided during heavier lifting tasks as they reduce stability margin while increasing spinal loads.