Physical Strength - Strength Capability

Strength Capability

There are various ways to measure physical strength of a person or population. Strength capability analysis is usually done in the field of ergonomics where a particular task (e.g. lifting a load, pushing a cart, etc.) and/or a posture is evaluated and compared to the capabilities of the section of the population that the task is intended towards. The external reactive moments and forces on the joints are usually used in such cases. The strength capability of the joint is denoted by the amount of moment that the muscle force can create at the joint to counter the external moment.

Skeletal muscles produce reactive forces and moments at the joints. To avoid injury or fatigue, when person is performing a task, such as pushing or lifting a load, the external moments created at the joints due to the load at the hand and the weight of the body segments must be ideally less than the muscular moment strengths at the joint.

One of the first sagittal-plane models to predict strength was developed by Chaffin in 1969. Based on this model, the external moments at each joint must not exceed the muscle strength moments at that joint.

Where, S_j is the muscle strength moment at joint, j, and M_j/L is the external moment at the joint, j, due to load, L and the body segments preceding the joint in the top-down analysis.

Top-down analysis is the method of calculating the reactive moments and forces at each joint starting at the hand, all the way till the ankle and foot. In a 6-segment model, the joints considered are elbow, shoulder, L5/S1 disc of the spine, hip, knee and ankle. It is common to ignore the wrist joint in manual calculations. Software intended for such calculation use the wrist joint also, dividing the lower arm into hand and forearm segments.