D2 Dynamic Force Analysis

Topic

Dynamic force analysis quantifies the mechanical loads acting on the musculoskeletal system during movement, going beyond kinematics to determine the mechanical causes of that movement rather than merely describing it. The central tool for this analysis is the multiaxial force platform—an instrument that uses piezoelectric or strain-gauge sensors to measure the three components of the ground reaction force experienced by the body during contact, along with the three moments associated with that force.

Inverse dynamics is applied using the ground reaction force measured by the platform and the movement's kinematics: by treating each body segment as a rigid body, the equations of motion allow for the calculation of the net moment that must have acted at each joint to produce the observed movement. This calculation is performed in a chain, starting at the distal end—the foot, where the ground reaction force is applied—and proceeding segment by segment toward the proximal end—the hip—such that the forces and moments calculated for each segment propagate to the next.

The net moment calculated at each joint, multiplied by the joint's instantaneous angular velocity, yields the joint power; the sign of this power reveals the joint's mechanical role: when power is positive, the joint delivers mechanical energy to the body (derived from metabolic processes), acting as a motor; when power is negative, the joint absorbs mechanical energy from the movement rather than delivering it, acting as a brake or shock absorber.

The distribution of the ground reaction force beneath the sole of the foot is determined using high-spatial-resolution pressure platforms; these allow for the calculation of the center of pressure beneath the foot at any given moment and the tracking of its trajectory throughout the contact phase, reflecting how the point of actual application of the ground reaction force shifts step by step. This same pressure distribution also allows for the calculation of a friction utilization coefficient, which quantifies, at any given moment, the fraction of the available sliding limit being demanded by the movement.

The behavior expected during a healthy gait—as measured by this dynamic force analysis—shows vertical ground reaction force peaks of approximately the animal's body weight (up to twenty percent higher) during walking, and between two and a half and three times the body weight during running. In pathological cases, however, asymmetries exceeding ten percent between the two sides of the body, or joint moments abnormally redistributed across joints, serve as diagnostic indicators that the mechanical loads quantified by this dynamic force analysis are being managed in a pathological manner.

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