Generalized measurement to describe the ground reaction force curves of a walking rat. Ground reaction forces were reduced into generalized measurements, as described by Howard and co-workers . The relative size of the ground reaction forces are described in the top left panel. Vertical (Fz) forces are the largest ground reaction forces; here, a loading, support, and unloading phase can be observed (open circles, top right). Vertical ground reaction forces were generalized into peak vertical force (the largest vertical force achieved) and vertical impulse (the area under the Fz-time curve). Braking-propulsion forces (Fx) resemble a negative sine wave (solid circle, bottom left). Braking forces (-Fx) slow the linear translation of the center of mass in the direction of travel. These forces are observed early within the Fx-time curve and can be generalized into peak braking force and braking impulse. Propulsive forces (+Fx) propel the center of mass along the direction of travel. These forces occur later in Fx-time curve and can be generalized into peak propulsive force and propulsive impulse. Finally, mediolateral forces are directed at the midline of the animal and tend to have two distinct peaks (shaded circles, bottom right). The first peak (at < 50% stance time) represents the slowing of the center of mass as it is being transferred from the contralateral limb to the ipsilateral limb; or in other words, the center of mass is being translated onto the ipsilateral limb such that the contralateral limb may enter its swing phase. This region is generalized as the first peak mediolateral force. The second peak (at > 50% stance time) represents the force that must be generated to propel the center of mass to the contralateral limb such that the ipsilateral limb may enter its swing phase. This region is generalized as the second peak mediolateral force. The mediolateral force curve can also be generalized by the mediolateral impulse.
Allen et al. Arthritis Research & Therapy 2012 14:R78 doi:10.1186/ar3801