Fig. 2.

Typical ground reaction forces for walking rodents. Ground reaction forces consist of three orthogonal components: vertical, braking/propulsive, and mediolateral. The vertical component carries the majority of weight, with the positive component directing out of the ground (+z). The vertical force curve for rodents tends to be semi-parabolic, with only one peak, in contrast to that of bipedal humans, which tends to have two peaks. The braking/propulsive curve aligns with the direction of travel and is composed of the negative braking component (180° from the direction of travel, −x) and the positive propulsive component (in the direction of travel, +x). While not shown, propulsive forces tend to be larger than braking forces for the hind limbs of rodents. However, this is flipped for the fore limbs, where braking forces tend to be larger than propulsive forces. The mediolateral curve is directed toward the animal's midline, typically defined as positive (+y). This convention is adapted for some models, as ground reaction forces for a left foot would not be defined by a right-handed coordinate system. The mediolateral curve consists of two positive peaks, where the first peak tends to be higher than the second. For unilateral medial meniscus tear in rats, we have observed decreased vertical force and propulsive forces on the injured limb [17••]