Force plate

From Biomch-W

A force plate is an instrument used to measure ground reaction forces during locomotion. The device is usually utilized in combination with motion analysis systems. Once the subject's foot hits the force plate, the loading is recorded using strain gages or piezoelectric sensors embedded in the device at various locations. The raw data is then converted into three force components (vertical, anterior-posterior shear, mediolateral shear), two center of pressure components (anterior-posterior and mediolateral) and free moment.

Considerable confusion often exists about the origin of the forces measured by a force platform. The best way to understand these is to think of the force platform as a whole-body accelerometer. Since Force = mass x acceleration (Newton's 2nd Law of Motion), any acceleration of the body will be reflected in a reaction when at least one foot is on the ground. An upwards acceleration (as occurs at push-off) will be reflected in an increase in the vertical load (weight) recorded, while a downwards acceleration (as occurs during mid-stance) will reduce the effective body weight. Note that a downwards deceleration (such as occurs at initial contact) is equivalent to an upwards acceleration, and vice versa. It is clear from the above that the ground reaction forces reflect accelerations of the body's centre of mass. They are not, as many people wrongly believe, influenced by changes in footwear. This is illustrated in the following set of curves taken from a subject walking three times barefoot and shod, then walking like Groucho Marx (minimal vertical accelerations) and as if on the moon (large vertical accelerations).

It can be seen that there are no significant differences in the curves according to footwear, but considerable differences when trunk accelerations are modified.

It's important not to get mixed up between displacement (position), velocity and acceleration. The three variables are out of phase, so don't expect the CoM to be high when the force is high, or the force to be related to velocity: the key variable for understanding force is ACCELERATION!