The speed, and gait (how the legs move) of animals can be calculated from trackways. The speed of an animal, such as a dinosaur or a spider, can be calculated when two factors are known.
The first factor is stride length, or the distance between two prints of the same foot. This can be measured directly from fossil trackways. However, the stride length is only useful for calculating speed if we know how big the animal was. This is because stride length is relative to the length of the legs. For example, large animals with long legs will have long stride lengths but may actually be moving slowly. If a small animal with short legs made the same sized trackway, it must be moving faster.
Matching up the track and the track maker
The second factor to add into the calculation is the size of the animal, or the height of the hips from the ground (equivalent to the length of the legs). This usually requires that the track maker can be identified by matching up the shape of the fossil feet to the fossil footprint. It is very rare to find the track maker at the end of the trail, but this can be done. Alternatively, we can also estimate the hip height is typically about four times the length of the footprint.
The Alexander formula
These measurements can be added into a mathematical equation called the Alexander formula, which gives an estimate of speed. This formula assumes that the leg is kept absolutely straight. This is very unlikely, so variations of the formula have been produced to account for factors such as bending knees.
Was T. rex actually a slow coach?
An alternative approach for estimating speed in extinct organisms has recently been applied to T. rex. This involves calculating the strength and size of muscles in the legs relative to the size of the animal. For instance, this showed that T. rex probably could not run because the muscles needed would be too bulky to fit into the legs!