The Biomechanics of Foot and Leg Problems
To understand lower-leg problems that runners encounter, you should be aware of how biomechanical abnormalities in the lower body are related to specific foot and leg problems. The following is a review of the gait cycle, which leads to a more complex discussion of foot and lower-leg biomechanics.
Many specific foot problems are caused by biomechanical faults within the foot or lower leg. In order to understand the cause of these problems, it helps to have a working knowledge of the normal anatomy, function, and biomechanics of the lower leg. By visualizing the events of the normal gait cycle during walking or running and breaking these down into phases and subphases, each action of the foot and leg can be evaluated at a specific sequential time period.
Anatomy of the Gait
Before understanding the specifics of gait, let's take a brief look at the bones involved. The key bone structures involved are the talus and calcaneus (located at the ankle and comprising the subtalar joint) and the navicular and cuboid (located in the midfoot just forward of the ankle).
The talus and navicular along with the calcaneus and cuboid make up what is known as the midtarsal joint. The leg bones of significance consist of the femur (thigh bone) and the tibia (the larger of the two lower-leg bones). The fibula is the smaller lower-leg bone. The kneecap is called the patella.
It is important to note that in running a third subphase is present called the float phase. During the float phase, neither foot is on the ground. This is what differentiates walking from running. Walking has no float phase; one foot is always in contact with the ground when you're walking.
Phases of Gait
The gait cycle of each leg is divided into the stance phase and the swing phase. The stance phase is the period during which the foot is in contact with the ground; the swing phase is the period in which the foot is off the ground and swinging forward.
In walking, the stance phase comprises approximately 60 percent of the gait cycle and the swing phase about 40 percent. The proportion of swing to stance phase changes as the speed of walking or running increases. As the speed is increased, the percentage of time spent in the stance phase decreases. Increased time is then spent in the swing phase, with a corresponding increase in the importance of swing phase muscles.