Biomechanics is the study of the body in motion. Foot biomechanics studies the relationship of the foot to the lower leg. During walking and running the musculoskeletal system generates forces to propel the body forward. The foot serves two main functions: to act as a mobile adaptor and adjust to varying terrain, and as a rigid lever for forward propulsion in locomotion. The two functions are time specific in that when the foot spends too much time being a mobile adaptor it is not spending enough time being a rigid lever, and vice versa.

Biomechanics of the foot analyzes how the various structures of the foot work together to perform specific functions (timing being of utmost importance). Excessive or prolonged motion or lack of motion will cause various deformities and pathologies discussed in the next section. Proper biomechanics allow human beings to walk, run, jump and move freely without pain or dysfunction.

The following are important terms used frequently in the study of the biomechanics of the foot:

1. Gait Cycle (See Figure 2.5) – a complete gait cycle when the foot is in contact with the ground (weightbearing)
2. Stance Phase (See Figure 2.6) – a portion of the gait cycle when the foot is in contact with the ground (weightbearing)
3. Swing Phase – the portion of the gait cycle when the foot is in the air (non-weightbearing)
4. Subtalar Joint – the neutral position, or the position of the subtalar joint when the foot is neither pronated nor supinated; when the hindfoot is neutral, the bisector of the calcaneus is the 90 to the supporting surface
    
5. Subtalar Joint Pronation – a coordinated tri-plane motion of the foot which involves three planes of motion: abduction, dorsiflexion and eversion.
   
6. Subtalar Joint Supination – a coordinated tri-plane motion of the foot which involves three planes of motion: adduction, plantarflexion and inversion
   
7. Pronated and Supinated – adjectives that describe the position of the foot (they do not describe the action of pronation or supination). Thus a “pronated foot” is one that is in a pronated position as compared to a neutral position.
8. Degrees of Pronation and/or Supination – measured by degrees of inversion (in the case of supination) or eversion (in the case of pronation) away from the neutral position.
9. Normal Centre of Pressure Line (Gait Line) – the average vector of all forces that act on the bottom of the normal foot as it goes through the stance phase of gait. Figure 2.5 shows the progression of the forces in the “normal centre of pressure line” as the foot goes from contact phase to propulsion.