A conceptual approach to the biomechanical assessment of the lower extremity is important, so that the Orthotist can design and select appropriate components for an orthosis.

Definitions:

Kinematics = Description of motion without regard for the forces and masses involved. Analyzed from heel strike to heel strike of the same foot. These include the phases of gait, important stages of gait, step length, cadence, translational and rotational motion.

Kinetics = Analysis of forces that causes motion. Based on physics (Newton's laws of motion). These include force and motions.

Force = Described as a push or pull.
Has four characteristics – magnitude; direction; point of application; line of action.
Within and from outside the human body there are forces occurring directly related to walking. These include: muscle forces, ligament forces, joint forces, gravitational forces, ground reaction forces, inertial forces and corrective forces.

Total Body Force (TBF) = Inertial Force + Gravitational Force

Ground Reaction Force (GRF) = The reaction force of the floor to the TBF. During each stage of the gait cycle movements at the hip, knee, ankle, and foot are in response to the GRF and the pull of muscles opposing it.

Moment = The tendency of a force to cause rotation about some axis. Defined as the
force x the perpendicular distance from the line of action to the center of rotation.


For static equilibrium to occur the sum of the forced must equal zero (F = 0) and the sum of the moments must equal zero (M = 0). If this condition dose not exists we have a
non-equilibrium condition.

Base of Support = The complete area under and between the supporting members in contact with the ground. The centre of gravity must be aligned vertically over the base of support. The stability of an object is affected by the height of the center of gravity (or center of mass), the weight and the base of support. The large these three factors are the more stable the object will be.


Energy = The capacity to do work. The total of a body is the sum of potential energy
(due to height) and the kinetic energy (due to motion).
For the human body to be efficient the changes in total energy must be minimized by decreasing the movement of the center of gravity (mass) to within a 5cm square both vertically and horizontally (potential energy changes). The maintenance of a constant velocity
(kinetic energy changes) must also be recognized. To accomplish this the body performs various motions at the pelvis, hip and ankle. They include
Pelvic Rotation
Pelvic Step
Knee Motion
Ankle / Foot Motion
Spinal Motion
The smoothness of the motion maximizes the conservation of energy. During a pathological gait pattern the movements of the center of gravity (mass) are exaggerated and have an inefficient jerky stop and start type of motion. Basically, efficiency is sacrificed for stability. An Orthosis that decreases these abnormal patterns will increase the efficiency and potential of the patient's activities.


Pressure = Force / unit area. The large the surface area the less pressure / area.
The further one separates the corrective three point pressure systems, the lesser the pressure / unit area. In general, the pressure applied by an Orthosis must be minimized.

Stress = The internal resistance of materials to externally applied loads. F / A

Newton's Laws of Motion
The first Law (Law of Inertia): A body tends to remain in motion or at rest unless acted upon by some unbalanced force.
The Second Law (Law of Acceleration): An unbalanced force acting on a body causes the body to accelerate in the direction of the force.
The Third Law (Law of Reaction) = For every action there is an equal and opposite reaction and both action and reaction are along the same straight line.