Sagittal Plane Analysis of GRFV during Stance Phase of Gait

Forces Acting on the Body during Gait

• External forces acting on the body during normal gait include ground reaction force, gravity, and muscle forces.
• The force equal in magnitude and opposite in direction to the force of gravity during walking is the ground reaction force.

Line of Gravity and Internal Forces

• The line of gravity (LOG) represents the vertical direction of gravity's force on the body.
• The primary structure creating internal forces during gait is the muscles.
• From a lateral view, the line of gravity (LOG) passes anterior to the ankle joint during standing.

Muscle Function and Force Resistance

• Muscles, assisted by tendons and ligaments, resist, transmit, and absorb forces during gait.
• The result of the gravitational torque passing anterior to the axis of the ankle joint is a dorsiflexion moment.
• The counterbalancing torque to the gravitational torque is mainly created by the tibialis anterior muscle.

Ground Reaction Force Vector (GRFV)

• The Ground Reaction Force Vector (GRFV) is composed of three forces: vertical, anterior/posterior, and medial/lateral.
• The GRFV and the Line of Gravity (LOG) are aligned along a common action line in double support posture.
• The body's center of pressure (COP) is located under the heel in unilateral stance.

Gait Cycle and Movement Patterns

• The COP produces a characteristic pattern of lateral to medial movement during gait.
• The pattern of ground reaction force vector (GRFV) starts at the heel during the stance phase of gait.
• A greater perpendicular distance of the GRFV from a joint's axis increases the moment or torque produced at that joint.

Joint Movement and Muscle Action

• When the GRFV is located anterior to the ankle joint, it produces a dorsiflexion moment.
• The GRFV needs to be lateral to the hip joint to cause an extension moment.
• The GRFV moves laterally to the midline at the end of the stance phase.
• The hamstrings muscle group is responsible for eccentric contraction to counterbalance the flexion moment behind the knee during gait.
• The hip flexor muscle group exhibits concentric contraction to aid in obtaining toe clearance during initial swing.

Limb Advancement and Muscle Function

• At midswing, the limb advances through hip flexion and knee extension.
• The primary muscle action at terminal swing is to prevent excessive knee hyperextension.
• During initial contact, the muscles counterbalancing the planar flexion moment created by the GRFV passing posterior to the ankle joint are the gastrocnemius and soleus.
• The main function of the pretibial muscles during the swing phase of gait is to aid in toe clearance.

Muscle Activity and Gait Phase

• The adductor muscles, mainly adductor magnus, show increased activity at initial contact in gait to assist in stabilizing the pelvis.
• The tibialis anterior muscle function at terminal swing is to counterbalance the gravitational torque.
• The gastrosoleus muscles reduce their intensity of action during the swing phase.
• The purpose of soleus and gastrocnemius eccentric contraction during gait is to control the descent of the body's center of mass.
• There needs to be an opposite moment balanced at the hip joint during gait to maintain posture and stability.