Muscle Activity During Gait

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Joint Moment

The body should be in equilibrium during gait, therefore the external ground reaction forces (GRF) (external moment) should be balanced by the internal muscle forces (internal moment). Different muscles perform different actions at every phase to create an internal moment against the external joint moment. Ground reaction force vectors create the direction of moment and the muscle will act in the opposite direction. If the ground reaction force falls anterior to the axis of the joint, the proximal segment of the joint will move anteriorly and if the ground reaction force falls posterior to the axis of the joint, the proximal segment of the joint with move posteriorly. Muscles can contract concentrically or eccentrically to overcome the external moment. During a concentric muscle contraction, the muscle shortens and there is movement at the joint, whereas during an eccentric contraction the muscle lengthens while producing force.

Read more:

• Types of Skeletal Muscle Actions
• Ground Reaction Forces

Breakdown of Gait Cycle Phases and Sub-Phases

Stance Phase

Initial Contact (or Heel Strike) - 0%

The moment the foot touches the ground and begins the first phase of double support. Its function is to establish contact with the ground surface and initiate weight acceptance.

Kinetics at Initial Contact

Joint	Kinetics
Ankle	External forces	Internal forces/ Muscle Action
	GRF posterior to axis of ankle joint External plantar flexion moment created	Internal dorsiflexion moment Eccentric contraction of dorsiflexors: tibialis anterior extensor digitorum longus extensor hallucis longus
Knee	GRF anterior to axis of knee joint External extensor moment created	Internal flexion moment Concentric contraction of knee flexors: hamstrings Eccentric contraction of knee extensors: quadriceps (range of motion change in knee 0° - 15°, this results in the shifting of GRF from anterior to posterior - this will result in external flexor torque and knee extensors will work eccentrically to control the knee)
Hip	GRF anterior to axis rotation of hip joint External flexion moment created	Internal extensor moment Concentric contraction of gluteal muscles (hip extensors): gluteus maximus

Foot Flat (Loading Response) - 0-10%

Begins with initial contact and continues until the contralateral foot leaves the ground. The foot continues to accept weight and absorb shock by rolling into pronation.

Kinetics at Foot Flat

Joint	Kinetics
	External forces	Internal forces/Muscle Action
Ankle	GRF posterior to axis of rotation of ankle joint External plantar flexion moment created	Internal dorsiflexion moment Concentric contraction of dorsiflexors Eccentric contraction of plantar flexors: gastrocnemius soleus Range of motion change in ankle 5° plantarflexion to 5° dorsiflexion, results in the shifting of GRF from posterior to anterior thus posterior musculature (plantarflexors) are used eccentrically to control dorsiflexion
Knee	GRF posterior to axis of rotation of knee joint External flexion moment created	Internal extensor moment Concentric contraction of knee extensors
Hip	GRF anterior to axis of rotation of hip joint External flexion moment created	Internal extensor moment Concentric contraction of hip extensors

Midstance - 10-30%

Begins when the contralateral foot leaves the ground and continues until ipsilateral heel lifts off the ground. The body is supported by a single leg and begins to move from force absorption at impact to force propulsion forward.

Kinetics at Midstance

Joint	Kinetics
	External forces	Internal forces/Muscle Actions
Ankle	GRF anterior to axis of rotation of ankle External dorsiflexion moment	Internal plantar flexor moment Concentric contraction of plantarflexors
Knee	GRF anterior to axis of rotation of knee joint External extension moment	Internal flexor moment Eccentric contraction of knee flexors
Hip	GRF posterior to axis of rotation External extension moment	Internal flexor moment Eccentric contraction of hip flexors

Heel Off (or Terminal Stance) - 30-50%

Begins when the heel leaves the floor and continues until the contralateral foot contacts the ground. In addition to single limb support and stability, this event serves to propel the body forward. Bodyweight is divided over the metatarsal heads.

Kinetics at Heel Off

Joint	Kinetics
	External forces	Internal forces/Muscle action
Ankle	GRF anterior to axis of rotation of ankle External Dorsiflexion moment	Internal plantarflexion moment Concentric contraction of plantarflexors
Knee	GRF anterior to axis of rotation of knee External extensor moment	Internal flexor moment Concentric contraction of knee flexors
Hip	GRF posterior to axis of rotation of hip External extensor moment	Internal flexor moment Eccentric contraction of hip flexors

Toe Off (or Preswing) - 50-60%

Begins when the contralateral foot contacts the ground and continues until the ipsilateral foot leaves the ground. Provides the final burst of propulsion as the toes leave the ground.

Kinetics at Toe Off

Joint	Kinetics
	External Forces	Internal Forces/Muscle Action
Ankle	GRF anterior to axis of rotation of ankle External dorsiflexion moment	Internal plantarflexion moment Eccentric contraction of plantarflexors Concentric contraction of dorsiflexors to propel the foot up
Knee	GRF posterior to axis of rotation of knee External flexion moment	Internal extensor moment Eccentric contraction of knee extensors Concentric contraction of knee flexors
Hip	GRF posterior to axis of rotation of hip External extensor moment	Internal flexor moment Concentric contraction of hip flexors

Swing Phase

Ground reaction forces are not applicable to the swing phase as there is no contact with the ground. However, the muscles still contract concentrically or eccentrically as needed.

Early Swing - 60-75%

Begins when the foot leaves the ground until it is aligned with the contralateral ankle. This event functions to advance the limb and shorten the limb for foot clearance.

Muscle Actions at Early Swing

	Kinematics/Joint motion	Muscle Action
Ankle	Ankle moves from 20 of plantarflexion at Toe Off to 10 of plantarflexion at early swing	Concentric contraction of dorsiflexors
Knee	Knee moves from 30 flexion to 60 flexion	Eccentric contraction of knee extensors and concentric contraction of knee flexors
Hip	Hip moves into flexion	Concentric contraction of hip flexors

Mid-Swing - 75-85%

Begins from the ankle and foot alignment and continues until the swing leg tibia is vertical. As in early swing, it functions to advance the limb and shorten the limb for foot clearance.

Muscle Action at Mid-Swing

	Kinematics/Joint motion	Muscle Action
Ankle	Ankle moves from 10 of plantarflexion to 0 of plantarflexion - moving into dorsiflexion	Concentric contraction of dorsiflexors
Knee	Knee moves from 60 of flexion to 30 of flexion	Eccentric contraction of knee flexors
Hip	Hip moves from 20 of flexion to 30 of flexion	Concentric contraction of hip flexors

Late Swing/Deceleration - 85-100%

Begins when the swing leg tibia is vertical and ends with initial contact. Limb advancement slows in preparation.

Muscle Action at Late Swing

	Kinematics/Joint motion	Muscle Action
Ankle	Maintains a neutral position	Concentric and isometric contraction of dorsiflexors
Knee	Knee moves from 3o flexion to o flexion	Eccentric contraction of knee flexors, then concentric contraction of knee flexors
Hip	Hip moves from 20 flexion to 30 flexion	Concentric contraction of hip flexors