Biomechanics of Gait: Phases and Involved Muscles

Walking is the most natural and fundamental mode of human locomotion. Although it appears to be a simple and automatic action, walking is actually a motor activity of extraordinary complexity that involves the central and peripheral nervous systems, over 200 muscles, numerous joints, and sophisticated energy-saving mechanisms.

The analysis of gait biomechanics (gait analysis) is a fundamental tool in rehabilitation, orthopedic, and neurological fields. It allows for the identification of gait alterations, understanding their causes, and setting up targeted treatment. Many musculoskeletal and neurological pathologies manifest early with a modification of the gait pattern, often even before the patient is aware of it.

Knowledge of the phases of the gait cycle, the muscles involved in each phase, and the energy-saving mechanisms is the basis for understanding gait alterations and designing effective rehabilitation pathways.

The Gait Cycle

The gait cycle is the sequence of lower limb movements from initial foot contact to the next contact of the same foot, involving coordinated muscle and joint actions. The gait cycle is the functional unit of walking. It is defined as the interval between two successive contacts of the same heel with the ground (heel strike). A complete cycle includes a right step and a left step.

The gait cycle is divided into two main phases:

During the gait cycle, there is a period of double support (both feet in contact with the ground) which lasts approximately 20-25% of the total cycle (10-12% initial + 10-12% final). Double support is an indicator of stability: as walking speed increases, double support decreases until it disappears in running, where it is replaced by a flight phase (both feet lifted).

Spatio-Temporal Gait Parameters

The Gait Phases in Detail

The most widely used subdivision in clinical and biomechanical fields is Perry’s, which identifies 8 sub-phases: 5 in the stance phase and 3 in the swing phase.

Stance Phase (60% of cycle)

1. Initial Contact — 0-2% of cycle

The heel touches the ground. This moment marks the beginning of the gait cycle.

• Position: hip flexed to about 30°, knee extended (or nearly so), ankle in neutral position (0°)
• Active muscles:

• Tibialis anterior: maintains the ankle in dorsiflexion to ensure heel contact (and not toe contact)

• Quadriceps: preparation for impact absorption

• Hamstrings: decelerate limb swing and stabilize the hip

• Gluteus maximus: begins contraction to control hip flexion

2. Loading Response — 2-12% of cycle

Body weight is transferred to the limb that has just made contact. The foot fully contacts the ground (foot flat). The first period of double support begins.

• Position: the foot is fully in contact with the ground; the knee flexes slightly (15-20°) to absorb impact (first “wave” of knee flexion)
• Active muscles:

• Quadriceps (eccentric): controls knee flexion, absorbing impact shock. This is the phase where the quadriceps works the most

• Tibialis anterior (eccentric): controls the lowering of the foot to the ground (prevents “foot slap”)

• Gluteus maximus: extends the hip and stabilizes the trunk

• Gluteus medius: begins stabilizing the pelvis in the frontal plane

3. Midstance — 12-31% of cycle

The contralateral limb lifts from the ground: single-limb support begins. The body advances over the stance foot.

• Position: the knee progressively extends; the ankle moves from neutral position to dorsiflexion; the hip progressively extends
• Active muscles:

• Gluteus medius and gluteus minimus: fundamental stabilization of the pelvis in the frontal plane. They prevent the pelvis from dropping on the opposite side (Trendelenburg sign if insufficient)

• Triceps surae (soleus + gastrocnemius): begins eccentric contraction to control tibial advancement over the ankle

• Tibialis posterior and peroneals: stabilize the foot and control pronation

• The quadriceps relaxes: the knee passively stabilizes due to the “locking” mechanism (extension with slight hyperextension)

4. Terminal Stance — 31-50% of cycle

The heel lifts from the ground (heel off). The body advances beyond the stance foot, which acts as a rigid lever for propulsion.

• Position: the hip is in maximum extension (10-20°); the knee is extended; the ankle is in dorsiflexion; the metatarsophalangeal joints extend (windlass mechanism)
• Active muscles:

• Triceps surae (concentric, then eccentric): generates propulsive thrust. The soleus is the main force generator in this phase

• Flexor digitorum longus and flexor hallucis longus: stabilize the toes and contribute to propulsion

• Iliopsoas: begins to flex in preparation for swing

The windlass mechanism is fundamental in this phase: toe extension tenses the plantar fascia, which raises the longitudinal arch of the foot and transforms it into a rigid lever for propulsion.

5. Pre-swing — 50-62% of cycle

The contralateral foot touches the ground: the second period of double support begins. The stance limb prepares for toe-off.

• Position: the knee rapidly flexes (35-40°); the ankle plantarflexes; the toes lift from the ground
• Active muscles:

• Iliopsoas: initiates hip flexion to begin swing

• Rectus femoris: contributes to hip flexion

• Hip adductors: contribute to hip flexion and limb advancement

• The triceps surae relaxes after generating propulsion

Swing Phase (40% of cycle)

6. Initial Swing — 62-75% of cycle

The limb lifts from the ground and begins to advance. The foot must clear the ground without tripping (clearance).

• Position: the hip flexes; the knee flexes to about 60° (the maximum in the gait cycle); the ankle moves from plantarflexion to neutral position
• Active muscles:

• Iliopsoas: hip flexion (main mover)

• Tibialis anterior: ankle dorsiflexion to lift the foot from the ground (clearance)

• Short head of biceps femoris: contributes to knee flexion

• The knee flexes largely passively, due to the inertia of the swing

7. Midswing — 75-87% of cycle

The limb advances in front of the body. The knee progressively extends as the limb “pendulums” forward.

• Position: the hip is in maximum flexion (about 30°); the knee progressively extends; the ankle is in neutral position
• Active muscles:

• Tibialis anterior: maintains dorsiflexion for foot clearance

• Quadriceps: begins to contract to control knee extension

• The hip flexors relax: the limb advances by inertia

8. Terminal Swing — 87-100% of cycle

The limb decelerates and prepares for ground contact. The knee fully extends and the ankle positions itself for heel contact.

• Position: the hip is flexed to 30°; the knee is fully extended; the ankle is in neutral position
• Active muscles:

• Hamstrings (eccentric): decelerate knee extension and hip flexion. This is the moment of maximum stress on the hamstrings (and the moment when hamstring muscle injuries most frequently occur in sprinters)

• Tibialis anterior: maintains dorsiflexion for heel contact

• Quadriceps: controls final knee extension

Summary: Muscles per Gait Phase

Center of Gravity and Energy Saving

The Displacement of the Center of Gravity

During walking, the body’s center of gravity (located approximately at S2 level, anterior to the vertebral column) moves following a three-dimensional sinusoidal trajectory:

• Vertical plane: oscillates up and down by approximately 5 cm. The highest point is reached during midstance (single-limb support); the lowest point during double support
• Lateral plane: oscillates laterally by approximately 4 cm, shifting towards the side of the stance limb
• Sagittal plane: moves forward with cyclic accelerations and decelerations

The Six Determinants of Gait (by Saunders)

The classic six determinants of gait are biomechanical mechanisms that minimize the vertical oscillation of the center of gravity, reducing energy consumption:

• Pelvic rotation: the pelvis rotates forward (about 4° per side) during swing, increasing step length without lowering the center of gravity
• Pelvic tilt: the pelvis tilts downward (about 5°) on the side of the swinging limb, lowering the highest point of the center of gravity’s trajectory
• Knee flexion in stance: the slight knee flexion during loading response lowers the center of gravity and absorbs impact
• Foot-ankle interaction: the “rocker” motion of the foot from heel to forefoot creates an arc that smooths the center of gravity’s trajectory
• Knee-ankle interaction: the coordination between knee flexion and ankle movement optimizes the trajectory
• Lateral pelvic displacement: controlled by the physiological valgus angle of the knee and step width

Energy Consumption

Normal walking is optimized for minimal energy consumption. The preferred speed (approximately 1.3 m/s) corresponds to the speed of minimum energy cost per meter traveled. The energy cost of normal walking is approximately 3.3 kcal/min (at 1.3 m/s).

Any alteration of the gait pattern (limp, joint stiffness, muscle weakness) increases energy consumption, sometimes dramatically:

Common Gait Alterations

Antalgic Gait

Antalgic gait (or pain-avoiding gait) is the most common gait alteration. It occurs when the patient reduces the stance time on the painful limb to limit pain.

• Characteristics: shortened stance phase on the painful limb, reduced step length, decreased speed
• Causes: osteoarthritis (hip, knee), fractures, tendinopathies, any painful pathology of the lower limb
• Compensation: the trunk leans towards the painful side during stance, to reduce the moment of force on the joint

Trendelenburg Gait

Trendelenburg gait is caused by insufficiency of the gluteus medius (hip abductor).

• Mechanism: during single-limb support, the gluteus medius must stabilize the pelvis, preventing the opposite side from dropping. If the gluteus medius is weak, the pelvis drops towards the contralateral side (swinging limb side)
• Positive Trendelenburg sign: in single-limb support, the pelvis drops on the side opposite to the stance limb (side of the weak gluteus)
• Duchenne compensation: the patient leans the trunk towards the side of the stance limb to shift the center of gravity over the hip, reducing the demand on the gluteus medius. This compensation is “Duchenne gait”
• Causes: superior gluteal nerve lesion (L5), hip osteoarthritis, hip prosthesis, myopathies, prolonged inactivity

Steppage Gait (Foot Drop)

Steppage gait is a gait characterized by excessive hip and knee flexion during swing, to compensate for the inability of the ankle to dorsiflex.

• Mechanism: paralysis or weakness of the tibialis anterior prevents ankle dorsiflexion. The foot “drops” into plantarflexion during swing and risks tripping on the ground
• Compensation: the patient excessively flexes the hip and knee to lift the foot from the ground (steppage = “high-stepping gait”). At initial contact, the foot touches the ground with the toes (instead of the heel) or with the entire sole (foot slap)
• Causes: common peroneal nerve lesion, L5 radiculopathy, peripheral neuropathy, amyotrophic lateral sclerosis

Stiff-Knee Gait

Stiff-knee gait occurs when the knee fails to flex adequately during swing.

• Mechanism: reduced knee flexion during swing (normally about 60°) prevents foot clearance
• Compensation: circumduction of the limb (the patient “swings the limb outwards” to advance), hip hiking, or vaulting (lifting onto the toes of the contralateral foot)
• Causes: quadriceps spasticity, advanced osteoarthritis, knee arthrodesis, pain

Other Alterations

Instrumental Gait Analysis

Instrumental gait analysis is a specialized examination that allows for an objective and detailed evaluation of the ambulation pattern:

• Spatio-temporal parameters: speed, cadence, step length, stance and swing times
• Kinematics: joint angles during the gait cycle (measured with infrared cameras and reflective markers)
• Kinetics: ground reaction forces (measured with force platforms), joint moments, powers
• Dynamic electromyography (EMG): temporal activation and intensity of muscles during walking
• Energy consumption: measurement of oxygen consumed during walking (metabolic cost)

Frequently Asked Questions (FAQ)

Frequently Asked Questions

Why is understanding one’s walking pattern important for health?

Understanding one’s walking pattern is crucial for identifying subtle changes that may signal underlying health issues, often before they become apparent. This insight is fundamental for guiding effective rehabilitation and optimizing movement.

What are the two main phases of the gait cycle?

The human gait cycle is primarily composed of two main phases: the stance phase and the swing phase. The stance phase involves the foot being in contact with the ground, while the swing phase describes the foot moving through the air.

How do physical therapists utilize gait analysis?

Physical therapists utilize gait analysis as a fundamental tool to systematically evaluate an individual’s walking pattern. This detailed assessment helps identify biomechanical inefficiencies or abnormalities, informing the development of personalized rehabilitation plans.

What makes human walking a complex motor activity?

Human walking, despite appearing automatic, is an extraordinarily complex motor activity. It involves the intricate coordination of the central and peripheral nervous systems, over 200 muscles, numerous joints, and sophisticated energy-saving mechanisms.

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Sources and Scientific References

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