3.4 Gait Analysis and Locomotion

Biomechanics of human gait

Gait analysis studies how humans walk and move, breaking down the complex, repeating motions of locomotion into measurable components. Understanding gait mechanics is essential in biomedical engineering because it connects statics and dynamics concepts to real clinical problems: designing prosthetics, diagnosing movement disorders, and planning rehabilitation.

Gait definition and characteristics

Gait is a repetitive sequence of limb motions that moves the body forward while maintaining stability. One full gait cycle runs from the initial contact of one foot to the next initial contact of that same foot.

The cycle divides into two main phases:

• Stance phase (~60% of the cycle): the foot is in contact with the ground. It subdivides into initial contact, loading response, mid-stance, terminal stance, and pre-swing.
• Swing phase (~40% of the cycle): the foot is off the ground and advancing forward. It subdivides into initial swing, mid-swing, and terminal swing.

Two other characteristics are central to gait analysis:

• Ground reaction forces (GRFs) are the forces the ground exerts on the body during stance. They have three components: vertical, anterior-posterior, and medial-lateral. The vertical GRF trace during walking typically shows a characteristic "double-hump" (M-shaped) pattern, with peaks during loading response and terminal stance.
• The center of mass (COM) follows a sinusoidal path in both the vertical and lateral directions. The COM is at its lowest during double support (both feet on the ground) and at its highest during single-limb support (mid-stance).

Inverted pendulum model of gait

The inverted pendulum model explains how walking conserves energy through exchanges between potential energy (PE) and kinetic energy (KE). Think of the stance leg as a rigid strut: the COM vaults over it like an inverted pendulum.

Here's how the energy exchange works across the cycle:

1. At mid-stance, the COM is at its highest point. PE is at a maximum, and KE is at a minimum (the body is momentarily moving slowest).
2. During late stance, the COM falls forward and downward. PE converts to KE, accelerating the body forward.
3. At double support (between stance phases), the COM is at its lowest point. KE is at a maximum.
4. During early stance of the next step, KE converts back to PE as the COM rises again.

This PE-KE exchange recovers roughly 60–70% of the mechanical energy of walking, which is why walking at a preferred speed is so efficient. Running, by contrast, uses a different model (the spring-mass model) and stores energy elastically in tendons.

Phases of the gait cycle

Stance phase

Each sub-phase has a distinct mechanical role. Pay attention to what the hip, knee, and ankle are doing at each stage:

• Initial contact (heel strike): The heel contacts the ground. The hip is flexed (~30°), the knee is near full extension, and the ankle is in a neutral (0°) position.
• Loading response: The body absorbs the impact of foot contact. The knee flexes slightly (~15°) to absorb shock, and the ankle plantarflexes as the foot lowers to the ground. This is when the first peak in vertical GRF occurs.
• Mid-stance: The body progresses over the stationary foot on a single limb. The hip and knee extend, and the ankle dorsiflexes as the tibia advances over the foot. The COM reaches its highest point here.
• Terminal stance: The heel rises off the ground. The hip moves into hyperextension, the knee flexes slightly, and the ankle plantarflexes to push the body forward. The second peak in vertical GRF occurs here.
• Pre-swing: This transitional sub-phase prepares for toe-off. The hip and knee flex, and the ankle dorsiflexes as the opposite limb accepts body weight.

Swing phase

The swing phase advances the limb forward for the next step:

• Initial swing: Begins at toe-off. Rapid hip and knee flexion lifts the foot off the ground.
• Mid-swing: The foot clears the ground. The hip and knee reach near-maximum flexion, and the ankle holds a neutral position to prevent the toes from dragging.
• Terminal swing: The swinging limb decelerates. The hip stays flexed, the knee extends, and the ankle remains neutral, positioning the limb for the next initial contact.

Kinematic and kinetic parameters

Gait analysis quantifies movement using two categories of parameters:

Kinematic parameters describe motion without reference to forces:

• Joint angles describe the relative position of two adjacent segments (e.g., knee flexion angle). These are the most commonly reported gait data.
• Angular velocities describe how fast joint angles change over time.
• Angular accelerations describe how fast angular velocities change.

Kinetic parameters describe the forces and energetics behind the motion:

• Joint moments are the net rotational effects at a joint, produced by muscles, ligaments, and external forces like GRFs. Calculated using inverse dynamics, they tell you which muscles are working at each phase.
• Joint power is the rate of energy generation or absorption at a joint: $P = M \times \omega$, where $M$ is the joint moment and $\omega$ is the angular velocity. Positive power means the muscles are generating energy (concentric contraction); negative power means they are absorbing energy (eccentric contraction).
• Joint work is the total energy generated or absorbed over a time interval: $W = \int P \, dt$.

Muscle activity and joint forces in gait

Muscle activity during the gait cycle

Different muscle groups activate at specific phases to control motion and provide propulsion. The major patterns are:

• Gluteus maximus and hamstrings: Active at initial contact and loading response. They extend the hip and eccentrically control knee flexion during shock absorption.
• Quadriceps femoris: Active during loading response and mid-stance. The quads eccentrically control knee flexion at loading response, then concentrically extend the knee to support the body during mid-stance.
• Gastrocnemius and soleus (triceps surae): Active during mid-stance and terminal stance. These are the primary ankle plantarflexors and provide the "push-off" force that propels the body forward. The soleus generates more force during walking than almost any other muscle.
• Tibialis anterior: Active during pre-swing and throughout the swing phase. It dorsiflexes the ankle to clear the foot from the ground and controls the lowering of the foot after initial contact.
• Hip abductors (gluteus medius and minimus): Stabilize the pelvis during single-limb stance, preventing the opposite side from dropping.
• Hip adductors (adductor longus, brevis, and magnus): Assist in limb advancement during the swing phase.

Joint forces during the gait cycle

Joint reaction forces result from the combined effects of muscle forces, GRFs, and inertial forces. These forces are much larger than body weight because muscles must generate large forces across short moment arms to balance external loads.

Typical peak joint forces during normal walking:

• Faster walking speed increases GRFs and muscle forces
• Higher body weight directly increases the loads on all joints
• Joint malalignment (e.g., varus or valgus knee alignment) redistributes forces unevenly across the joint surface, which can accelerate cartilage degeneration

These force estimates are clinically important for designing joint replacements and predicting implant wear.

Gait analysis techniques for assessment

Observational and instrumented gait analysis

Observational gait analysis involves a clinician visually examining the patient's walking pattern for deviations in joint angles, timing, and symmetry. Standardized protocols like the Rancho Los Amigos Observational Gait Analysis System guide the clinician to assess gait systematically across the sagittal, frontal, and transverse planes. This approach is quick and requires no special equipment, but it can miss subtle deviations and has limited reliability between different observers.

Instrumented gait analysis uses technology to quantify gait parameters objectively:

• Motion capture systems track the position and orientation of body segments. Optical (marker-based) systems use infrared cameras to track reflective markers placed on the skin. Inertial (sensor-based) systems use accelerometers and gyroscopes attached to body segments.
• Force plates embedded in the walkway measure GRFs and moments during stance, providing magnitude and direction data for all three force components.
• Electromyography (EMG) records the electrical activity of muscles, revealing the timing and relative intensity of muscle activation. Surface EMG is most common in gait labs; fine-wire EMG can target deeper muscles.

Instrumented gait analysis produces objective, quantitative data that can be compared to normative databases, tracked over time, or used to evaluate the effects of an intervention.

Assessment of pathological gait patterns

Pathological gait patterns arise from pain, weakness, spasticity, or structural abnormalities. Three commonly tested examples:

• Antalgic gait: The patient shortens the stance phase on the painful limb to minimize loading time. Seen in conditions like osteoarthritis, fractures, or tendinopathies. The result is an asymmetric gait with a characteristic "limp."
• Trendelenburg gait: The pelvis drops on the swing side because the stance-side hip abductors (gluteus medius/minimus) are too weak to stabilize it. Associated with hip dysplasia, stroke, or cerebral palsy. You can test for this with a single-leg stance test.
• Circumduction gait: The patient swings the leg outward in an arc during swing phase to clear the foot, compensating for an inability to adequately dorsiflex the ankle or flex the knee. Common in stroke, multiple sclerosis, or peripheral neuropathy.

Gait analysis identifies the underlying cause of these patterns, whether it's muscle weakness, joint instability, or a neurological impairment, and helps guide treatment decisions such as strengthening programs, orthotic prescription, or surgical planning.

Gait retraining interventions

Gait retraining uses feedback and specific movement cues to help patients adopt improved walking patterns:

• Biofeedback provides real-time information about the patient's gait using visual displays (mirrors, video, or augmented reality), auditory cues (metronomes, verbal prompts), or tactile feedback (vibration). The patient uses this information to consciously modify their movement toward a target pattern.
• Gait modification strategies use specific verbal or visual cues to alter a particular aspect of gait. Examples include increasing step length, widening the base of support, or reducing knee adduction moment to decrease medial compartment loading in knee osteoarthritis.

These interventions have demonstrated effectiveness in conditions like knee osteoarthritis, patellofemoral pain syndrome, and running-related injuries (e.g., iliotibial band syndrome, tibial stress fractures). Success depends on patient adherence, the specificity of the feedback provided, and whether the modified pattern transfers into habitual daily walking.