Walking Gait Analysis | mysimulator.uk

The gait cycle: one stride, two phases

Clinical gait analysis breaks one full stride of a single leg — heel strike to the next heel strike of the same foot — into a repeating cycle split into stance (the foot is on the ground, roughly 60 percent of the cycle) and swing (the foot is in the air, roughly 40 percent). Stance itself subdivides further: initial contact, loading response, mid-stance, terminal stance and pre-swing, each with a characteristic joint angle pattern at the hip, knee and ankle that a healthy walker repeats almost identically stride after stride.

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The inverted-pendulum model: why walking is cheap

During single-leg stance, the simplest useful model of the body is an inverted pendulum: a point mass (the centre of mass, roughly at the hips) balanced on top of a rigid strut (the stance leg) that pivots at the ankle. As the body vaults forward over the planted foot, the centre of mass rises and slows near mid-stance, converting kinetic energy into gravitational potential energy, then falls and speeds up again toward the far side, converting it back. This exchange is a nearly free ride — measurements put the recovered energy at up to about 65 percent of what would otherwise require active muscle work, which is why walking at a natural, moderate pace is metabolically so much cheaper than a faster or slower gait that departs from this pendulum-like rhythm.

Energy recovery ≈ 65% at a comfortable walking speed
KE_max (mid-stance)  ↔  PE_max (near heel-strike / toe-off)   — pendulum exchange
Running abandons this: both KE and PE rise and fall together — a spring-mass model, not a pendulum

Ground reaction forces: reading the M-shaped curve

A force plate embedded in a clinical walkway measures the ground reaction force (GRF) — Newton's third law made visible: the ground pushes back on the foot exactly as hard as the foot pushes down. The vertical component over one stance phase traces a characteristic M shape: a first peak around 110–120% of body weight shortly after heel strike as the leg absorbs the impact of loading, a dip near or slightly below body weight at mid-stance as the centre of mass passes directly overhead, and a second peak, again around 110–120%, as the calf muscles push off before toe-off.

Vertical GRF (as % of body weight) over one stance phase:

120% |      *                          *
     |    *   *                      *   *
100% |  *       * _______mid-stance_*       *
     | *                                      *
  0% |________________________________________________
     0%   heel strike        mid-stance        toe-off  100%
                       time through stance

A flattened, single-humped or asymmetric curve compared with the opposite leg is one of the simplest, most widely used objective signs of an abnormal gait — it can indicate an antalgic pattern where a patient unconsciously shortens the painful leg's stance phase to minimise load, weakness in the push-off muscles, or a joint that cannot tolerate the normal loading peaks.

Why clinics measure this instead of just watching someone walk

The human eye is good at spotting that someone limps but bad at quantifying exactly where in the gait cycle, by how much, and whether it is improving. Instrumented gait labs combine force plates, motion-capture markers on anatomical landmarks and electromyography of muscle activity to produce objective, repeatable numbers — joint angle trajectories, step timing, force symmetry between limbs — that can be compared against age-matched normative data and tracked visit to visit. That turns rehabilitation from "does this look better" into "the knee flexion deficit at mid-swing dropped from 15 degrees to 6 degrees," which is exactly the kind of measurable target physiotherapy programmes are built around.

Frequently asked questions

What are the two double-bump peaks in a ground reaction force curve?

The vertical ground reaction force during one stance phase typically shows an M shape: a first peak around 110 to 120 percent of body weight as the leg absorbs impact just after heel strike, a mid-stance dip closer to body weight as the centre of mass passes over the supporting foot, and a second peak, also around 110 to 120 percent, as the calf muscles push off before toe-off. A flattened or asymmetric M shape is one of the simplest clinical signs used to spot an abnormal or antalgic (pain-avoiding) gait.

Why is walking modelled as an inverted pendulum instead of just legs moving?

Because the inverted-pendulum model captures the dominant energy-saving mechanism of normal walking: during single-leg stance, the body's centre of mass vaults over a nearly straight, rigid leg like a pendulum swinging over its pivot, continuously trading kinetic energy for gravitational potential energy and back. This exchange recovers up to about 65 percent of the mechanical energy that would otherwise have to come from muscle work, which is the main reason walking is so much cheaper metabolically than an equivalent distance of, say, deep-knee shuffling.

How does gait analysis actually help in rehabilitation?

It turns a subjective impression ('they are limping') into objective, repeatable numbers — joint angles through the gait cycle, force plate data, timing of each phase, symmetry between left and right — that can be compared against normative data and tracked over successive visits. That lets a clinician quantify how much a knee replacement, an ACL repair or a stroke recovery has actually changed someone's walking pattern, and target therapy at the specific phase of the gait cycle where the deviation is largest rather than treating the whole gait generically.

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Walking Gait Analysis: The Inverted Pendulum and Ground Reaction Forces