Normal gait

The development of bipedal gait may have allowed humans to use their upper
limbs, but it creates problems of stability. The centre of mass in the upright adult
lies in front of the S2 vertebra, and must remain over the support base if a person
is to stay upright. Normal gait is the result of a complex series of muscle actions
that produce efficient forward movement of the body without sacrificing
stability.
Gait is defined in terms of the gait cycle. A single gait cycle encompasses all the
events between the first contact with the ground of one foot (initial contact)
and the next time the same foot contacts the ground. Figure 26.1 shows the
key events and their approximate relative duration.
Certain concepts need to be understood before the events of the gait cycle can be
fully appreciated:
• Muscle contraction may be concentric, eccentric or isometric. When a
muscle contracts concentrically, the muscle–tendon unit shortens and kinetic
energy is released. When contracting eccentrically the overall unit lengthens
and energy is stored.
• The moment about a joint is a measure of the turning effect produced by a
force about the joint. The magnitude of the moment is the product of the force
and the perpendicular distance from the centre of rotation of the joint axis to
the line of action of the force.
• The ground reaction force (GRF) is defined as the reaction to the force that
the body exerts on the ground. It combines gravity’s effect on the body and the
effects of the body’s movement and acceleration (Figure 26.2a).
The position of the GRF in relation to each lower limb joint at any point in the gait cycle
determines whether the overall moment at that joint is a flexion or extension
moment, and this in turn will dictate which muscles will need to act, and in
what manner, to maintain stability. The GRF is key to determining the forces
required to make forward progress whilst at the same time maintaining
stability

With these basic concepts in mind, we can now look at the sequence of normal gait in more
detail.

The gait cycle comprises a stance phase and a swing phase. The stance phase starts on
initial contact and ends when the last part of the foot leaves the ground.

This marks the start of the swing phase, which continues until the next initial
contact. In normal walking, stance comprises 60% and swing 40% of the cycle.

From this, it can be seen that there must be periods when both feet are in contact with the
ground; these double-support periods occur either side of the swing period and each
comprise about 10% of the cycle. Stance and swing are further divided into subphases,
described below.
Stance
• Initial contact (IC): this is the point at which the leading foot strikes the
ground; in normal gait, this contact is made with the heel. Since the other foot
is still on the ground at this time, IC marks the start of the period of doublesupport.
Heel contact means that the
• GRF will be behind the ankle and knee joints, creating plantarflexion and knee
flexion moments, respectively. These are resisted by controlled eccentric
activity of tibialis anterior, toe extensors and the vasti. Immediately after
heel-strike, the dorsiflexors ‘pay out’ eccentrically to lower the rest of the foot
to the ground (Figure 26.2b, First).
• Loading response (LR): this period starts when the whole foot comes into
contact with the ground, and finishes at the end of double-support when the
contralateral foot leaves the ground. In this period there is slight flexion of the
knee, which is a weight acceptance or shock-absorption mechanism. Perry
described IC and LR as the periods of the first rocker (Figure 26.2a), when
the fulcrum and hence the GRF is at the heel; subsequent limb movement
brings the GRF progressively further forward in the foot, as described below.
• Mid-stance (MS) – second rocker: as the body moves forward over the static
foot, the lower leg or shank also moves forward so that the ankle is in passive
dorsiflexion. The amount of forward movement is restrained by eccentric
contraction of the calf complex or triceps surae, and in particular the
soleus component (Figure 26.2b, Second). In this, the second rocker, the
GRF moves forward in the foot and in front of the knee. There is thus an
overall extension moment at the knee, maintaining the joint in extension with
minimal muscle activity being required. As the body moves further forward a
stabilizing extension moment at the hip also occurs, further reducing the
amount of muscle activity required at this energy-conserving stage of gait.
• Late or terminal stance (TS) – third rocker: the heel begins to lift under the
powerful concentric contraction of the triceps surae (particularly
gastrocnemius) (Figure 26.2b, Third). This is the stage of the third rocker,
with the pivot point now moving still further forward to the metatarsal heads.
With the hip now extended, the ankle plantarflexes and the knee starts to flex
as the GRF passes behind the knee joint (Figure 26.2a).
The end of terminal stance is signalled by pre-swing or toe-off as the limb is thrust
forwards into swing phase, the calf contraction coming to an end and being replaced by a
short burst of concentric contraction of the hip flexors. This has the effect not
only of flexing the hip but also increasing knee flexion by induced
acceleration. Thus, the knee flexes rapidly as a combined result of gastrosoleus
and hip flexor activity, reaching the 60° of flexion necessary to achieve
clearance of the foot in the swing phase. In late pre-swing, the ankle
dorsiflexors are activated to prevent the foot dropping into plantarflexion in the swing
phase

Figure 26.2 a: Gait rockers and the ground reaction force; b: the three ankle

rockers.

Swing

This is conventionally divided into initial, mid-swing and terminal swing

(ISW, MSW and TSW). Throughout much of the swing phase, the limb moves as
a pendulum under its own momentum, the hip flexors and ankle dorsiflexors

having switched off by the end of initial swing. By terminal swing, the brakes

are applied to the pendulum by the isometric activity of the hamstrings, and just

prior to IC, the ankle dorsiflexors switch on again to control the foot on its

‘landing flare’.

Coronal and transverse plane movement

The above descriptions all apply to sagittal plane movement. It is also necessary

to consider how the body moves when viewed from in front or behind, and from

above. A key feature of coronal movement in normal gait is the action of the hip

abductors on the stance limb to correct the adduction caused by the pelvis falling

on the unsupported swing side. In the transverse plane, the pelvis is seen to

rotate forwards on the side of the swinging limb while the ipsilateral hip

externally rotates to maintain normal foot progression. This strategy not only

serves to lengthen the step but also limits the vertical excursion of the pelvis,

thereby reducing energy expenditure (see below).

Prerequisites of normal gait

Perry (1985) described five prerequisites of normal gait, a concept

subsequently popularized by Gage:

Stability in stance: this requires a stable foot position and good control of

body segments above the lower limbs whilst still achieving clearance and

forward propulsion.

Adequate foot clearance in swing: this requires appropriate positioning

of hip, knee and ankle in the stance limb with adequate flexion at the hip

and knee and adequate ankle dorsiflexion in the swing limb.

Adequate step length: this follows from good balance and stability on the
stance side with appropriate flexion on the swing side.

Appropriate pre-positioning of the foot in terminal swing: this follows

5.

from all the above.

Energy conservation: this is achieved by minimizing vertical excursion of

the body during gait and by ensuring that joint stability is achieved

wherever possible by the position of the GRF rather than by muscle

activity.

Abnormal gait is characterized by failure to meet one or more of these

requirements.

Gait analysis

The assessment of gait and the identification of any deviation from normal is known as gait
analysis. In its simplest form, it is a clinical or observational

analysis, in which a subject’s gait is observed in sagittal and coronal planes in a

clinic setting. Video recordings make it possible to study the gait pattern more

effectively and without tiring the subject.

A more objective method of assessing gait involves instrumented gait analysis,

which in addition to video recordings in two planes, involves the following

evaluations:

• Linear gait parameters.

• Kinematics.

• Kinetics.

• Electromyography (EMG).

• Energy expenditure.

• Pedobarography.
Linear gait parameters:

• Step length: distance between point of contact of one foot to the next point of contact of
the other foot (metres).

• Stride length: distance between point of contact of one foot and the next point of contact
of the same foot (i.e. one complete gait cycle) (metres).

• Cadence: number of steps per minute.

• Velocity: equals step length × cadence (metres/min).

• Foot progression angle: angle made between the direction of progression and

the long axis of the foot (degrees).

Kinematics: kinematics is the measurement of the movement of the body

segments involved in walking. In instrumented analysis, it is usually achieved using infra-

26.3). Kinetics: the measurement of forces, moments, energy and power associated

with body movements during gait. In order to determine these, data from a force

plate are required in addition to kinematic data.

EMG: in gait analysis, EMG is most useful in determining the timing of

individual muscle activity during the gait cycle. Either surface or fine-wire electrodes may
be used.