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Muscle activity during the gait cycle

The movement pattern that we observe in the lower limbs during walking results from theinteraction between external forces (joint reaction and ground reaction) and internal forces(produced by muscles and other soft tissue). Knowledge of the ground reaction force is

especially helpful to therapists who must understand how muscle activity and timing contributesto stability and propulsion.

Muscle activity is typically studied using electromyography (EMG). EMG records differbetween individuals, and differ for a single individual according to variables such as velocity.

The following summary draws on the findings of reliable investigators.

Loading Response (0 to 12 percent ofgait cycle)

This is a period of extensive muscle activity. The ankle dorsiflexors act eccentrically toprevent slapping of the foot on the ground. The quadriceps act eccentrically to control

knee flexion. Hip flexion is controlledby isometric action ofthe hamstrings(primarily biceps femoris) and gluteus maximus (primarily its lower portion).

In the frontal plane, activity in the hip abductors, tensor fascia lata, and upperportions ofthe gluteus maximuscontrol drop of the contralateral pelvis, which is

relative hip adduction. While activity in the the anterior gluteals (gluteus medius andminimus) might appear eccentric, these muscles simultaneously move the hip joint into

internal rotation. In a closed chain, this hip rotation causes the pelvis to rotate forward on

the opposite side. Thus, gluteusm

edius activitym

ay be nearly isom

etric. Alsocontributing to both internal rotation and extension of the hip joint are the muscles of theadductor group.

The erector spinae are also active during loading response. Their activity during this

period has been characterized classically as a mechanism to stabilize the trunkduringweight transfer, and to prevent its forward flexion during the rapid slowing of forward

movement which occurs at initial contact. Recent theory (Gracovetsky 1988) attributes tothe paraspinal muscles a more active role in producing important trunk and pelvic

rotation.

Midstance (12 to

31 percentof

gait cycle)

As the body moves over the stance limb, activity in the foot's intrinsic muscles (which are

primarily subtalar supinators) activate to convert the foot into an increasingly rigidstructure. This supination force is augmented by activity in the ankle plantar flexors,

which act eccentrically to control closed chain ankle dorsiflexion in the form of tibialadvancement over the stable foot.

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The quadriceps act concentrically to initiate knee extension, and the hip abductorsare isometric as they halt contralateral pelvic drop.

Terminal Stance (31 to 50 percent ofgait cycle)

Foot intrinsics and ankle plantarflex

ors continue to function as during midstance,becoming isometric at around 35 to 40 percent of the gait cycle, when continued forward

momentum in the body's upper part causes the heel to rise from the floor.

Similarly, the hip abductors move from eccentric to isometric to concentric activity,elevating the pelvis in preparation for swing. The iliopsoas becomes active,

eccentrically controlling the rate of hip extension.

The quadriceps are inactive during this phase, as ground reaction forces, as well asactivity in the plantar flexors, maintain knee extension.

Preswing (50 to

62 percentof

gait cycle)

Along with loading response, this is a period of widespread muscle activity. The foot is inits most supinated and rigid position. Acting on this rigid base, the plantar flexors act

concentrically, producing a propulsive "pushoff." The iliopsoas also contributes topropulsion as it shifts from eccentric to concentric activity which will advance the

extremity into swing phase.

At typical to faster walking speeds, the rectus femoris also acts in a nearly isometricfashion, to limit knee flexion and augment hip flexion. Only at slower walking speeds,

when ground reaction and joint reaction forces are too small to initiate knee flexion, must

knee flexors like the short head of the biceps femoris, or the gracilis, actually work to flexthe knee directly.

The erector spinae are active on the preswing side, and produce greater EMG activitythan during their previous period of activity during loading response; a vital debate

concerns whether this assymetrical activity functions simply to control unwanted trunkmovement or if it helps initiate forward pelvic rotation, through the mechanism of

coupled motion, and thereby helps drive the extremity into swing.

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Initial Swing

During this very brief phase, the hip flexors and knee extensors (primarily rectusfemoris)concentric continue their preswing activity. The dorsiflexors act concentrically

to permit the forefoot to clear the ground. While their activity varies widely among

individuals, the hip adductors can also assist during preswing and initial swing to assist inhip flexion.

Midswing

Muscle activity virtually ceases except for the dorsiflexors as the extremity's inertia

carries it through swing like a pendulum.

Terminal Swing

The hamstrings (primarily Semi tendin/membran) act eccentrically to decelerate the

swinging extremity, while the do

rsiflex

ors is

ometrically hold the ankle in position forinitial contact. Just before the foot touches the ground, the quadriceps and the hip

abductors initiate activity, disclosing the existence of a feedforward mechanism by which

the body prepares for the large ground reaction its joints will encounter at initial contact.

Sources:

Gage, J.R. (1990). An overview of normal walking. Instructional Course Lectures, 39,

291-303.

Inman, V.T., Ralston, H.J., & Todd. F. (1981). Human Walking. Baltimore: Williams and

Wilkins.

Rodgers, M.M. (1995). Dynamic foot biomechanics. Journal of Orthopedic and SportsPhysical Therapy, 21, 306-316.

Winter, D.A. (1987). The Biomechanics and Motor Control of Human Gait. Waterloo,

Ontario: Univ. of Waterloo Press.