The practical biomechanics of running

ROBERT S. ADELAAR,* MD

From the Division of Orthopaedic Surgery, Department of Surgery, Medical College of Virginia,
Virginia Commonwealth University, Richmond, Virginia

ABSTRACT equivalent to 60% and jogging to 70% of the walking cycle

time.’ With increased velocity (Fig. 1), the stance phase
The foot and ankle is a complex structure made of
decreases, a double unsupported phase or float phase occurs,
many small bones with capsular and ligamentous con- and the double support limb phase vanishes. As the gait
straints. The physiology, kinematics, and muscle inter-
velocity increases, the stresses on the other shortened single
action of the walking, jogging, and running cycles will limb support phase or stance increases two to three times.
be discussed and the current biomechanical literature A 68 kg human puts 35 kg/m on one foot when walking,
reviewed. To analyze the pathologic state, one must
compared to 110 tons for running.’
be aware of the normal stresses and functions of the The phases of gait in walking are the key to understanding
running cycle. This knowledge establishes a rational the running cycle. Heel strike is the first phase and is
basis for the interpretation of problems in providing
equivalent to about 15% of the stance phase. At heel strike
medical and orthotic treatment. the tibia is being internally rotated from an externally
rotated position, and the ankle is moving passively toward
plantar flexion. During this period the anterior tibialis is
To evaluate gait and clinical problems of running, an active while the rest of the musculature are inactive.
understanding of the biomechanics of the foot in both static During the second part of stance phase, relationships
and dynamic conditions is important. It is important in occur between hindfoot eversion and inversion that are
analyzing foot and ankle symptoms to evaluate the axial important in midfoot pronation, distribution of forces, and
alignment of the pelvis, lumbar spine, knee, and tibia. Var- positioning of the plantar grade forefoot surface. The walk-
iations in leg length, lumbar scoliosis, changes in normal ing cycle is a primarily passive event which occurs because
lumbar lordotic curve, knee ligament pathology, and abnor- of the constraints of the multiple bone articulations, liga-
mal alignment of the femur, tibia, and os calcis can alter the mentous and capsular restraints. In the stance phase of gait,
mechanics and distribution of forces in the foot and ankle. the center of the body is passing over the center of the axis
Problems such as varus or valgus alignment of the knee and of the ankle and hindfoot. This phase encompasses 15% to
hindfoot can frequently alter the stress distribution about 45% of the stance phase. During this phase the hindfoot will
the heel and plantar surface. start to invert. This leads to supination of the midfoot and
support of the longitudinal arch (Fig. 2); also, the plantar
GAIT CYCLE flexors and intrinsic muscles start to become active.
In the third interval of the stance phase, the arch is
Variation in the velocity of running, jogging, and walking stabilized by the hindfoot position and plantar fascia wind-
causes different gait patterns (Fig. 1). Walking has been lass mechanism (Fig. 2). The long and short flexors and
defined as moving at a speed of 5.63 km/h 6; a pace faster intrinsics are also very active in initiating foot-off. It is
than 201 m/min is considered running. The walking cycle is during this phase that the foot supinates and raises the arch
divided into stance phase (65%) and swing phase (35%). in preparation for the swing phase4(Fig. 2). In the following
The components of the walking cycle are heel strike, mid- sections, biomechanical activities of the muscle, ligaments,
stance, forefoot off, swing phase, and opposite heel strike and complex bone structure of the foot and ankle are re-
(Fig. 1). As the velocity of the gait increases, the cycle length, viewed.
with respect to time, decreases. Running is approximately
MUSCLE ACTIVITY
*
Address correspondence and repnnt requests to: Robert S. Adelaar, MD,
Division of Orthopedic Surgery, Medical College of Virginia, Virginia Common- Running speed is dependent on stride length, gait technique,
wealth University, Richmond, VA 23298 muscle capacity, body weight, and running surface. Walking
497
498

Figure 1. Comparison of the phases of the walking and running cycle. The running gait cycle is different from walking because
of the increase in the double limb unsupported time or float phase, decrease in stance phase, and increase in swing phase.

is passive activity, depending on the shift of body weight

a foot-off. In running, this muscle group is much more active,
over the foot. In running the muscles are more active and operating in 70% of the running cycle.8,9, 12 The foot flexors
must accelerate the limb against the forces of gravity and are important for acceleration. This group also assists the

the running surface.8 intrinsics of the foot which tend to be active throughout
The gluteus maximus and hamstring are active in the stance phase. Intrinsics coordinate hindfoot inversion with
walking cycle from the midstance to foot-off. In the running stabilization of the longitudinal arch and supination of the
cycle they increase their activity 30% to 50%9 to decelerate forefoot.1,4 This action is also assisted by the windlass action
the stance phase limb. The hip adductors stabilize the stance of the plantar fascia, which is really an extension of the
leg at ground contact. In the walking cycle they are active triceps surae muscle.
only from swing phase to midstance phase, but they are In the running cycle the center of gravity must pass over
active throughout the running cycle.9 the hindfoot in the stance phase, which requires a relative
The knee extensors are active in walking to control knee leg shortening. The shortening consists of knee flexion
flexion in swing phase and to initiate knee extension during against the quadricep resistance, ankle dorsiflexion, and
early stance phase. As a walker increases velocity, the quad- pronation of the foot, which allows depression of the longi-
ricep activity increases and as there is a greater arc of knee tudinal arch. During the acceleration phase, as the foot
motion needed to clear the foot from the ground. The comes off the ground, the limb must extend. Limb extension

strength and fatigue capacity of the quadricep is very im- adds to the thrust of the body as it propels itself into the
portant because of its prolonged activity in the running double unsupported limb phase of gait. The extension of the
cycle. knee in acceleration is resisted by the hamstrings. The
The foot and ankle intrinsics, plantar flexors, and pero- plantar flexors of the ankle and foot supinators are active
neals are important to stabilize the plantar surface and in this important phase of gait.4 The coordinated muscle
hindfoot during the foot-flat phase. This group assists plan- activity must interface with the complex ligament and bone
tar flexion at heel strike and resists a dorsiflexion group at architecture of the foot.
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over the hindfoot. It has been noted that with increased heel
height there is an increase in plantar flexion and a decrease
in total motion.7, 10 As the velocity of gait increases, the peak
plantar flexion is reached faster with the same dorsiflexion.
The forces transmitted to the ankle on walking have been
found to be two times body weight. With an increase in gait
velocity, there is increased activity of the gastrocnemius
muscle, which increases ankle force up to five times body
weight in the late stance phase.

Subtalar

The subtalar axis is very complex and has been described as

an oblique hinge or a screw with an angle of 42° to the floor
pointing toward the forefoot.’ A normal subtalar joint can
invert up to 10° on the average and can evert 20°.6 This joint
functions to assist the midfoot in placement of the plantar
surface on rough and smooth ground and in passive prona-
tion or supination of the midfoot. The subtalar joint also
helps in dampening the stresses which are placed upon the
heel by transmitting them to the midfoot and forefoot. The
architecture of the subtalar joint is complex, with a posterior,
middle, and anterior facet. The posterior articular facet has
a sagittal plane similar to the ankle joint. There is a con-
nection between the anterior facet and the talonavicular
joint, which may be the reason why the subtalar fusion will
result in arthritic degeneration of the talonavicular joint.

Transverse tarsal joint

The transverse tarsal joint (talonavicular, calcaneal-cuboid)

links the subtalar joint and the forefoot. Motion occurs in
all planes with the purpose of allowing the forefoot to remain
Figure 2. This chart demonstrates the changes that are in the plantigrade position independent of the ankle joint.
occurring in the subtalar joint, gastrocnemius, plantar fascia, The axis of the transverse tarsal joint is 15° lateral and 9°
and longitudinal arch during the running cycle.
on the anteroposterior view.’ When the axes of these joints

STRUCTURAL COMPONENTS are parallel, the hindfoot is everted. Pronation or depression

of the longitudinal arch is allowed. As the axes converge or
Ankle diverge, there is decreased motion with the heel usually in
the inverted position. The longitudinal arch is stabilized by
In the walking cycle, the foot and ankle ligaments and bone the bone architecture, plantar fascia, and ligament re-
complex can operate passively without significant muscle straints.
activity. This cycle is dependent on the shift of the body
weight over the hindfoot. The ankle joint is a transverse Forefoot
joint, allowing dorsiflexion and plantar flexion. The ankle
axis is slightly posterior because the fibula is posterior to The tarsometatarsal and intermetatarsal joints are relatively
the tibia. The average range of motion in the ankle is 45°, rigid because of bone architecture and firm ligamentous
with normal range of motion being 10° to 20° of dorsiflexion attachments. The second metatarsal is usually the longest,
and 25° to 35° of plantar flexion.’ A kinematic analysis of and forms the keystone of the foot. It is firmly mortised into
walking demonstrates sliding motion, except at the extremes the base of the first and third metatarsal cuneiform joint.
of motion, with compression in dorsiflexion and distraction Because of its length and rigidity, high stress can occur,
in plantar flexion. These compression and tension forces in resulting in stress fractures. In the tarsometatarsal joint, the
the extremes of ankle motion explain why the talotibial first ray and lateral four rays can move independently of
osteophyte and traction problems of the os trigonum occur each other. There is also increased mobility of the lateral
in distance runners. three metatarsals in the anteroposterior and lateral plane.
With heel strike in the walking cycle, passive ankle plan- A metatarsal break or oblique axis has been described2, 3,6
tar flexion occurs, increasing until midstance. After mid- in which the longer medial rays are elevated. The sesamoids
stance, dorsiflexion occurs as the center of gravity shifts keep the first metatarsal head elevated. The axis can be
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appreciated in the crease of a firm leather shoe. It is impor- Pronation allows the foot to distribute the energy of heel
tant in shoe fabrication to add semirigid constraints to the strike to the entire midfoot and forefoot. If pronation is
metatarsal break area for motion. limited, a great deal of the impact is transferred to the heel.
The normal force distribution for the plantar surface in Conditions which limit the mobility of the subtalar joint,
walking and jogging starts with lateral heel pressure at heel such as traumatic arthritis or peroneal spasm, also limit the
strike. The force is then transmitted up the lateral border ability of the plantar surface to adjust to uneven terrain.
of the foot to the metatarsals. The metatarsals then distrib- The subtalar joint acts to alter the plantar surface; through
ute the load, with the largest load going to the first metatar- its action the midfoot can maintain an even pressure on the
sal sesamoid complex. Disorders such as hallux valgus de- plantar surface. The person with restricted subtalar mobility
crease the amount of stress that the first metatarsal bears, has a difficult time walking on uneven surfaces.
due to the sesamoid rotation. Therefore, these stresses are In the orthotic management of foot and ankle, the impor-
transmitted to the adjacent metatarsal heads and transfer tance of the hindfoot cannot be minimized. To decrease the
stress lesions are created. There are also abnormalities in amount of pronation one tries to block the amount of
the metatarsal heads themselves, with an enlarged fibula eversion of the hindfoot with a medial based hindfoot or-
component to the condyle, which allow increased stress on thosis and longitudinal arch. The height of the heel is also
certain metatarsal heads during the walking process. important because it determines the amount of plantar
flexion the ankle goes through at heel strike.10 Increasing
Plantar fascia heel pad height decreases plantar flexion, but does not alter
the dorsiflexion.
The plantar fascia forms a passive restraint originating from It is also important to maintain a flexible gastrocsoleus
its gastrocnemius extension at the medial tubercle of the os complex.’2 Tightness of the gastrocsoleus alters the way the
calcis. It inserts around the metatarsal heads to the proximal body can move over the center of gravity of the hindfoot
phalanges. This structure helps stabilize the longitudinal during stance phase. It decreases the amount of dorsiflexion
arch and acts as a windlass. With increased tension in the which is also necessary for clearance of the foot, and thus
structure, the longitudinal arch is stabilized. Increased tight- increases the activity of the knee musculature to increase
ness causes increased flexion in the toes. With elongation flexion. Stretching and flexibility of the gastrocsoleus and
of the toes, the plantar fascia flatten onto the ground and hamstring should be an important component of rehabili-
pronation of the forefoot occurs. tation after injury. Muscle conditioning of the peroneals,
posterior tibialis, and intrinsic musculature is vital in main-
DISCUSSION taining a smooth stance phase and accelerating the limb at
foot-off. The peroneal musculature is also critical to lateral
ankle instability syndromes.
In order to analyze the muscle ligament and bone interac-
tions operating during the running cycle, one must assume
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