Analysis of Human Movement 491sion, knee extension, and ankle plantar flexion to
help produce the backward push of the foot against
the ground, which combined with rapid forward
swing of the opposite leg will help propel the body
upward and forward (Hay, 1993; Mann, Moran, and
Dougherty, 1986).
The next phase, the recovery phase, begins as
the foot leaves the ground (table 8.6E, right leg).
The primary function of the muscles in this phase
is to bring the leg forward in preparation for the
beginning of the next cycle in which the foot again
comes in contact with the ground. This phase can
be pictured by looking at the right leg in table 8.6, E
and F, and then realizing that it would continue to
swing forward until just before it contacts the ground,
as pictured with the left leg in table 8.6, A through E.
So, the recovery phase is accompanied by hip flexion
produced by concentric contraction of the hip flex-
ors. At the beginning of this phase, the knee flexes
(largely passively due to the transfer of inertial force)
and the ankle dorsiflexes via concentric contraction
of the ankle-foot dorsiflexors. This knee flexion
and ankle-foot dorsiflexion shorten the distance
from the hip to the toes (functionally reducing limb
length), allowing the leg to clear the ground and
swing forward more rapidly (decreased moment of
inertia). Later in the recovery phase, when the hip
has almost reached full flexion, the knee rapidly
undergoes extension (left leg—table 8.6, D to just
before foot contact in F), allowing a longer distance
to be covered (increased stride length). The initial
extension is passive (transfer of momentum) as the
thigh decelerates, followed by concentric contraction
of the knee extensors.
To understand the relationship between the right
and left legs during running, one must know that for
the sake of simplicity table 8.6 only shows one half of
a stride. So, to picture a full stride, one must realize
that following what is pictured in table 8.6F, the left
leg (shown in gray) would now go through what is
shown for the right leg (shown in white) in A through
F. Similarly, after what is pictured in table 8.6F, the
right leg would now go through what is shown for the
left leg in A through F. Some key points that clarify
the relationship between the right and left leg during
running follow. When one leg is in the early part of
the recovery phase, the body is airborne (table 8.6E
for the right leg) with both feet out of contact with
the ground (flight phase). When knee flexion brings
the recovery knee almost to the center of mass of the
body (table 8.6A for the left leg), the opposite leg
contacts the ground and enters its support phase;
and when the recovery hip reaches about maximum
hip flexion in the late recovery phase (table 8.6, just
following D for the left leg), the opposite leg enters
its recovery phase and the body is again airborne
(Slocum and James, 1968).
Although useful information can be gathered
from this simplified analysis, research has demon-
strated additional elements that complement these
basic elements. For example, at the end of the recov-
ery phase and very beginning of the support phase,
the hip actually undergoes a very brief period of
concentric hip extension and the knee a very brief
period of concentric flexion. This helps decrease
the undesired braking force that would tend to push
the body backward if the hip was still undergoing
flexion and the knee still undergoing extension as
the foot contacted the ground. At the beginning of
the support phase this concentric contraction of the
hip extensors, especially the hamstrings, then serves
to pull the body over the point of contact of the foot
with the ground; and in sprinting, the strength of
these muscles to effect this motion is considered
essential for success (Kyrolainen, Belli, and Komi,
2001; Mann et al., 1984). Brief ankle plantar flexion
controlled by eccentric contraction of the dorsiflex-
ors has also been noted just as the foot strikes the
ground to help position the foot at the same time the
ankle plantar flexors are co-contracting for stability
(Elliot and Blanskby, 1979).
Similarly, at the very beginning of the recovery
phase, concentric contraction of the hip extensors
continues briefly and the hip flexors actually work
eccentrically first, to decelerate hip extension, prior
to working concentrically to bring the leg forward.
At the ankle, the concentric contraction of the ankle
plantar flexors at the end of the support phase is fol-
lowed by a brief eccentric contraction of the ankle
dorsiflexors to decelerate this plantar flexion of the
foot prior to the previously described concentric
contraction of the dorsiflexors in the support phase
(McGinnis, 2005). And at the end of the recovery
phase the hip extensors (hamstrings) work eccentri-
cally to decelerate hip flexion and knee extension,
prior to working concentrically briefly at foot strike
as previously described. So, an eccentric contrac-
tion is commonly used to decelerate a limb, just
prior to reversal of the direction of movement. This
common use of an eccentric contraction imme-
diately preceding a concentric contraction (i.e.,
the stretch–shortening cycle discussed in chapter
2) makes running much more efficient, requiring
markedly less energy than if it were composed of only
sequential concentric muscle contractions (Biewener
and Roberts, 2000; Ito et al., 1983; Kram, 2000).
For simplicity this analysis has been limited to
sagittal plane movements at the hip, knee, and ankle.