Analysis of Human Movement 493forms of dance, the term “jump” is used more loosely
and includes single and double takeoffs or landings
or both.
This grand jeté movement can be divided into the
following phases: preparation, takeoff, flight, and
landing. For purposes of simplicity, this discussion
will focus on the hip, knee, and ankle. The prepara-
tion phase involves hip flexion, knee flexion, and
ankle dorsiflexion of the takeoff leg (right leg in table
8.7A). Since this movement is primarily produced by
gravity, eccentric contraction of the opposite muscles,
the hip extensors, knee extensors, and ankle plantar
flexors, is necessary to control these movements.
This movement is important to bring the center of
gravity of the body over the foot of the support leg
(Adrian and Cooper, 1989), and a moderate amount
of hip and knee flexion is necessary to allow time to
generate sufficient force (impulse) during takeoff
for an effective jump (Ryman, 1978).
During the takeoff phase, as with running, the
takeoff leg (right leg) undergoes rapid hip exten-
sion, knee extension, and ankle plantar flexion,
against gravity, via concentric contraction of the hip
extensors, knee extensors, and ankle plantar flexors,
respectively. Proper timing and adequate force gen-
eration associated with these joint movements are
essential for producing the forces that will propel
the body. As the right leg extends, the left leg swings
forward, utilizing concentric contraction of the hip
flexors to bring the thigh forward, isometric contrac-
tion of the knee extensors to maintain the knee in
extension, and concentric contraction of the ankle-
foot plantar flexors to help point the foot. The right
arm also swings forward (concentric contraction of
the shoulder flexors) as the left arm raises to the
side (concentric contraction of the shoulder abduc-
tors). These movements of the right leg (takeoff leg
via the resultant ground reaction force), combined
with the forward movements of the left leg (lead
leg) and right arm, propel the center of gravity of
the body up and forward for the flight phase. The
takeoff phase ends just before the foot of the takeoff
leg loses contact with the ground (just before B in
table 8.7, where the toes would still be in contact
with the ground).
When the takeoff leg no longer is in contact with
the ground, the flight phase (table 8.7, just prior to B
through D) begins. This takeoff leg is rapidly brought
backward and upward, ideally to the height of the
lead leg, via rapid concentric contraction of the hip
extensors. The knee of this back leg (right leg) is
maintained in extension by utilizing a contraction of
the knee extensors, while concentric contraction of
the right ankle-foot plantar flexors serves to slightly
increase the “point” of the foot. For many dancers,
the initial flight phase is also accompanied by a very
brief continuation of the forward motion of the lead
leg (left leg) produced by concentric contraction
of the hip flexors while the knee extensors contract
isometrically to maintain knee extension and the
ankle-foot plantar flexors contract isometrically to
help keep the foot pointed. Then, further forward
motion of the leg is arrested, and this hip angle is
maintained isometrically by the hip flexors while
transfer of momentum from the leg to the torso
facilitates desired forward motion of the center of
gravity of the body.
During the flight phase, the center of mass of
the body follows a parabolic path (as shown in table
8.7). The shape of this trajectory is determined by
the angle (angle of projection), speed, and height
of the body at takeoff. Hence, one of the key deter-
minants of achieving the desired height or distance
in a given jump is maximizing the velocity of the
body at takeoff.
In accordance with the laws of physics, once in the
air the center of mass of the body must follow this
given parabolic path. However, to achieve the illusion
of “suspension,” skilled dancers often manipulate the
relative position of the center of mass within the body
by lifting or lowering the limbs. For example, during
a grand jeté, a well-timed extra lift of the legs or lift
of the arms near the peak of the jump will cause the
center of mass to move up within the body and allow
a very brief moment in which the head and torso
move approximately horizontally, giving the illusion
of floating (Laws, 2002; Ryman, 1978).
During the landing phase (similar to the sup-
port phase in running), the primary function of
the muscles is to control and arrest the downward
motion of the body caused by gravity, with eccentric
contraction of the hip extensors, knee extensors,
and ankle plantar flexors to control the respective
hip flexion, knee flexion, and ankle dorsiflexion pro-
duced by gravity on the landing leg (left leg in table
8.7E). This phase is also important for appropriately
positioning the body for the next movement in the
given choreography. If the body changes direction
or stays in place, this landing phase also functions to
halt the forward motion of the body; and on landing,
the center of gravity needs to be behind the body so
that the push of the landing foot on the floor slows
the forward motion of the body. This is different
from what occurs in sprinting or dance choreography
such as repetitive forward leaps in which the goal is
to have the center of gravity almost over the foot so
that forward motion is maximized. As with sprinting,
it appears that some dancers may use the hamstrings