488 Dance Anatomy and Kinesiology
regarding the pressures on the plantar surface
of the foot. Force platforms are used to provide
information about the ground reaction forces in
vertical, mediolateral, and anteroposterior directions
(dynamography). When the foot presses against the
ground during movement, the ground applies equal
and opposite forces on the foot and body (Newton’s
third law) termed ground reaction forces. The
recordings of ground reaction forces from force
platforms can be used to determine many important
aspects of movement, such as weight placement,
pronation, the forces associated with takeoff, and
the forces associated with landing (impact). Figure
8.10 provides an example of the vertical ground reac-
tion force associated with landing from a forward
leap (grand jeté devant) by an elite ballet dancer
(Clippinger and Novak, 1981). One way to get a
more practical idea of the meaning of this informa-
tion is to compare the forces to body weight. In this
example, the dancer’s maximum vertical ground
reaction force in landing from this jeté was almost 5
times (485%) her body weight.
Although electromyography (EMG) is not a direct
measure of muscle force, some texts also consider
EMG a source of kinetic data. Electromyography
can provide information about the onset, duration,
and peak of muscle activity, as well as the relative
activity of the same muscle in different phases
of a movement or with different trials of a given
movement. Examples of EMG records when stand-
ing in first position were given in chapter 2 (Tests
and Measurements 2.1, p. 64), and examples ofEMG records when performing a second-position
plié were given in chapter 5 (figure 5.28 and 5.29,
p. 273). Because factors such as electrode placement
dramatically influence the records, these records are
often presented as a percentage of that seen with a
maximum voluntary contraction of a given muscle
so that more meaningful comparisons can be made
between muscles and between subjects. EMG data
are often used in conjunction with other data such
as joint angles and positions of key body landmarks.
In figure 8.11, a camera, mirrors, and a control frame
were used to allow a three-dimensional analysis (3-D
cinematography) of the knee with special consider-
ation to muscle activation and patellar positioning
in different angles of a plié. EMG studies can also be
used to support or refute muscle activity predicted
from qualitative movement analyses.Mechanical Analysis
While beyond the scope of this book, integration of
concepts related to the laws of motion and kinetic
data provides very valuable information that can
be utilized in biomechanical analysis of movement.
Interested readers are referred to the books by
Kreighbaum and Barthels (1996), Hall (1999), Laws
(2002), and Hamilton and Luttgens (2002). The
latter authors use a recommended approach in which
an anatomical analysis is followed by a brief mechani-
cal analysis and brief discussion of mechanical prin-
ciples as they relate to optimizing performance of
the movement being analyzed.123450Vertical force (body weight)TimeFIGURE 8.10 Example of use of recordings from a force platform to investigate forces associated with landing from a
grand jeté.