Fundamentals of Biomechanics

288 FUNDAMENTALS OF BIOMECHANICS
hypertrophy: the increase in size of muscle
fibers
hysteresis: the energy loss within a deformed
material as it returns to its normal
shape
impulse: the mechanical effect of a force
acting over time (vector); J = F • t
impulse–momentum relationship: principle
which states that the change in momentum
of an object is equal to the net
impulse applied; the original language
of Newton's second law, and equivalent
to the instantaneous version: F = ma
inertia: the property of all matter to resist a
change in its state of motion
inertial force: the mass acceleration (ma)
term in Newton's Second Law (dynamics);
the effect of inertia and acceleration
on dynamic movement, but it is
important to remember that its effect is
not a real force acting on an object from
another object
inertia principle: A biomechanical application
principle which states that inertial
resistance to changes in state of motion
can be used to advantage in resisting
motion or transferring energy
information: observations or data with unknown
accuracy
in situ: Latin for “in place”, or structures
isolated by dissection
integrated EMG (IEMG): the area under
a rectified EMG signal; correctly,
the time integral reported in units of
amplitude time (mV•s); unfortunately,
some studies employ outdated
equipment and incorrect terminology,
so that reported IEMGs are not really
integrated but filtered or smoothed
EMG values (mV), which is essentially
a linear envelope detector
interdisciplinary: the simultaneous integrated
application of several disciplines
to solution of a problem
internal force: a force within an object or
between the molecules of an object
internal work: work done on body segments
by internal forces (muscles, ligaments,
bones)
inverse dynamics: biomechanics research
technique for estimating net forces and
moments in a linked-segment model
from measured kinematics and anthropometric
data
in vitro: Latin for “in glass,” or tissues removed
from the body but preserved
in vivo: Latin for “in the living,” or during
natural movement
isokinetic (“same, or constant, motion”):
the condition where activated muscles
create constant joint angular velocity
isometric (“same, or constant, length”): the
condition where activated muscles create
a torque equal to the resistance
torque, so there is no joint motion
isotonic (“same, or constant, tension”): the
condition where activated muscles
work against a constant gravitational
resistance; muscle tension is not constant
in these conditions
jerk: the third derivative of displacement
with respect to time
joint center: an approximation of the instantaneous
center of rotation of a joint
joint reaction forces: the net forces acting at
joints calculated from inverse dynamics;
these forces do not represent the actual
bone-on-bone forces acting at
joints, but a combination of bone, muscle,
and ligament forces
Joule: the unit of mechanical energy and
work