Introduction to Sports Biomechanics: Analysing Human Movement ...

ISOKINETIC DYNAMOMETRY
THE ANATOMY OF HUMAN MOVEMENT
The measurement of the net muscle torque at a joint using isokinetic dynamometry
is very useful in providing an insight into muscle function and in obtaining muscle
performance data for various modelling purposes (‘isokinetic’ is derived from Greek
words meaning ‘constant velocity’). Isokinetic dynamometry is used to measure the net
muscle torque (called muscle torque in the rest of this section) during isolated joint
movements, as in Figure 6.26. A variable resistive torque is applied to the limb segment
under consideration; the limb moves at constant angular velocity once the preset
velocity has been achieved, providing the person being measured is able to maintain
that velocity in the specified range of movement. This allows the measurement of
muscle torque as a function of joint angle and angular velocity, which, at certain joints,
may then be related to the length and contraction velocity of a predominant prime
mover, for example the quadriceps femoris in knee extension. By adjusting the resistive
torque, both muscle strength and endurance can be evaluated. Isokinetic dynamometers
are also used as training aids, although they do not replicate the types and speeds of
movement in sport.
Passive isokinetic dynamometers operate using either electromechanical or hydraulic
components. In these devices, resistance is developed only as a reaction to the applied
muscle torque, and they can, therefore, only be used for concentric movements.
Electromechanical dynamometers with active mechanisms allow for concentric and
eccentric movements with constant angular velocity; some systems can be used for
concentric and eccentric movements involving constant velocity, linearly changing
acceleration or deceleration, or a combination of these.
Several problems affect the accuracy and validity of measurements of muscle torque
using isokinetic dynamometers. Failure to compensate for gravitational force can
result in significant errors in the measurement of muscle torque and data derived from
those measurements. These errors can be avoided by the use of gravity compensation
methods, which are an integral part of the experimental protocol in most computerised
dynamometers. The development and maintenance of a preset angular velocity
is another potential problem. In the initial period of the movement, the dynamometer
is accelerated without resistance until the preset velocity is reached. The resistive
mechanism is then activated and slows the limb down to the preset velocity. The
duration of the acceleration period, and the magnitude of the resistive torque required
to decelerate the limb, depend on the preset angular velocity and the athlete being
evaluated. The dynamometer torque during this period is clearly not the same as the
muscle torque accelerating the system. If the muscle torque during this period is
required, it should be calculated from moment of inertia and angular acceleration
data. The latter should be obtained either from differentiation of the position–time
data or from accelerometers if these are available.
Errors can also arise in muscle torque measurements unless the axis of rotation of the
dynamometer is aligned with the axis of rotation of the joint, estimated using anatomical
landmarks (see, for example, Box 6.2). For normal individuals and small misalignments,
the error is very small and can be neglected. Periodic calibration of the
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