DEVELOPM ENT
1.1 INTRODUCTION AND RATIONALE
Over recent decades, isotonic, isokinetic, hydrodynamic and other systems have
been designed and used to measure the dynamic contractile properties of human
muscle. However, it is the acceleration of inertial loads, which most closely
resemble the demands placed upon skeletal muscles in everyday exertions. A.V.
Hill was one of the first to realise the potential use of inertia in the form of a
flywheel to study muscle in the 1920s (Hill 1920). More recently others (Aagaard
et al. 1994, Berg and Tesch 1994) have also described systems for studying
human muscular exertions against inertial loads, which involve the use of strain
gauges to directly measure applied torque and goniometers to measure joint
rotation.
In a purely rotational inertial system, such as a flywheel, the angular acceleration
of the wheel is directly proportional to the applied torque. From determination of
the inertia and rotational movement of the flywheel alone it should therefore be
possible to infer the mechanical properties of the muscular exertions accelerating
the flywheel, assuming that the body and the machine are tightly coupled.
Recently Bassey and Short (1990) developed a tool for measuring the average
muscle power developed during a lower limb extensor thrust, this is known as the
Nottingham Power Rig (NPR). Here a heavy flywheel o f fixed inertia represented
the load. The inferred value of average power applied was determined by
calculating the fixed value of the flywheel inertia and by determining the motion
second. The method employed involved the calculation of the terminal velocity of
the flywheel (thereby obtaining its kinetic energy). Average power output was
estimated on the assumption that the wheel’s acceleration (and therefore the
applied torque) was constant throughout the exertion. The calculation of average
power is simply then the kinetic energy divided by the exertion time. The kinetic
energy being derived from the equation ÆE = , where I is the flywheel
inertia and (O is the final flywheel angular velocity. The exertion time being
estimated from the time required for the flywheel to rotate one full turn after the
exertion phase (see Appendix A).
As the assumptions of constant acceleration and hence linear velocity increase
during the exertion phase were of fundamental importance to the calculation of
the exertion time, these factors were examined as part of this study (see Appendix
A). Furthermore, because the NPR only allowed a figure of average power to be
determined, an experimental analysis of the velocity and acceleration during the
exertion phase was undertaken. This experiment was undertaken using a CODA
motion analysis system. The results showed that the underlying assumptions
regarding the constant acceleration and linear velocity increase during the exertion
phase were not true therefore a suitable method of monitoring the exertion phase
was required (see section 2 - system description).
When measuring either peak or average power during an exertion, the values
obtained will depend upon the test protocol used. However in a test where the
inertial load as in the NPR, may not be optimal for all individuals in terms of
maximising the power output; this can be illustrated best by examination of the
hypothetical force-velocity characteristics of two skeletal muscles which differ in
their force producing capabilities (see Figure 1.3).
In Figure 1.3 a simple example is shown where it can be seen that the possible
force generated at a fixed load as represented by the arrow, allows a certain level
of velocity to be generated corresponding to the particular characteristics of each
force-velocity curve. For a weaker person, the level of force will represent a
relatively higher proportion of their maximum force generating ability. In terms of
power output this translates into disproportionately lower power for the weaker
muscle, because a slower velocity of contraction is required to generate the
required force. Therefore using this rationale it is hypothesised that a system,
which uses a fixed inertial load as a resistance, may not allow the optimal
Weak Strong
Fixed load
Lower relative power and velocity for weaker muscle
Velocity
Figure 1.3 Hypothetical force-velocity curves and power characteristics for a relatively weak muscle and a stronger muscle. The arrows indicate a slower velocity of movement required by a weaker muscle to shorten at a fixed absolute load resulting in a relatively lower power output
In light of the experimental observations of the non-linearity of velocity during
the exertion phase (see Appendix A), a suitable method of monitoring the exertion
was needed in order to obtain accurate measures of muscle output values actually
during an exertion itself. Furthermore, it is clear that an inertial system whose
inertial load could be varied in order for power output to be optimised for each
individual was also needed if an objective measure of maximal muscle power
output was to be obtained from an inertial system.
Aims
1) To develop a system which accurately monitors an exertion against an
inertial load and which also allows discrete measures of power, torque,
velocity, and acceleration to be made.
2) To design a loading system whose inertia could be varied in order to allow