Introduction to Sports Biomechanics: Analysing Human Movement ...

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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 273
INTRODUCTION TO SPORTS BIOMECHANICS Figure 6.26 Use of an isokinetic dynamometer for: (a) knee extension; (b) elbow flexion. 274 dynamometer system is necessary for both torque and angular position, the latter using an accurate goniometer. Torque calibration should be carried out statically, to avoid inertia effects, under gravitational loading. Accurate assessment of isokinetic muscle function requires the measurement of
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Introduction to Sports Biomechanics
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First edition published 1997 This e
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Contents List of figures x List of
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Study tasks 216 Glossary of importa
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FIGURES 1.26 Underarm throw - femal
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FIGURES 5.9 Typical path of a swimm
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Tables 2.1 Examples of slowest sati
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Preface Why have I changed the cove
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Introduction MISSING TEXT The first
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INTRODUCTION principal movements in
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INTRODUCTION TO SPORTS BIOMECHANICS
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Figure 2.2 ‘Principles’ approac
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Figure 2.13 Identifying critical fe
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Page 245 and 246: INTRODUCTION TO SPORTS BIOMECHANICS
Page 265 and 266: Figure 6.8 Main skeletal muscles: (
Page 295: INTRODUCTION TO SPORTS BIOMECHANICS
Page 305 and 306: INDEX 282 balls air flow past 173 b
Page 307 and 308: INDEX 284 energy 219 kinetic 219 mi
Page 309 and 310: INDEX 286 isokinetic contraction 24
Page 311 and 312: INDEX 288 muscle length-tension rel
Page 313 and 314: INDEX 290 three-dimensional 146-7,
Page 315: INDEX 292 validity 220 vantage poin
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