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torques from inverse dynamics in sporting movements can be larger than those seen in isokinetic testing because of antagonist activity in isokinetics testing, segment interaction in dynamic movements, the stretch-shortening cycle, and eccentric muscle actions. Most isokinetic norms are normalized to bodyweight (e.g., lb •ft/lb) and categorized by gender and age. Recall

Figure 7.3. Calculating the torque created by a person pushing on a merry-go-round involves multiplying the force times its moment arm. This torque can be converted to other units of torque with conversion factors (Appendix B).

Figure 7.4. Joint torque-angle diagrams represent the strength curves of muscle groups. The shapes of joints vary based primarily upon the combined effect of changes in muscle length properties and muscle moment arms. Reprinted by permission from Zatsiorsky (1995).

that the shape of the torque-angle graphs from isokinetic testing reflects the integration of many muscle mechanical variables. The angle of the joints affects the torque that the muscle group is capable of produc ing because of variations in moment arm, muscle angle of pull, and the force-length relationship of the muscle. There are several shapes of torque-angle diagrams, but they most often look like an inverted "U" because of the combined effect of changes in muscle moment arm and force-length relationship (Figure 7.4).

Torque is a good variable to use for expressing muscular strength because it is not dependent on the point of application of force on the limb. The torque an isokinetic machine (T) measures will be the same for either of the two resistance pad locations illustrated in Figure 7.5 if the subject's effort is the same. Sliding the pad toward the subject's knee will decrease the moment arm for the force applied by the subject, increasing the force on the leg (F2) at that point to create the same torque. Using torque instead of force created by the subject allows for easier comparison of measurements between different dynamometers.

Figure 7.5. Isokinetic dynamometers usually measure torque because torque does not vary with variation in pad placement. Positioning the pad distally decreases the force the leg applies to the pad for a given torque because the moment arm for the leg is larger.

Application: Muscle-Balance and Strength Curves

Recall that testing with an isokinetic dynamometer documents the strength curves (joint torque-angle graphs) of muscle groups. Normative torques from isokinetic testing also provide valuable information on the ratio of strength between opposing muscle groups. Many dynamometers have computerized reports that list test data normalized to bodyweight and expressed as a ratio of the peak torque of opposing muscle groups. For example, peak torques created by the hip flexors tend to be 60 to 75% of peak hip extensor torques (Perrin, 1993). Another common strength ratio of interest is the ratio of the quadriceps to the hamstrings. This ratio depends on the speed and muscle action tested, but peak concentric hamstring torque is typically between 40 and 50% of peak concentric quadriceps torque (Perrin, 1993), which is close to the physiological cross-sectional area difference between these muscle groups. Greater emphasis has more recently been placed on more functional ratios (see Aagaard, Simonsen, Magnusson, Larsson, & Dyhre-Poulsen, 1998), like hamstring eccentric to quadriceps concentric strength (Hecc:Qcon), because hamstrings are often injured ("pulled" in common parlance) when they slow the vigorous knee extension and hip flexion before foot strike in sprinting. In conditioning and rehabilitation, opposing muscle group strength ratios are often referred to as muscle balance. Isokinetic (see Perrin, 1993) and hand dynamometer (see Phillips, Lo, & Mastaglia, 2000) testing are the usual clinical measures of strength, while strength and conditioning professionals usually use one-repetition maxima (IRM) for various lifts.These forms of strength testing to evaluate muscle balance are believed to provide important sources of information on the training status, performance, and potential for injury of athletes. In rehabilitation and conditioning settings, isokinetic and other forms of strength testing are useful in monitoring progress during recovery.Athletes are cleared to return to practice when measurements return to some criterion/standard, a percentage of pre-injury levels, or a percentage of the uninvolved side of their body. It is important for kinesiology professionals to remember that the strength (torque capability) of a muscle group is strongly dependent on many factors: testing equipment, protocol, and body position, among others, affect the results of strength testing (Schlumberger et al., 2006). If standards in testing are being used to qualify people for jobs or athletic participation, there needs to be clear evidence correlating the criterion test and standard with safe job performance.

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