V

Figure 6.20. The in vitro mechanical power output of skeletal muscle. Note that peak power in concentric actions does not occur at either the extremes of force or velocity.

Interdisciplinary Issue: Efficiency

One area of great potential for interdisciplinary cooperation is in determining the efficiency of movement.This efficiency of human movement is conceptually different from the classical definition of efficiency in physics. physics defines efficiency as the mechanical work output divided by the mechanical work input in a system, a calculation that helps engineers evaluate machines and engines. For endurance sports like distance running, adjusting a formula to find the ratio of mechanical energy created to metabolic cost appears to be an attractive way to study human movement (van Ingen Schenau & Cavanagh, 1999). progress in this area has been hampered by the wide variability of individual performance and confusion about the various factors that contribute to this movement efficiency (Cavanagh & Kram, 1985). Cavanagh and Kram argued that the efficiency of running, for example, could be viewed as the sum of several efficiencies (e.g., biochemical, biome-chanical, physiological, psychomotor) and other factors. Examples of the complexity of this area are the difficulty in defining baseline metabolic energy expenditure and calculating the true mechanical work because more work is done than is measured by ergometers. For instance, in cycle ergometry the mechanical work used to move the limbs is not measured. Biomechanists are also struggling to deal with the zero-work paradox in movements where there no net mechanical work is done, like in cyclic activities, co-contracting muscles, or forces applied to the pedal in an ineffective direction. Figure 6.21 illustrates the typical forces applied to a bicycle pedal at 90° (from vertical).The normal component of the pedal force does mechanical work in rotating the pedal (FN), while the other component does no work that is transferred to the bike's flywheel. Movement efficiency is an area where cooperative and interdisciplinary research may be of interest to many scientists and may be an effective tool for improving human movement.

Figure 6.21. Only some of the force applied to a bicycle pedal creates work and mechanical power. Note how the angle of the pedal illustrated means that a small component of FT actually resists the normal force (FN) creating rotation.

ties (Funato et al., 1996, 2000; Newton et al., 1996). The best conditioning for "explosive" movements may be the use of moderate resistances (just less than strength levels that are usually >70% 1RM), which are moved as quickly as possible. Oftentimes these exercises use special equipment like the Plyometric Power System, which allows for the resistance to be thrown (Wilson et al., 1993). The disadvantage of high-speed exercise is that it focuses training on the early concentric phase, leaving much of the range of motion submaximally trained. Even slow, heavy weight training exercises have large submaximal percentages (24-52%) of range of motion due to negative acceleration of the bar at the end of the concentric phase (Elliott, Wilson, & Kerr, 1989).

There are several field tests to estimate short-term explosive leg power, but the utility and accuracy of these tests are controversial. The Margaria test (Margaria, Aghemo, & Rovelli, 1966) estimates power from running up stairs, and various vertical jump equations (see Johnson & Bahamonde, 1996; Sayers, Harackiewicz, Harman, Frykman, & Rosenstein, 1999) have been proposed that are based on the original Sargent (1921) vertical jump test. Companies now sell mats that estimate the height and power of a vertical jump (from time and projectile equations). Although mechanical power output in such jumps is high, these tests and devices are limited because the resistance is limited to body mass, the many factors that affect jump height, and the assumptions used in the calculation. There has been a long history of criticism of the assumptions and logic of using vertical jump height to estimate muscular power (Adamson & Whitney, 1971; Barlow, 1971; Winter, 2005). Instantaneous measurements of power from force platforms or kinematic analysis are more accurate but are expensive and time-consuming. Future studies will help determine the role of mechanical power in various movements, how to train for these movements, and what field tests help coaches monitor athletes.

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