Direct measurement of power during one single sprint on treadmill
Introduction
Maximal power output has been explored by means of ergometers, typically with stationary bicycle on which subjects produce mechanical power against friction loads. Along with maximal power, the muscular function has been studied through force–velocity (F–V) relationships, that were first determined by plotting the maximal velocity reached against various resistive loads during multiple sprints (5–8) (multiple sprint methods, Vandewalle et al., 1987a, Vandewalle et al., 1987b). It has later been shown that F–V relationships could be determined by plotting the velocity and the force produced not over multiple sprints, but over the multiple pedal downstrokes of one single sprint (single sprint method, Martin et al., 1997, Seck et al., 1995). In both cases, the decrease in force with increasing velocity was described by linear relationships.
Another way to measure maximal power output requires subjects to accelerate a treadmill belt, while their waist is tethered backwards to a fixed point. Power output is then measured as the product of the treadmill belt velocity and the horizontal component of the force exerted on the tether, measured by force transducers and goniometers. These treadmills are either non-motorized (Belli and Lacour, 1989, Cheetham et al., 1985, Funato et al., 2001, Lakomy, 1987, Nevill et al., 1989) or motorized (Chelly and Denis, 2001; Falk et al., 1996; Jaskolska et al., 1999b; Jaskolski et al., 1996) and in this case, the default motor torque is set to compensate for the friction of the treadmill belt-bed due to subjects’ body weight.
The main issues put forward concerning sprint treadmills are the following, most of them being discussed in the basic paper of Lakomy (1987). First, the “tethered” method implies approximations, in that the tethering device is not warranted a horizontal orientation. As a consequence, when the tether is not close to the horizontal (during the up and down motion of the subject) and short in length, the vertical ground reaction force affects the tether horizontal force. Propulsive power is thus the product of the horizontal component of force and velocity, though these two entities are not applied/measured at the same location (the end of the tether and the ground, respectively). Further, an additional torque is needed to compensate the friction due to subjects’ weight on the belt and allow them to reach maximal running velocities close to free running ones (Chelly and Denis, 2001; Falk et al., 1996; Jaskolska et al., 1999b; Jaskolski et al., 1996; McKenna and Riches, 2007).
Second, existing methods propose averaging of instantaneous values of force, velocity and power over arbitrary time windows (0.25, 1 s or longer). This is a drawback influencing power measurements (Lakomy, 1986), which could be avoided thanks to high sampling frequencies allowing averaging over appropriate and variable time windows.
Beyond these limitations, sprint treadmills have been shown accurate and reliable for measuring maximal power output in humans (McKenna and Riches, 2007; Sirotic and Coutts, 2008), and to calculate linear F–V relationships according to the multiple sprint method (Jaskolska et al., 1999a). In parallel, instrumented treadmill dynamometers have been developed, validated and used to measure three-dimensional ground reaction forces during walking and running (e.g. Belli et al., 2001; Kram et al., 1998), forces being in these cases measured at the same location than velocity: the contact between the foot and the belt.
Our aims were to: (1) test the validity of an instrumented treadmill modified for sprint use to measure maximal propulsive power output avoiding some of the drawbacks of the existing methods, and (2) determine if maximal propulsive power output and linear F–V relationships could be calculated using only one single sprint, as it is the case during cycle ergometer sprint. We therefore tested whether the default load described earlier could be considered an optimal load.
Section snippets
Sprint treadmill dynamometer and load setting
The device presented is not a novel device; it is a three-dimension treadmill ergometer (ADAL3D-WR, Medical Developpement – HEF Tecmachine, Andrézieux-Bouthéon, France) that had already been used in our research group but it was modified for sprint use. It is similar to that validated for walking by Belli et al. (2001), with only one large belt. This device (Fig. 1) has been used in studies about running mechanics (Divert et al., 2005; Morin et al., 2005, Morin et al., 2007, Morin et al., 2009
Results
Typical traces of instantaneous (Fig. 2) and averaged (Fig. 3) velocity, horizontal and vertical forces are shown along with the corresponding typical linear force–velocity relationship obtained from the data of one single sprint (Fig. 3). For all subjects and all loading conditions, linear F–V relationships were observed, and Fmax, Pmax and Vmax values occurred systematically in this order.
No statistical difference was found between the three load conditions for Pmax (P=0.695), whereas Fmax
Discussion
The present method brings novelty and advantages compared to existing methodologies reported in introduction: force and velocity are measured at the same location, and therefore no assumption or computations of force components are made, and no vertical component of subjects’ horizontal pulling force on the 2 m rope interfered with their effort. Further, contrary to what could be observed with force measured along the tethering system (as discussed by Lakomy (1987), no ground reaction force was
Conflict of interest statement
The authors do not have any conflict of interests or personal relationship with other people or organisations that could inappropriately influence this work.
Acknowledgement
We are grateful to Dr Nicolas Peyrot from the Laboratory of Exercise Physiology, University of Saint-Etienne, for his stimulating and helpful comments on this work.
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