Estimating the energy cost of 5-m shuttle running through positive and negative work : a pilot study

Introduction The metabolic energy required to cover a shuttle distance is larger than that of linear running at constant speed [1]. However, an affordable method to estimate the energy cost of running when an athlete performs frequent Changes of Direction (CoD) is still unavailable. CoDs require a modification of the locomotor pattern to direct the momentum of straight running in a new direction and it is possible only applying an additional force to the ground. In this pilot study we assessed the accuracy of an algorithm to estimate the metabolic cost of shuttle running based on the computation of external positive and negative mechanical work. Materials and Methods The 3D position of 23 body landmarks applied on the body of four male participants (age: 26.3±4.8 y, weight: 65.5±5.7 kg, height: 173.5±6.6 cm) were recorded by means of a motion capture system (BTS Spa, Milano, Italy) while they were running over a 5-m shuttle course. After obtaining the instantaneous position of the body centre of mass [2], external mechanical positive and negative work were computed [3]. The oxygen uptake during the exercise (VO!) was measured using a K4b2 metabolimeter (Cosmed, Roma, Italy); the net metabolic cost was computed subtracting the standing metabolic rate. Two 5-min trials were arranged, with average speeds of 6.0 and 8.5 km/h. In the fifth minute, synchronized metabolic and kinematic data were recorded. Rate of perceived exertion (RPE) was assessed by Borg 6-20 scale. The proposed algorithm considers negative work only in the ‘braking phases’, i.e. when at least one foot is on the ground and the knee is flexing (flexion angular velocity < 0); in the remaining ‘propulsive phases’, only positive work was considered. Negative and positive work were supposed to have an efficiency of 6/5 and 1/4, respectively [4]. The estimated metabolic cost was compared with the classical positive-external-work-only (Wext-only) approach to mechanical work estimation. To compare results, a Wilcoxon signed rank test followed by Bonferroni-adjusted post-hoc comparisons was performed (alpha level: 0.05). Results Both the Wext-only and our approaches overestimated the normalized energy cost during shuttle running (Figure 1). However, post-hoc tests showed only a significant difference between measured and Wext-only estimation (p=0.001), while measured vs. estimated metabolic costs were not statistically different (p=0.445). Our method returned a mean error of 2.63±1.52 J/Kg∙s. Reported RPE was always between 10 and 13. Figure 1: comparison between measured (black), estimated (gray) and classically derived (white) metabolic cost during 5-m shuttle running. P1-4: participants; S (slow): 6.0 km/h; F (fast): 8.5 km/h. Discussion The better estimation we obtained considering negative work under certain conditions indicates the importance of elastic energy recovery in sharp CoD maneuvers. However, the contribution of anaerobic lactic and anaerobic alactic sources to the overall energy expenditure was not considered, even though the assumption of purely aerobic exercise couldn’t have hold for every participant. In particular, it would be interesting to consider both one exercise load (running speed) below and one over the aerobic threshold (approximately 50% and 80% of !!"#). What is more, the cutting technique may have a considerable influence on running economy, and should somehow be considered. Nevertheless, the current results are promising and encourages further developments of this approach. References 1. Hatamoto et al, Open Acces J Sport Med, 4:117-122, 2013. 2. Mapelli et al, Gait Posture, 39:460-465, 2013. 3. Willems et al, J Exp Biol, 393:379-393, 1995. 4. Hill, Proc R Soc Biol Sci, 126:136-195, 1938.