Monitoring Locomotor Load in Soccer: Is Metabolic Power, Powerful?

The aim of the present study was to examine the validity and reliability of metabolic power (P) estimated from locomotor demands during soccer-specific drills. 14 highly-trained soccer players performed a soccer-specific circuit with the ball (3 x 1-min bouts, interspersed with 30-s passive recovery) on 2 different occasions. Locomotor activity was monitored with 4-Hz GPSs, while oxygen update (VO2) was collected with a portable gas analyzer. P was calculated using either net VO2 responses and traditional calorimetry principles (P-VO2, W.kg(-1)) or locomotor demands (P-GPS, W.kg(-1)). Distance covered into different speed, acceleration and P zones was recorded. While P-GPS was 29+/-10% lower than P-VO2 (d<-3) during the exercise bouts, it was 85+/-7% lower (d<-8) during recovery phases. The typical error between P-GPS vs. P-VO2 was moderate: 19.8%, 90% confidence limits: (18.4;21.6). The correlation between both estimates of P was small: 0.24 (0.14;0.33). Very large day-to-day variations were observed for acceleration, deceleration and > 20 W.kg(-1) distances (all CVs > 50%), while average Po-2 and P-GPS showed CVs < 10%. ICC ranged from very low-(acceleration and > 20 W.kg(-1) distances) to-very high (P-VO2). P-GPS largely underestimates the energy demands of soccer-specific drills, especially during the recovery phases. The poor reliability of P-GPS > 20 W.kg(-1) questions its value for monitoring purposes in soccer.