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 (3×1-min bouts, interspersed with 30-s passive recovery) on 2 different occasions. Locomotor activity was monitored with 4-Hz GPSs, while oxygen update (VO 2 ) was collected with a portable gas analyzer. P was calculated using either net VO 2 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 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 P o2 and P GPS showed CVs  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.