Is playerload related to fatigue following different types of repeated maximal sprint exercises in recreationally active young men?

Introduction: GPS units are commonly used by sports clubs to estimate the ?external load? [e.g. Catapult Playerload^TM (PL)] experienced by the athlete during exercise to predict fatigue response, despite a lack of evidence for a relationship between PL and exercise fatigue. The aim of this study was to (i) compare PL and fatigue during and after two different maximal sprint protocols and (ii) examine relationships between PL and fatigue/recovery. 
Methods: Eighteen healthy, recreationally active, males (age: 20.7 ± 3.0 yr; height: 168 ± 41 cm; body mass: 73.7 ± 11.0 kg) performed either 15 x 30m maximal sprints (SP, n=11) or 15 x (3 x 10m) maximal shuttle sprints (3T, n=7) with 90 s rest between repetitions and 180 s rest after repetition five and 10. PL, heart rate (HR) and rate of perceived exertion (RPE) were measured for the duration of the exercise. Maximum isometric voluntary contraction knee extensor/flexor torque (MVC), range of motion (ROM), quadriceps/hamstring muscle soreness (algometer and visual analogue scale, VAS), (in)voluntary rate of torque development (RTD) and vastus lateralis/biceps femoris long head EMG were measured during knee extension/flexion pre, immediately post and 48 h post exercise. Voluntary muscle activation (VA) and the torque-frequency relationship (TFR) were measured in the hamstring muscle group using electrical stimulation at the same time points. Blood samples were drawn from the antecubital vein pre, post and post 48 hr completion of the protocols
Results: PL (126.8 ± 13.8 vs. 107.6 ± 12.7 AU; P<0.05), HR (163.4 ± 5.2 vs. 149.6 ± 4.6 bpm; P=0.01)and RPE (13.0 ± 0.3 vs. 12.0 ± 0.6 AU; P=0.02) were all higher in SP vs. 3T. PL correlated positively withHR (R2=0.497) and RPE (R2=0.570) in both protocols combined. There was a main effect for time forhamstring MVC, quadriceps MVC, creatine kinase concentration, interleukin-6 concentration, lactate,range of motion, RTD and TFR (P≤0.04) but there were no group x time interactions for these variables.There was a group x time interaction for soreness, with SP exhibiting higher VAS values at post and 3post 4h hr compared to 3T (P≤0.04). Moreover, PL did not correlate with changes in MVC, RTD, ROM,TFR or soreness in either protocol or in both protocols combined (P>0.05). 
Conclusions: Our data demonstrate that two different types of repeated maximal sprinting exercises lead to a similar impairment of quadriceps and hamstring muscle function. Post exercise measures of fatigue do not differ between repeated 30 m straight line sprinting and repeated 3 x 10 m shuttle sprinting, except for subjective measures of muscle soreness, which were higher after straight line sprinting compared to shuttle sprinting. PL was positively related with physiological load during the exercise but did not correlate with changes in indices of central or peripheral fatigue. Therefore, our data support the use of GPS units to predict physiological load during maximal sprinting but not to predict rate of recovery following these types of maximal exercise.