Author(s): REIMANN, M., ROSENTHAL, T., FERRAUTI, A., Institution: RUHR-UNIVERSITY BOCHUM, Country: GERMANY, Abstract-ID: 1270

INTRODUCTION:
Optimizing the recovery management of athletes is a crucial topic in sports because of steadily increasing training loads and competition frequency [1]. A sensitive monitoring of fatigue and recovery could reduce the risk of injury and counteract a non-functional overreaching and overtraining [2]. Therefore, the aim of this study was to validate a new field-based portable measurement based on electrostimulations of the m. quadriceps as a marker of fatigue introduced by high-intensive strength, endurance, and sprint protocols.
METHODS:
Twelve healthy sport students (age 22.5 ± 2.5 yrs., n=6f, 6m) participated in this cross-over study. Subjects completed three different training interventions followed by 1 week recovery. The training protocols consisted of either strength training (ST: eccentric overload squat 4x6 reps), endurance training (EN: 4x10 min running at 4mmol lactate threshold) or sprint training (SP: 4x6x5 sec all-out sprints) and were applied in a randomized counter balanced order. Low frequency fatigue (LFF) was assessed at baseline, pre- and post-exercise and after 24h and 48h using a device that measures evoked force of low- and high-frequency stimulation at the m. quadriceps (Myocene, Liège, Belgium). Changes of the low- and high-frequency force ratio (Powerdex (PD)) were compared between the different trainings and related to changes in countermovement jump height (CMJ), serum creatine kinase (CK) and delayed-onset muscle soreness (DOMS). Repeated measures ANOVA was used to detect changes of PD, CMJ and CK between interventions, measurements and gender. Reliability of PD was calculated by ICC between baseline measures. Significance level was set at p<0.05.
RESULTS:
PD baseline reliability was high [r=0.854]. PD changed significantly between measurements (p<0.001), while no differences between interventions (p=0.217) and gender (p=0.653) was found. PD decreased from pre to post training and ST had the strongest effect (ST: -20.2, p<0.001; SP: -10.6, p<0.001; EN: -8.0; p=0.1). Similar changes were found for CMJ (ST: -5.4 cm, p=0.029; SP: -2.7cm; EN: -0.2cm) while for CK the highest increase was found in SP (SP: +305 U/L, p=0.098; EN: +150 U/L, p=0.037; ST: 103 U/L, p=0.931).
CONCLUSION:
PD seems to be a reliable tool to measure the acute muscle fatigue induced by intensive training as well as the follow-up recovery, both under practical field conditions. It has to be considered that strength, endurance and repeated sprint training lead to different effects on PD which can be attributed to different mechanisms of fatigue. The highest sensitivity was shown for neuromuscular fatigue (ST) while sensitivity for muscle damage and CK activity (SP) was lower. Metabolic and cardiorespiratory fatigue (EN) seems to be less relevant for PD changes. In conclusion, PD seems to be most valid for strength and sprint training including intensive quadriceps work.

1. Hecksteden et al. (2016) 2. Thorpe et al. (2017)