The dose–response relationship between training-load measures and aerobic fitness in elite academy soccer players

The aim of the current study is to examine the dose–response relationships between training load (TL) measures and the consequent changes in aerobic fitness. Data were collected over the 6-week pre-season period in elite youth soccer players. Participants completed a lactate threshold test to identify changes in treadmill speed at 2 mmol · l−1 (S2) and 4 mmol · l−1 (S4). Internal TL was quantified with the following training impulse (TRIMP) methods: Banister TRIMP, Edwards TRIMP, Lucia TRIMP, individual TRIMP (iTRIMP) and rate of perceived exertion was also collected. External TL measures were total distance, PlayerLoad, high speed running (14.4–19.8 km · h−1), very high-speed running (19.8–25.2 km · h−1) and maximal sprint distance (>25.2 km · h−1). Individual high-speed distance was derived from each participants treadmill speed at S4. Different Bayesian regression models were run with different likelihood functions. The best-fitting models with both the lowest out-of-sample prediction error and the highest variance explained (R 2) were used. iTRIMP had the strongest relationships with changes in S2 (r = 0.93, R2 = 0.90) and S4 (r = 0.88, R2 = 0.82). Explained variance ranged from 10%–69% and 11%–38% for all other internal TL measures and external measures, respectively. In summary, the iTRIMP method demonstrates a dose–response relationship with changes in aerobic fitness in elite youth soccer players.