The aim of this study was to analyse the influence of fatigue on sprint biomechanics. Fifty-one football players performed twelve maximal 30 m sprints with 20 s recovery between each sprint. Sprint kinetics were computed from running speed data and a high-frequency camera (240 Hz) was used to study kinematic data. A cluster analysis (K-mean clustering) was conducted to classify individual kinematic adaptations. A large decrease in maximal power output and less efficiency in horizontally orienting the ground reaction force were observed in fatigued participants. In addition, individual changes in kinematic components were observed, and, according to the cluster analysis, five clusters were identified. Changes in trunk, knee, and hip angles led to an overall theoretical increase in hamstring strain for some players (Cluster 5, 20/51) but to an overall decrease for some others (Cluster 1, 11/51). This study showed that the repeated sprint ability (RSA) protocol had an impact on both kinetics and kinematics. Moreover, fatigue affected the kinematics in a different way for each player, and these individual changes were associated with either higher or lower hamstring length and thus strain.

Resisted sprint training (RST) is widely used to enhance acceleration capacity; however, evidence concerning on long-term effects to RST in professional women’s remains limited. Methods: This study examined the chronic effects of a six-week resisted sled sprint training intervention using a single-group longitudinal pilot design in professional female soccer players. Twenty-two players were assessed at baseline (T1), with fourteen completing the post-intervention assessment (T2) and seven available at the two-month follow-up (T3). Athletes completed one weekly RST session with loading progressively increasing from 20% to 80% of body mass and total sprint volume ranging from 100 to 200 m per session. Sprint performance and kinematic variables of the first three acceleration steps for both limbs were assessed before the intervention, immediately after and at a two-month follow-up. Within-group changes across time were analyzed using a one-way ANOVA with Bonferroni post hoc comparisons, with the level of significance set at p ≤ 0.05. Results: Sprint times significantly improved following the intervention (T1-T2), with a 2.61% improvement in acceleration performance over the 0-20 m phase. This improvement was accompanied by increases in center of mass projection angle and toe-off distance, resulting in a more forward-oriented sprint posture. At follow-up, sprint performance showed partial retention of these changes. These adaptations were accompanied by greater hip and knee extension of the ipsilateral limb at toe-off, without evidence of adverse sagittal-plane kinematic alterations during the early acceleration phase. Conclusions: The results indicate that once-weekly RST was associated with preliminary improvements in acceleration performance in professional female soccer players and induces technical adaptations that did not appear to negatively affect sprint mechanics during the initial acceleration phase. Given the absence of a control group and the substantial attrition at follow-up, these findings should be interpreted as exploratory.