Neuromuscular asymmetries have been previously associated with reduced performance. Similarly, maturation has shown that youth athletes may experience a loss of motor control, which could also lead to compromised physical performance. The present study aimed to evaluate and quantify the level of asymmetry among chronological and maturational groups. Forty-one youth tennis players performed the single leg countermovement jump (SLCMJ), star excursion balance test (SEBT) and a change of direction speed (CODS) test. Differences were found between the strongest and weakest limbs across all tests (p < 0.001), and also for SEBT in the posteromedial direction (p = 0.02), SEBT composite score (p < 0.01) in maturation groups, and for SEBT posterolateral direction (p = 0.03) and SEBT composite score (p = 0.01) in chronological groups. The SLCMJ showed the largest inter-limb asymmetries for the circa peak height velocity (PHV) group (19.31 ± 12.19%) and under-14 (U14) group (17.55 ± 9.90%). Chronological and maturation groups followed similar trends for inter-limb asymmetries, but the biological index showed larger asymmetry scores in the jumping test at PHV compared to that found in the chronological group (U14). These results show that inter-limb differences may be heightened during PHV. Practitioners can use this information to inform the decision-making process when prescribing training interventions in youth tennis players.

Deficits in hamstring muscle strength and in hip range of motion (ROM) have been considered risk factors for hamstring muscle injuries. However, there is a lack of information on how chronic exposure to regular football training affects hamstring muscle strength and hip ROM. The aim of this study was to examine the longitudinal effect of football training and competition during a complete season on hamstring muscle strength and hip ROM in football players. A total of 26 semi-professional football players underwent measurements of isometric hamstring muscle strength and passive hip flexion/extension, and internal/external hip rotation (IR/ER) ROM during the football season (pre-season, mid-season, end-season). Compared to pre-season, hamstring muscle strength increased in the dominant (+11.1%, p = 0.002) and non-dominant (+10.5%, p = 0.014) limbs in the mid-season. Compared to mid-season, hamstring strength decreased in the dominant (−9.3%, p = 0.034) limb at end-season. Compared to the pre-season, hip extension ROM decreased in mid-season in the dominant (−31.7%, p = 0.007) and non-dominant (−44.1%, p = 0.004) limbs, and further decreased at end-season (−49.0%, p = 0.006 and −68.0%, p < 0.001) for the dominant and non-dominant limbs. Interlimb asymmetry for hip IR ROM increased by 57.8% (p < 0.002) from pre-season to mid-season. In summary, while hamstring muscle strength increased during the first half of the football season in football players, a progressive reduction in hip extension ROM was observed throughout the season. The reduced hip extension ROM suggests a reduced mobility of the hip flexors, e.g., iliopsoas, produced by the continuous practice of football. Consequently, hip-specific stretching and conditioning exercises programs should be implemented during the football season.

The COVID-19 pandemic has produced a major disruption for professional football leagues that has affected the physical preparation of both football players and referees. In Spain, health authorities decreed home confinement for eight weeks, supressing the normal training routines of professional referees. After home confinement, referees had four weeks to retrain as the national football league was set to resume matches to complete the 11 games remaining. The aim of the present investigation was to assess changes in eccentric hamstring muscle strength during football competition suspension/resumption due to the COVID-19 pandemic in 21 professional football referees (mean ± SD, age: 33.4 ± 5.1 years; height: 182.4 ± 5.0 cm; body mass: 75.1 ± 4.4 kg). Eccentric hamstring muscle strength was measured with the Nordic hamstring exercise at four time points. During home confinement, referees presented the lowest value of bilateral eccentric muscle strength (300 ± 14 N). Eccentric muscle strength increased by 13.2 ± 3.7% one week after the end of home confinement (339 ± 16 N; p = 0.001, effect size (ES) = 2.8) and remained stable before the first match (343 ± 17 N; p = 0.001, ES = 3.1) and after the end of the national league (328 ± 13 N; p = 0.001, ES = 2.0). In summary, home confinement produced detraining effects in professional football referees associated with hamstring muscle weakness. In this regard, strength-based activities with body loads may be insufficient to avoid muscle weakness and other means (e.g., weights) may be necessary to maintain muscle strength. However, the 4-weeks retraining period was sufficient to resolve hamstring muscle weakness induced by the restrictions of home confinement. This information may be helpful in the case of future sport competition suspension or home quarantine due to new waves of COVID-19 pandemic.