Bilateral vs. unilateral countermovement jumps: comparing the magnitude and direction of asymmetry in elite academy soccer players

The aims of the present study were to compare the magnitude and direction of asymmetry in comparable bilateral and unilateral countermovement jumps (CMJ). Forty-five elite academy soccer players from under-23 (n = 15), under-18 (n = 16) and under-16 (n = 14) age groups performed bilateral and unilateral CMJ as part of their routine pre-season fitness testing. For the magnitude of asymmetry, no significant differences were evident for any metric between tests. However, eccentric impulse asymmetry was significantly greater than mean force and concentric impulse in both bilateral and unilateral tests (p < 0.01). For the direction of asymmetry, Kappa coefficients showed poor levels of agreement between test measures for all metrics (mean force = -0.15; concentric impulse = -0.07; eccentric impulse = -0.13). Mean jump data was also presented relative to body mass for each group. For the bilateral CMJ, significant differences were evident between groups, but showed little consistency in the same group performing better or worse across metrics. For the unilateral CMJ, eccentric impulse was the only metric to show meaningful differences between groups, with the under-18 group performing significantly worse than under-23 and under-16 players. This study highlights that despite the magnitude of asymmetry being similar for each metric between comparable bilateral and unilateral CMJ, consistency in the direction of asymmetry was poor. In essence, if the right limb produced the larger force or impulse during a bilateral CMJ, it was rare for the same limb to perform superior during the unilateral task. Thus, practitioners should be aware that bilateral and unilateral CMJ present different limb dominance characteristics and should not use one test to represent the other when measuring between-limb asymmetries

Seasonal variation of physical performance, bilateral deficit and inter-limb asymmetry in elite academy soccer players: which metrics are sensitive to change?

This study aimed to report seasonal variations for: 1) physical performance, 2) inter-limb asymmetry and, 3) BLD data over the course of a competitive soccer season, and determine which metrics are sensitive to change. This study used a repeated measures observational design for 19 under-20 elite academy soccer players (age: 17.58 ± 0.61 years; height: 1.81 ± 0.09 m; body mass: 74.36 ± 7.58 kg), who conducted bilateral and single leg countermovement jumps (CMJ and SLCMJ), linear speed (5, 10, 20 and 30-m) and 505 change of direction (COD) speed tests, at pre, mid and end of season time points. For jump tests, jump height, reactive strength index modified (RSI-Mod), time to take-off and countermovement depth metrics were monitored, with inter-limb asymmetry and the bilateral deficit (BLD) also calculated for each. Significant improvements (p < 0.05) in performance were evident in all fitness tests: CMJ (ES: 0.61 to 1.03), SLCMJ (ES: 0.60 to 2.25), linear speed (ES: -0.54 to -1.96) and COD speed (ES: -0.68 to -1.14). Significant reductions in asymmetry (ES: -0.68 to -1.07) and significant increases in the BLD (ES: 1.15 to 1.57) were also evident throughout the season. Additionally, Kappa coefficients were used to determine consistency in limb dominance throughout the season, but only poor to fair levels of agreement (K: -0.17 to 0.37) were evident, highlighting the fluctuating nature of limb dominance throughout the season. Despite all tests exhibiting meaningful change, the SLCMJ and linear speed showed the most frequent and largest differences in performance, highlighting their usefulness in the ongoing monitoring process of physical capacities in elite male academy players across a competitive soccer season

Effects of pre-season strength training on bilateral and unilateral jump performance, and the bilateral deficit in premier league academy soccer players

The aim of this study was to determine the effects of pre-season strength training on bilateral and unilateral jump performance and the bilateral deficit (BLD) in Premier League academy soccer players. Fourteen male academy players (age: 16.36 ± 0.50 years; body mass: 73.45 ± 8.43 kg; height: 1.80 ± 0.09 m) performed one upper body and one lower body strength session per week, for eight weeks. Bilateral and unilateral countermovement jumps (CMJ) were assessed pre and post-intervention with jump height, mean force, reactive strength index modified (RSI-Mod) and time to take-off reported for each test. In addition, the BLD was also computed for each metric. Results showed meaningful and significant increases in unilateral jump height on the left (g = 1.00; p = 0.02) and right (g = 0.77; p = 0.039) legs, and for the BLD for jump height (g = 0.67; p = 0.046). No other significant changes in jump performance were evident. Despite numerous non-significant changes in jump performance at the group level, individual analysis showed numerous players exhibited percentage improvements greater than the variance in the test (as depicted by the coefficient of variation), across all metrics. Thus, the results illustrate that a single lower body strength session per week is enough to elicit positive improvements in jump performance for all metrics when assessed on an individual basis, but only for unilateral jump height, when assessed at the group level. Significant increases were also evident for jump height BLD. Despite the majority of players exhibiting improvements in jump performance, some small reductions in performance were also evident. Given the requirement for soccer players to be competent during unilateral movement patterns, an increase in the BLD may be favourable, as it shows greater improvements in unilateral test scores compared to bilateral

Jumping Asymmetries are Associated with Speed, Change of Direction Speed, and Jump Performance in Elite Academy Soccer Players

The aim of the present study was to establish inter-limb asymmetries across different age groups in elite academy male soccer players and to examine any relationships between asymmetry and measures of physical performance. Fifty-one players from an English Premier League soccer academy were split into under-23 (n = 21), under-18 (n = 14) and under-16 (n = 16) groups and performed bilateral and unilateral countermovement jumps, 5, 10 and 20 m sprints and a 505 change of direction speed tests. All tests showed low variability (CV ≤ 2.5%) and good to excellent reliability (ICC = 0.80-0.99). A one-way ANOVA showed that the under-23 group were significantly faster than the under-16 group during the 20 m sprint (2.90 vs 2.98s; p = 0.02; ES = 0.94). No other significant differences were present between groups. Inter-limb asymmetry was quantified from the single leg countermovement jump and no significant differences in the magnitude of asymmetry were present between groups. However, multiple significant correlations were present in each age group between asymmetry and physical performance tests, all of which were indicative of reduced athletic performance. Results from this study show that although inter-limb asymmetry scores are comparable across age groups in elite academy soccer players, differences as low as 5% are associated with reduced physical performance during jumping, sprinting, and change of direction speed tasks. This study suggests the importance of monitoring jump height asymmetries in elite academy soccer players