Maximal Strength, Eccentric Knee-Flexor Force, Dynamic Balance and Movement Quality in Professional Versus Amateur Soccer Players: A Cross-Sectional Comparison of Physical Profiles and Their Internal Structure

Background/Objectives: Maximal strength, eccentric knee-flexor force, dynamic balance and movement quality are routinely profiled in soccer, yet which of them discriminate competitive level under multiplicity control, and whether they are similarly organized across levels, is unclear. We compared these attributes in professional versus amateur male soccer players and exploratorily examined level-related differences in correlation structure. Methods: Male outfield players (n = 144; 72 per group) were tested in-season, 48 h post-match: 1RM back squat and deadlift, eccentric knee-flexor force (NordBord), squat- and countermovement-jump (SJ, CMJ) height, Y-Balance Test (YBT) composites, and the seven-item Functional Movement Screen (FMS). Sixteen primary comparisons were false-discovery-rate (FDR)-controlled (Benjamini–Hochberg) with effect sizes and 95% CIs; a pre-specified secondary family covered body-mass-normalized strength and inter-limb asymmetry, and within-group correlations were compared by Fisher’s r-to-z. Results: Professionals were superior for 1RM back squat (d = 0.73), 1RM deadlift (r = 0.41), eccentric knee-flexor force (r = 0.34/0.26) and FMS composite, in-line lunge and deep squat (r = 0.20–0.25; q < 0.05); jump height did not differ, and shoulder mobility, rotary stability and left-limb YBT composite did not survive FDR control. Differences persisted, with similar magnitude, after normalization to body mass (relative deadlift 1.84 vs. 1.66 kg·kg−1, d = 0.90, r-equivalent = 0.41), and on a common r scale the largest effect was eccentric inter-limb asymmetry, threefold greater in amateurs (12.5% vs. 3.8%, r = 0.55). Correlation structure also differed (all p < 0.01; FDR q < 0.01): squat–deadlift near-zero in professionals but moderate in amateurs, eccentric symmetry higher in professionals, SJ–CMJ coupling stronger in amateurs (z = −3.04, 6.84, −4.64). Conclusions: Competitive level was robustly associated with maximal and eccentric strength, both absolute and relative to body mass, and most strongly with inter-limb symmetry, but not with jump height or dynamic balance. These observational findings are hypothesis-generating.