Power Indices Through Rotational Inertial Devices for Lower Extremity Profiling and Injury Risk Stratification in Professional Soccer Players: A Cross-Sectional Study

Abstract

Background/Objectives: Power indices may provide valuable information for performance and injury prevention in soccer players, so increasing the knowledge about them seems essential. Therefore, this study aimed to establish limb-specific normative values for flywheel-derived power indices in professional soccer players, while accounting for limb performance or ability, to explore the relationships between power indices across variables and to compare the power outcomes related to these indices between injured and non-injured players within four months post-assessment. Methods: Twenty-two male professional soccer players (age: 26.6 ± 4.6 years; competitive level: Belgian second division) were recruited from a single elite-tier club to participate in this cross-sectional diagnostic study. Participants underwent a standardized assessment protocol, executed in a rotational inertial device, comprising six unilateral exercises focused on the lower limbs: hip-dominant quadriceps (Qhip), knee-dominant quadriceps (Qknee), hip-dominant hamstrings (Hhip), knee-dominant hamstrings (Hknee), adductor (Add), and abductor (Abd). The testing session incorporated a randomized, counterbalanced design, with each exercise comprising two sets of eight maximal concentric-eccentric repetitions per limb. Leg dominance was operationally defined as the self-reported preferred limb for ball-striking tasks. Power indices were calculated from these exercises. Results: No significant differences in flywheel-derived power indices were found between limbs or between injured and non-injured players. However, significant correlations between indices were found in all power variables, with the Qhip:Qknee and Hhip:Hknee concentric ratios emerging as the most clinically actionable biomarkers for rapid screening. Conclusions: These results suggest the necessity of including more variables for injury prediction. Moreover, power indices could be considered based on the classification of limbs as "strong" or "weak".

Keywords: dominant limb; injury prevention; lower musculoskeletal strength; power indices; professional soccer players.

Figure 1

Exercises by muscle group.

Figure 2

Correlations between indices for each power variable. MP_CON = mean power concentric; MP_ECC = mean power eccentric; PP_CON = peak power concentric; PP_ECC = peak power eccentric. The colors indicate the strength of the correlation, with darker shades representing stronger relationships. Purple tones indicate positive correlations; orange tones indicate negative correlations. *: significance level at p < 0.05; **: significance level at p < 0.01; ***: significance level at p < 0.001.