Do age and body size affect the eccentric knee flexor strength measured during the Nordic hamstring exercise in male soccer players?

Assessment of Strength and Power Capacities in Elite Male Soccer: A Systematic Review of Test Protocols Used in Practice and Research

BACKGROUND

Strength and power represent two crucial physical qualities for the attainment of a high level of performance considering the frequency and the importance of explosive actions occurring during elite soccer match-play. Evaluation of strength and power is a multifaceted concept involving a vast array of tests and outcome variables. Nevertheless, a comprehensive and systematic search of strength and power assessment procedures in elite soccer has yet to be undertaken.

OBJECTIVES

The aims of this systematic review were to: (1) identify the tests and outcome variables used to assess strength and power of elite male soccer players; (2) provide normative values for the most common tests of strength and power across different playing levels; and (3) report the reliability values of these strength and power tests.

METHODS

A systematic review of the academic databases MEDLINE, CINAHL, SPORTDiscus, Web of Science and OVID for studies published until August 2023 was conducted, following the Preferred Reporting Items of Systematic Reviews and Meta-Analyses (PRISMA) guidelines. Studies were eligible for inclusion if they: (1) were original research studies, published in a peer-reviewed journal, and written in English language; (2) had the primary aim to assess strength and/or power; (3) players were male and older than 17 years of age (i.e., mean age of the group); and (4) their playing level was defined as "professional", "international" or "elite".

RESULTS

Regarding strength testing, 115 studies and 29 different tests were identified. The three most frequent strength tests were the knee extensor isokinetic strength test (58 studies), the knee flexor isokinetic strength test (55 studies) and the Nordic hamstring strength test (13 studies). In terms of power testing, 127 studies with 31 different tests were included. The three most frequent power tests were the countermovement jump with hands fixed on hips (99 studies), the squat jump (48 studies) and the vertical jump with arm swing (29 studies).

CONCLUSIONS

The wide range of different tests and outcome variables identified in this systematic review highlights the large diversity in the employed testing procedures. The establishment of a hybrid testing approach, combining standardised and widely accepted tests for establishing normative standards and enabling comparisons across different contexts, with flexible context-specific testing batteries, has the potential to maximise the impact of testing information for practitioners. In addition, the limited reporting of reliability data across studies highlights the need for practitioners to establish their own reliability measure within their specific contexts, informing the selection of certain tests and outcome variables.

Does an Adjusted Kinematic Model Predict the Relative Eccentric Force During Nordic Curl?

This study aimed to assess the combination of video-based kinematic variables adjusted by intrinsic covariates to predict the relative eccentric force (RelF) during the Nordic curl. The participants (n = 21) performed Nordic curls (3 trials; 3-min rest) on a device measuring the eccentric force. The peaks were normalized by body weight. Kinovea software was used to track angular and linear velocity and acceleration from recorded videos. Two prediction models with multiple linear regression equations associated kinematic, anthropometric, and age variables to adjust the actual RelF. The equations obtained the predicted RelF. The actual RelF was inversely correlated with height (r = -.52), tangential (r = -.50) and centripetal accelerations (r = -.715), and angular velocity (r = -.70). The best prediction models combined angular velocity with age (F2,18 = 15.1, P = .001, r = .792, r2 = .627) and with height (F2,18 = 14.5, P = .001, r = .785, r2 = .616). No differences were observed between actual and predicted values (P = .993-.994), with good levels of agreement and consistency (intraclass correlation coefficient = .77-.78; Cronbach α = .86-.87). Bland-Altman results showed high levels of agreement and low biases. The standard error of measurement and minimal detectable change ranges were 0.46 to 0.49 N/kg and 1.28 to 1.36 N/kg, respectively. Also, the percentage of standard error of measurement was below 10% (7.92%-8.35%). The coefficient of variation analysis returned a 14.54% and 15.13% for each model, respectively. Kinematic analysis offers portability and low cost to current expensive or technical impaired dynamometry-based techniques to assess the RelF.