Isometric Trunk Strength Assessment of Athletes: Effects of Sex, Sport, and Low Back Pain History.

CONTEXT: The importance of isometric trunk strength (ITS) among sport science professionals is higher than its actual reported effect size on either performance or low back pain (LBP) occurrence.

OBJECTIVE

To provide normative values of ITS and strength ratios, and to evaluate the effect of sex, sports discipline, and LBP status.

DESIGN

Crossover study.

SETTING

University research laboratory.

PARTICIPANTS

Five hundred and sixty-seven elite athletes (186 females) with and without a history of LBP from different sports.

MAIN OUTCOME MEASURE: Participants underwent ITS testing for trunk flexors, extensors, and lateral flexors. Normalized maximal strength (in newton meter per kilogram) and strength ratios were calculated. Differences between sex, LBP, and sport disciplines were assessed with 3-way analysis of variance (sex × LBP status × 7 sport categories) and partial eta-squared (ηp2) effect size. The predictive validity of ITS for LBP was checked with receiver operating characteristics (area under the curve).

RESULTS

The authors found significant differences in extensor and flexor ITS in favor of male athletes (medium ηp2, P < .05), while sex differences in lateral flexion ITS had a low size effect (P < .05). A low size effect was also observed for the differences in strength ratios extensors/flexors (mean 1.47; 95% confidence interval, 1.45-1.50) and left flexors/right flexors (mean 0.99; 95% confidence interval, 0.98-1.01) among sexes. The sport discipline-related differences generally had a low size effect. No significant differences in ITS were found between LBP and LBP-free athletes. Only 50% to 58% of athletes (area under the curve, 0.501-0.582) were correctly classified as LBP or LBP-free using different ITS and strength ratio variables.

CONCLUSIONS

ITS and strength ratios have low predictive validity for LBP history but may discriminate between sex and sport disciplines. Our data are a useful reference point for meaningful individual results interpretation when athletes are evaluated during training or rehabilitation.