Reliability and Usefulness of the 30-15 Intermittent Fitness Test in Rugby League.

This study examined the reliability and usefulness of the 30-15 Intermittent Fitness Test (30-15(IFT)) within rugby league. Fifty-five young rugby league players participated in the study. These included representative players from Under 16s (n = 19; 15.6 ± 0.3 years; 78.1 ± 10.9 kg), Under 18s (n = 21; 17.4 ± 0.5 years; 86.9 ± 11.2 kg), and Under 20s (n = 15; 19.4 ± 0.5 years; 95.9 ± 8.7 kg) squads within a professional rugby league club. Players performed the 30-15(IFT) twice within 9 days of each other. Maximal intermittent running velocity (V(IFT)) and heart rate at exhaustion (HR(peak)) were collected for both tests. Intraclass correlation coefficients (ICCs) for the "Combined" and Under 20s were very large (r > 0.7), whereas the ICCs for Under 16s and Under 18s were almost perfect (r > 0.9). Coefficients of variation were 1.9% (95% confidence interval, 1.6-2.4) for the combined test-retest of the 30-15(IFT) and 0.6% (0.5-1.0) for HR(peak). As the typical error of measurement (TE) (0.36 km·h⁻¹) was greater than the smallest worthwhile change (SWC) (0.21 km·h⁻¹) value, the usefulness of the V(IFT) was rated as "marginal." The TE for HR(peak) was similar to the SWC, rating the usefulness of this variable as "OK." Despite the usefulness of the 30-15(IFT) being deemed Marginal, a change as small as 0.5 km·h⁻¹ (1 stage) in V(IFT) could be considered substantial or "real." As a consequence, the 30-15(IFT) presents as both a reliable and useful field test in the assessment of intermittent fitness for rugby league players.