Background Lower limbs muscle flexibility remains a common assessment in sports therapy. Nevertheless no clear consensus exists about the methods for performing reliable measurements.

Objective The aim of the present study was to examine the reliability of muscle flexibility measures in adolescent athletes.

Design Randomised test-retest study.

Setting Sports academy physiotherapy laboratory.

Participants Height young athletes (age: 15.1±1.6, body mass: 65.4±26.2 kg, height: 171.7±8.8 cm) training in a national sports centre.

Interventions Flexibility of the main lower limb muscles was assessed, using a standardised force of stretching for each muscle and a video analysis software for measuring the angles. Each participant was tested on both sides by three operators and on two separate occasions, 4 days apart. Final measurements were reported in degrees.

Main outcome measurements Reliability of flexibility measures was assessed by 1) calculating the between-day difference expressed as Cohen's d (90% CI), no substantial difference was considered if Cohen's d<0.2 and the 90% CI included zero, and 2) the typical error of measurement, expressed as a coefficient of variation (CV, 90% CI).

Results Force of stretching (in N), mean angle (SD), mean between-day difference and CV were 78.5, 132.2 (10.1), −0.05 (−0.3;0.2) and 8.3% (7.5;9.3) for quadriceps, 68.7, 70 (11.2), −0.7 (−1.7;0.4) and 3.3% (3.0;3.7) for hamstrings, 39.2, 53.5 (9.4), 0.05 (−0.2;0.3) and 7.2% (6.5–8.0) for adductors, 147.2, 16 (6.3), 0.10 (−0.1;0.3) and 5.7% (5.1;6.3) for gastrocnemius, 147.2, 30.6 (8.9), 0.01 (−0.2;0.2) and 4.5% (4.0;5.0) for soleus, 98.1, 22.7 (5.9), 0.01 (−0.2;0.2) and 2.6% (2.3;2.9) for hip flexors, 49.1, 54.7 (14.5), −0.04 (−0.2;0.2) and 9.6% (8.6;10.8) for hip medial rotator and 49.1, 43.5 (14.9), −0.08 (−0.3;0.1) and 18.2% (16.3;20.5) for hip lateral rotator.

Conclusion The present method for assessing lower limb muscles flexibility has a moderate-to-good level reliability and may integrate prevention programs.