Participants 18 developmental athletes, mean age 13.93 ± 1.37 years, (range 11.9 -16.3 years), all members of an elite athletics project. Assessment Each participant performed a 40cm and a 30cm side-hop test for each leg. Athletes were tested during a regular training session, sufficient resting time (! 4min) granted after each trial. All tests conducted within 10 days. Tests were supervised and evaluated by two coaches, one counting number of jumps during the trial with a customary hand-held

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... chanical clicker, the other one filming each attempt with a latest generation smartphone high-speed camera. Errors clickcounted on home PC with videos running at half speed and error-rate calculated. Main Outcome Measurements Comparison of total number of jumps and error-rates for both distances. Evaluation of correlations with time to/from peak height velocity (PHV), height, and chronological age. Results Lower number of jumps for 40cm (59.33 ± 8.66) vs. 30cm (66.8 ± 9.91), but higher error-rate for 40cm (0.27 ± 0.13) compared with 30cm (0.22 ± 0.12). Paired t-tests show significant differences (p<0.02) for both. Based on cohen's d, effect is large for comparing number of hops (d=0.81), but small when comparing error-rates (d=0.40). Pearsons's correlations of error-rates with age, height, and PHV are stronger for 30cm (r age =-0.664; r height =-0.344; r PHV =-0.351), than for 40cm (r age =-0.537; r height =-0.145; r PHV =-0.155), however, respective effect sizes based on cohen's q are small (q age =0.201; q height =0.213; q PHV =0.210). Conclusions While results remain statistically inconclusive when comparing 40 cm and 30 cm hop-tests in developmental athletes, 30 cm width may be favourable in a clinical context due to higher number of jumps and lower error rates. Further research is warranted to provide clarity.