Aim  The purposes of this study were to investigate if the load-velocity (L-V) profile parameters – force capacity and efficiency - differ between swimmers of different performance level, and to investigate if efficiency is the key performance indicator between international elite and national elite level swimmers.  Method  Fifty-four swimmers (27 female and 27 male) of either regional level, national elite or international elite level, participated in this study. The swimmers performed three 25 m semi- tethered maximum effort swims with ascending loads (1 kg, 5% and 10% of body mass). Mean velocity during three stroke cycles mid-effort was calculated and plotted as a function of the external added load. A linear regression was established, expressing the relationship between load and velocity, with the intercepts between the axes and the regression line being defined as the theoretical maximum velocity (V0) and load (force capacity, L0). The slope of the regression line (slopeLV) serves as an index of efficiency. Results A statistically significant difference was found between the three performance levels for all L- V profile variables for front crawl: V0 (F [2, 51] = 7.76, p<0.001), L0 (F [2, 51] = 5.18, p=0.009), and slopeLV (F [2, 51] = 3.36, p=0.043). A paired t-test revealed no difference in slopeLV between matched international elite and national elite level swimmers (t [9] = 1.42, p=0.188), but a near significant difference in L0 (t [9] = 2.11, p=0.064) . Both slopeLV and L0 for front crawl had a strong correlation with personal best in 100 m front crawl (PB100). Conclusion Efficiency was not found to be the key performance indicator between matched international elite and national elite swimmers in this study, and neither was force capacity. Nevertheless, a significant difference in all front crawl L-V profile parameters was found between performance level groups, but post hoc analyses indicated no difference between adjacent performance levels neither in L0 nor slopeLV. There was however a strong correlation between both slopeLV, and L0, to the swimmers’ PB100. All these findings imply that efficiency and force capacity seem to be of equal importance for high performance, but swimmers use different strategies to reach the high swim velocity. / Longitudinal development of performance determining factors in swimming (NIH)