Elite performance evaluation has been mainly conducted through physiological or mechanical approaches separately. However, it is constructive to gather those fields to understand the major factors that explain outstanding performances. That velocity is dependent on the maximal total energy expenditure corrected for body mass and the energy cost (C), being C associated with intra-cyclic variations of the horizontal velocity (dv) of the body. Theoretically, dv are the result of intra-cyclic variations of the horizontal force (dF) of the body. Although higher variations lead to lower performances, it is not known if and how dv and dF are related and their role for very high velocities. 23 elite swimmers were tested (males, 18.6±2.3years of age; 1.79±0.09m of height; 69.9±9.2kg of body mass; 56.7±2.9s of 100m PB). On separate days, all-out 50m front crawl (26.7±1.50s) was performed to calculate dv by a speed-meter cable (Swimsportec, Hildesheim, Germany) attached to the swimmer’s hip, and a 30s all-out fully tethered swimming was completed to determine dF by a load-cell system (Globus, Codognè, Italy). Increase in blood lactate concentration (ΔBLa) was measured using a portable analyzer (Lactate Pro, Arkay, Japan). Heart rate was continuously recorded by a HR monitor (RS800CX, Polar Electro Oy, Kempele, Finland). Rate of perceived exertion (RPE) was assessed verbally (REF). SR (Hz) was determined using a portable SR counter (Seiko, Tokyo, Japan). ICC were between 0.94 (0.90-0.98) and 0.98 (0.96-0.99) for the measurements (n=8). Values are means ± S.D., compared by repeated measures. Both dv and dF exhibited similar patterns in the instantaneous curves; with very high different magnitudes (8.8±2.3% vs. 65.0±10.1%, p<0.05 respectively). There were no differences in ΔBLa, HR, RPE, or SR within the tests, with a very strong agreement of ΔBLa and SR (average differences were rather low, with limits of agreement (average±1.96 S.D.) ranging from -0.067 to 0.107 for SR and from -1.036 to 1.235 for ΔBLa). Thus, tethering the swimmer did not alter any physiological or mechanical responses compared with free swimming of similar duration and intensity. dv showed a high but non-linear relationship with swimming mean velocity (r=-0.78, p<0.01), whereas dF presented a linear relationship (r=-0.84, p<0.001). Hence, dF should be assessed for elite performance evaluations; higher dF induced a higher dv, leading to lower performances, suggesting that higher variations leads to an increase in C to overcome inertia and drag force. The present study showed that gathering the theoretical hypothesis with experimental data the demands of front crawl elite performance can be explained through a hybrid approach; ie. combining physiological and mechanical knowledge.