The Free Leg Position During Unilateral Squat Jumps Influences the Bilateral Deficit.

Schmidt, C, Lligona, J, Müller, M, Langenauer, J, Heer, M, and Malatesta, D. The free leg position during unilateral squat jumps influences the bilateral deficit. J Strength Cond Res 40(8): 964-972, 2026-The aim of this investigation was twofold: (a) to compare bilateral deficit (BLD), assessed using vertical jump performance with 2 different unilateral jump techniques in squat jump (SJ): flexed (Classic technique) and straight (FC Luzern technique) free leg; and (b) to examine whether age affected BLD assessed in players' age categories of the same professional club. Eighty-one soccer players of a Super League professional club participated in this study and were separated by age categories: Senior (older than 21 years), under-21 (U21), under-18 (U18), under-16 (U16), and under-15 (U15) categories/teams. The mean (P mean ) and peak (P peak ) vertical mechanical powers were measured during the push-off phase of bilateral and unilateral (Classic and FC Luzerne [FCL] techniques) SJs using twin force plates. Compared with FCL technique, BLD was significantly higher for the Classic technique ( p < 0.001) and presented higher values when the P peak (BLD Ppeak ; +8.7%), P mean (BLD Pmean ; +10.2%), and F mean (BLD Fmean ; +3.1%) were used to evaluate BLD. There were no significant differences in BLD Ppeak ( p = 0.898), BLD Pmean ( p = 0.427), and BLD Fmean ( p = 0.089) across age categories for either of the unilateral jump techniques used to calculate BLD. We demonstrated that BLD is influenced by the unilateral SJ technique used. Using FCL unilateral SJ with a straight free leg ensures similar biomechanical conditions in both unilateral and bilateral SJs, allowing for a more accurate assessment of BLD. In addition, although there were some significant differences in biomechanical SJ variables across age categories, our findings showed that age did not influence BLD for either jump technique.