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

Area of Science:

• Sports Science
• Biomechanics
• Human Movement Analysis

Background:

• The bilateral deficit (BLD) quantifies the difference in force or power output between unilateral and bilateral limb contractions.
• Understanding factors influencing BLD is crucial for optimizing training and performance in athletes.
• Previous research has explored various determinants of BLD, but the specific impact of unilateral jump technique requires further clarification.

Purpose of the Study:

• To compare the bilateral deficit (BLD) using two distinct unilateral squat jump (SJ) techniques: a classic flexed free leg and an FC Luzern (FCL) straight free leg.
• To investigate the influence of age categories on BLD measurements in soccer players.

Main Methods:

• Eighty-one soccer players from different age categories (Senior, U21, U18, U16, U15) participated.
• Vertical jump performance was assessed using bilateral and unilateral (Classic and FCL techniques) squat jumps.
• Vertical mechanical power (mean and peak) and mean force were measured using twin force plates during the push-off phase.

Main Results:

• The classic technique with a flexed free leg resulted in a significantly higher bilateral deficit (BLD) compared to the FCL technique (p < 0.001).
• BLD was notably higher for peak power (+8.7%), mean power (+10.2%), and mean force (+3.1%) when using the classic technique.
• No significant differences in BLD were observed across the different age categories for either unilateral jump technique.

Conclusions:

• The technique employed during unilateral squat jumps significantly influences the assessment of the bilateral deficit.
• The FC Luzern technique, utilizing a straight free leg, offers a more biomechanically consistent approach for evaluating BLD by minimizing differences between unilateral and bilateral movements.
• Age was not found to be a significant factor influencing the bilateral deficit in this cohort of soccer players, irrespective of the unilateral jump technique used.