Bilateral deficit magnitude increases with velocity during a half-squat exercise

Area of Science:

• Biomechanics
• Human Movement Science
• Exercise Physiology

Background:

• The bilateral deficit (BLD) describes the phenomenon where the sum of forces produced by unilateral limb contractions exceeds the force produced during bilateral contractions.
• Movement velocity is hypothesized as a key determinant of the BLD, but empirical evidence across different exercise types and velocities remains limited.
• Understanding the interplay between movement velocity, muscle activation, and BLD is crucial for optimizing training strategies.

Purpose of the Study:

• To investigate the influence of movement velocity on the magnitude of the bilateral deficit during a half-squat exercise.
• To assess the role of muscle excitation, specifically in the vastus lateralis and vastus medialis, in modulating the BLD.
• To determine the relationship between theoretical maximum force (F0) and velocity (v0) in bilateral versus unilateral contractions.

Main Methods:

• Twelve male soccer players performed a half-squat exercise with incrementally increasing loads.
• Force and velocity were measured during the pushing phase in both bilateral and unilateral conditions to calculate the bilateral index (BI).
• Surface electromyography root mean square (sEMG_RMS) was used to assess vastus lateralis and medialis excitation, calculating a sEMG BI.

Main Results:

• Maximum voluntary force (F0) was significantly greater in bilateral compared to unilateral contractions (+43%).
• Theoretical maximum velocity (v0) was similar between bilateral and unilateral conditions.
• The magnitude of the bilateral deficit (BI) increased significantly with movement velocity, ranging from -34% at 50%v0 to -70% at 90%v0.
• No significant bilateral deficit was observed in muscle excitation (sEMG BI) for either the vastus lateralis or medialis.

Conclusions:

• Movement velocity is a critical determinant of the bilateral deficit during half-squat exercises, with larger deficits observed at higher velocities.
• The bilateral deficit appears to be a force production phenomenon rather than a deficit in neural drive or muscle activation.
• These findings provide valuable insights for manipulating contraction velocities to either enhance or attenuate the bilateral deficit as a training stimulus.