Bilateral Deficit: Definition, Mechanisms, Measurement, and Training Implications

What is bilateral deficit?

Bilateral deficit is a phenomenon in exercise science where the sum of forces produced by each limb unilaterally (one limb at a time) is greater than the force produced by both limbs simultaneously during a bilateral (two-limb) movement.

Defining Bilateral Deficit

The concept of bilateral deficit describes a reduction in maximal force production when both limbs are engaged in a movement concurrently, compared to the combined maximal force produced by each limb acting independently. For instance, if an individual can perform a single-leg press with 100 kg on their right leg and 100 kg on their left leg, a perfect bilateral effort would theoretically yield 200 kg during a two-leg press. However, in cases of bilateral deficit, the two-leg press might only achieve, for example, 180 kg. This indicates a 20 kg deficit (200 kg - 180 kg) or a 10% deficit relative to the theoretical maximum.

It is important to note that not all individuals exhibit a bilateral deficit; some may display bilateral facilitation, where the combined force of both limbs is equal to or even greater than the sum of unilateral forces. This often depends on training status, movement type, and specific muscle groups.

The Physiological Mechanisms Behind Bilateral Deficit

While the exact mechanisms are complex and still an area of research, several theories attempt to explain the occurrence of bilateral deficit:

• Neurological Inhibition: This is the most widely accepted explanation. When both limbs are working together, the central nervous system (CNS) may reduce the neural drive to each individual limb's musculature.
  • Reduced Motor Unit Recruitment and Firing Rate: The CNS might not be able to optimally activate the full complement of motor units, or maintain their peak firing rates, in both limbs simultaneously, leading to less efficient force production.
  • Inter-limb Inhibition: There might be inhibitory signals sent across the corpus callosum (the bridge connecting the two brain hemispheres) that reduce the excitability of the motor cortex controlling the contralateral limb. This could be a protective mechanism to prevent excessive force or to improve coordination.
• Stabilization Demands: Bilateral movements, especially those involving free weights, often require greater stabilization of the core and surrounding joints. The need to maintain balance and control the movement might divert neural resources away from maximal force production in the prime movers. Unilateral movements, while also requiring stability, often allow for more focused effort on the working limb.
• Muscle Fiber Recruitment: Some theories suggest that there might be differences in the recruitment patterns of fast-twitch and slow-twitch muscle fibers during bilateral versus unilateral actions, although this area requires more conclusive evidence.

Measuring Bilateral Deficit

Bilateral deficit is typically quantified using a simple formula:

Bilateral Deficit (%) = [((Unilateral Left Peak Force + Unilateral Right Peak Force) - Bilateral Peak Force) / (Unilateral Left Peak Force + Unilateral Right Peak Force)] x 100

Common tests used to assess bilateral deficit include:

• Leg Press: Measuring maximum force output for single-leg and double-leg presses.
• Knee Extension/Flexion: Using isokinetic dynamometers to measure torque.
• Vertical Jump: Comparing single-leg jump height/force to two-leg jump height/force.
• Grip Strength: Comparing individual hand grip strength to simultaneous two-hand grip strength.

Factors Influencing Bilateral Deficit

The presence and magnitude of bilateral deficit can vary significantly based on several factors:

• Training Status:
  • Untrained Individuals: Often exhibit a larger bilateral deficit, as their nervous system is less adapted to coordinating maximal force output bilaterally.
  • Strength-Trained Athletes: May show a reduced deficit, or even bilateral facilitation, especially in movements specific to their sport. Powerlifters and weightlifters, who frequently train with heavy bilateral movements, often demonstrate bilateral facilitation.
• Movement Type:
  • Open-Chain Movements: Tend to show a larger deficit (e.g., knee extension machine).
  • Closed-Chain Movements: May show a smaller deficit or even facilitation (e.g., squats, deadlifts), as these movements often involve greater stability demands and inter-limb coordination.
• Muscle Group: The deficit can vary between different muscle groups (e.g., lower body vs. upper body, hip extensors vs. knee extensors).
• Velocity of Movement: The deficit tends to be more pronounced during slow, maximal contractions and may be less evident during faster, more dynamic movements.
• Sport Specificity: Athletes in sports requiring high levels of unilateral power (e.g., sprinters, jumpers, soccer players) may exhibit a more pronounced bilateral deficit, as their training often emphasizes single-limb actions.

Practical Implications for Training

Understanding bilateral deficit has significant implications for designing effective strength and conditioning programs:

• Benefits of Unilateral Training:
  • Improved Single-Limb Strength and Power: Directly translates to better performance in activities like running, jumping, kicking, and throwing.
  • Enhanced Balance and Stability: Challenges the core and stabilizing muscles more effectively, improving proprioception and coordination.
  • Addressing Asymmetries: Helps identify and correct strength imbalances between limbs, reducing injury risk.
  • Rehabilitation: Often a cornerstone of injury rehabilitation, allowing for targeted strengthening of an injured limb without overstressing it with bilateral loads.
  • Reduced Spinal Loading: In some cases, unilateral exercises can allow for high loads on the working limb with less compressive force on the spine compared to heavy bilateral lifts.
• Benefits of Bilateral Training:
  • Higher Absolute Loads: Bilateral exercises allow for the lifting of heavier total loads, which is crucial for maximizing overall strength, muscle mass (hypertrophy), and systemic adaptations.
  • Sport-Specific Movements: Many athletic movements and daily activities are inherently bilateral (e.g., jumping with two feet, pushing a sled, deadlifting).
  • Efficiency: Allows for training multiple muscle groups simultaneously, making workouts more time-efficient.
• Integrating Both: A well-rounded training program should incorporate both unilateral and bilateral exercises.
  • Use bilateral movements (e.g., squats, deadlifts, bench press) as foundational strength builders for overall power and muscle mass.
  • Integrate unilateral movements (e.g., lunges, single-leg RDLs, step-ups, single-arm rows) to address specific strength deficits, improve balance and coordination, and enhance sport-specific performance.

Conclusion

Bilateral deficit is a fascinating neurophysiological phenomenon that highlights the complex interplay between the central nervous system and muscular force production. While it suggests that the sum of parts can be greater than the whole in terms of maximal force, this does not diminish the importance of bilateral training. Instead, understanding bilateral deficit empowers fitness professionals and enthusiasts to strategically incorporate both unilateral and bilateral exercises into their programs, optimizing strength, power, balance, and injury prevention for a more comprehensive and effective training approach.