Bilateral Deficit: What It Is and How to Train Around It

The bilateral deficit is a fascinating neuromuscular phenomenon that affects how much force you can produce with two limbs versus one. Understanding it can help you make smarter exercise selections and program design decisions.

What Is the Bilateral Deficit?

The bilateral deficit (BLD) refers to the observation that:

The force produced by both limbs working together is less than the sum of each limb working independently.

Example

Right leg single-leg press: 100 kg
Left leg single-leg press: 100 kg
Expected bilateral leg press: 200 kg
Actual bilateral leg press: 180 kg
Bilateral deficit: 10%

You're "losing" force when both limbs work simultaneously.

The Formula

BLD% = [1 - (Bilateral / (Left + Right))] × 100

Or simplified:

BLD% = [(Sum of unilateral) - Bilateral] / (Sum of unilateral) × 100

A positive number indicates deficit (bilateral < sum of unilateral). A negative number indicates facilitation (bilateral > sum of unilateral).

Why Does the Bilateral Deficit Exist?

Several mechanisms contribute:

1. Neural Inhibition

When both limbs contract simultaneously, the nervous system appears to inhibit maximum activation. This may be a protective mechanism or simply a limitation of neural coordination.

2. Interhemispheric Interference

The two brain hemispheres controlling opposite limbs may interfere with each other during bilateral tasks, reducing overall output.

3. Attention/Focus Division

When performing bilateral movements, attention is divided between two limbs. During unilateral work, all focus is on one limb.

4. Stabilization Demands

Bilateral exercises often require more trunk and core stabilization, potentially diverting neural resources from the prime movers.

The Bilateral Facilitation Phenomenon

Interestingly, some people show bilateral facilitation instead of deficit:

Their bilateral force exceeds the sum of unilateral forces.

Who Shows Facilitation?

Trained weightlifters and powerlifters
Athletes who primarily train bilaterally
Those with extensive bilateral movement experience

What This Tells Us

The bilateral deficit is not fixed—it's trainable. Your training history determines whether you show deficit or facilitation.

Factors Affecting the Bilateral Deficit

Training Background

| Background | Typical Pattern | |------------|-----------------| | Untrained | Moderate deficit (8-15%) | | General fitness | Small deficit (3-8%) | | Bilateral sports (weightlifting) | Facilitation (0-5% enhancement) | | Unilateral sports (cycling, skating) | Large deficit (10-25%) |

Muscle Groups

The deficit varies by muscle group:

Knee extensors: 5-15% deficit common
Elbow flexors: 3-10% deficit
Hip extensors: Variable, training-dependent

Contraction Type

Isometric: Most pronounced deficit
Concentric: Moderate deficit
Eccentric: Smallest or no deficit

Movement Velocity

Slow contractions: Larger deficit
Fast/explosive contractions: Smaller or no deficit

Practical Implications for Training

When Bilateral Training Has Advantages

Maximum absolute load
- Even with a deficit, bilateral allows heavier absolute loads
- A 400 lb squat (bilateral) stresses the spine and trunk more than 225 lb single-leg work
Systemic/hormonal response
- Heavier loads may trigger greater hormonal response
- More total muscle mass working simultaneously
Sport specificity
- If your sport requires bilateral force production (jumping, weightlifting)
- Transfer is specific to training mode
Time efficiency
- Work both limbs simultaneously
- Important when training time is limited

When Unilateral Training Has Advantages

Greater per-limb activation
- Each limb works harder during unilateral exercises
- Better for addressing weak limbs
Correcting asymmetries
- Can identify and fix left-right imbalances
- Important for injury prevention
Sport specificity for unilateral athletes
- Running, skating, kicking all involve single-limb force
- May transfer better to these activities
Reduced spinal loading
- Single-leg squat with 100 lbs vs back squat with 200 lbs
- Similar leg stimulus, much less axial loading
Balance and stability
- Challenges proprioception
- Develops functional stability

The Cross-Education Effect

Related to bilateral deficit is cross-education:

Training one limb improves strength in the untrained opposite limb.

How Much Transfer?

Typical: 8-12% strength gain in untrained limb
Can be up to 20% in some studies
Effect is primarily neural (not hypertrophy)

Practical Applications

Injured limb: Train the healthy side to minimize strength loss
Dominant side weak: Train the strong side and get some transfer
Rehabilitation: Useful for immobilized limbs

Programming Considerations

For General Strength Development

Include both bilateral and unilateral work:

Lower Body Day:
1. Back squat (bilateral): 4×5
2. Romanian deadlift (bilateral): 3×8
3. Bulgarian split squat (unilateral): 3×8 each
4. Single-leg hip thrust (unilateral): 3×10 each

This approach:

Gets benefits of heavy bilateral loading
Addresses per-limb strength and imbalances
Develops stability and balance

For Athletes with Bilateral Deficits

If testing reveals a significant deficit:

Option 1: Accept it and train specifically

If sport is unilateral (running, skating), the deficit may be functional
Continue unilateral emphasis

Option 2: Reduce the deficit

Increase bilateral training volume
Focus on bilateral explosive work
May take 8-12 weeks to see changes

For Athletes Needing Bilateral Power

If sport requires bilateral force (jumping, weightlifting):

Emphasize bilateral training:

Bilateral exercises as primary movements
Unilateral exercises for accessory/assistance
Goal: Develop bilateral facilitation

Addressing Asymmetries

If one leg is significantly stronger:

Identify the deficit: Test single-leg strength bilaterally
Train weak side first: When fatigued, you'll naturally equalize volume
Use unilateral exercises: Can't compensate with the strong side
Consider extra volume on weak side: 1-2 additional sets

Testing for Bilateral Deficit

Single-Leg Strength Tests

Leg press:

Find single-leg max for each leg
Find bilateral max
Calculate: (Left + Right) vs Bilateral

Isokinetic dynamometer:

Most accurate method
Requires specialized equipment

Isometric testing:

Measure peak force unilaterally and bilaterally
Simple with force plate or strain gauge

Jump Testing

Single-leg vs bilateral jumps:

Measure single-leg vertical jump (each leg)
Measure bilateral vertical jump
Compare sum of single-leg to bilateral

This is more sport-specific for jumping athletes.

Exercise Selection Guide

Best Bilateral Exercises

Lower body:

Back squat
Front squat
Conventional/sumo deadlift
Leg press
Hip thrust
Bilateral jumps

Upper body:

Bench press
Overhead press
Barbell rows
Pull-ups/chin-ups

Best Unilateral Exercises

Lower body:

Bulgarian split squat
Single-leg Romanian deadlift
Step-ups
Single-leg leg press
Pistol squats
Single-leg hip thrust
Lunges

Upper body:

Single-arm dumbbell press
Single-arm rows
Single-arm overhead press
Single-arm pulldown

Hybrid Exercises

Some exercises challenge each limb somewhat independently:

Dumbbell exercises (each arm holds separate weight)
Trap bar deadlift (hands work independently)
Split stance variations

Common Questions

Should I Only Train Unilaterally?

No. Both have value:

Bilateral for maximum loading and systemic stress
Unilateral for per-limb strength and balance work
Most programs benefit from both

Will Unilateral Training Fix My Bilateral Deficit?

No—it may actually increase it. The deficit is training-specific. To reduce a bilateral deficit, you need more bilateral training.

Is a Bilateral Deficit Bad?

Not necessarily. It depends on your sport:

Sprinter with a deficit: Probably fine (sprinting is alternating unilateral)
Olympic lifter with a deficit: May want to address it
General fitness: Doesn't matter much

How Long to Change the Deficit?

Research suggests 8-12 weeks of specific training can shift the deficit toward facilitation, or vice versa. Neural adaptations are the primary mechanism.

Sample Program: Balanced Approach

Day 1: Bilateral Emphasis Lower

Back squat: 4×5
Romanian deadlift: 4×6
Leg curl: 3×10
Calf raises: 4×12

Day 2: Upper Push/Pull

Bench press: 4×6
Barbell row: 4×6
Single-arm DB press: 3×10 each
Single-arm cable row: 3×10 each

Day 3: Unilateral Emphasis Lower

Bulgarian split squat: 4×8 each
Single-leg RDL: 3×10 each
Step-ups: 3×10 each
Single-leg hip thrust: 3×12 each

Day 4: Upper Push/Pull

Overhead press: 4×6
Pull-ups: 4×8
Single-arm OH press: 3×10 each
Single-arm pulldown: 3×10 each

This provides:

Heavy bilateral work for absolute strength
Unilateral work for per-limb development
Balance between movement patterns

Key Takeaways

Bilateral deficit is real: Most people produce less force bilaterally than the sum of unilateral efforts
It's trainable: Bilateral training can create facilitation; unilateral training can increase deficit
Neither is inherently better: Exercise selection should match your goals and sport demands
Include both in most programs: Get the benefits of heavy bilateral loading AND unilateral strength development
Use unilateral work for asymmetries: Can't compensate with the stronger side
Cross-education exists: Training one side benefits the other (useful for rehabilitation)

Understanding the bilateral deficit helps you make informed decisions about exercise selection rather than blindly following trends. Choose exercises that match your goals, sport demands, and individual needs.