Bilateral Deficit in Strength Training: Research Review

When athletes perform a bilateral leg press, they rarely produce the combined force of both legs working independently — a systematic shortfall first documented by Henry & Smith (1961) and replicated across hundreds of studies since. The bilateral deficit (BLD) averages 5–15% in untrained populations (Hay et al., 1992) but can exceed 20% in highly sport-trained individuals, representing a hidden bottleneck in maximal power output. Understanding its mechanisms, magnitude, and trainability is essential for any coach designing a lower-body power programme.

The bilateral deficit describes the phenomenon where the combined force produced during simultaneous bilateral exertion is less than the sum of forces recorded during matched unilateral conditions. Henry & Smith (1961) coined the term after observing reaction-time interference between limbs; subsequent work shifted focus to neural inhibition as the primary driver.

Key mechanistic hypotheses include: (1) interhemispheric inhibition reducing corticomotor drive to both limbs simultaneously, (2) crossed inhibitory pathways suppressing high-threshold motor unit recruitment bilaterally, and (3) reduced afferent feedback from contralateral proprioceptors. Electromyographic (EMG) evidence consistently shows lower bilateral EMG amplitude in the vastus lateralis and gluteus maximus compared with the sum of unilateral efforts at equivalent loads (Škarabot et al., 2016).

The deficit is not universal. Elite sprinters and weightlifters — who train extensively with bilateral movements — often show a bilateral facilitation rather than a deficit, suggesting the BLD is substantially trainable over months to years of targeted bilateral loading.

A 2016 meta-analysis by Škarabot, Behm, & Strojnik synthesised 26 studies and reported a mean BLD of −9.2% (95% CI: −12.1 to −6.3%) for isometric leg press tasks. Dynamic ballistic tasks showed smaller deficits (approximately −5%), while slow isometric conditions amplified it toward −15%. Key findings are summarised below.

Importantly, Botton et al. (2016) found that 8 weeks of bilateral resistance training alone did not eliminate the deficit, while 8 weeks of unilateral-emphasised training reduced it by approximately 40%. This has direct programming implications.

Accurate BLD quantification requires matched conditions. The standard formula is:

BLD% = [(Bilateral Force − (Left Unilateral + Right Unilateral)) / (Left Unilateral + Right Unilateral)] × 100

A negative result indicates a deficit; positive indicates facilitation. Methodological pitfalls that inflate apparent BLD include: using peak force from bilateral tests versus mean force from unilateral, unequal rest intervals, and failing to account for fatigue order effects. Best practice is a randomised crossover design with ≥5 minutes rest between bilateral and unilateral conditions, and three trials per condition.

In the field, force plates are the gold standard. When force plates are unavailable, isometric mid-thigh pull with a dual-handle barbell and load cells provides a practical alternative. Jump-based BLD assessment — comparing bilateral CMJ height to the average of left and right single-leg CMJ heights — has a test-retest ICC of 0.84–0.91 (Janssen et al., 2022) and is feasible with portable IMU devices.

The relevance of the BLD varies considerably by sport. In activities that are inherently bilateral at peak force production — bilateral takeoff in volleyball, clean & jerk in weightlifting, block starts in swimming — a large BLD directly limits performance. In contrast, sports with predominantly unilateral peak demands (sprinting, soccer, basketball) are affected indirectly through the relationship between bilateral strength base and unilateral reactive strength.

A large BLD in a sprinter may indicate insufficient bilateral strength base rather than a motor-unit coordination problem — corrected by increasing squat or leg-press volume. Conversely, a large BLD in a volleyball blocker is more likely a neural inhibition issue requiring single-leg plyometric and reactive drills.

• Team sports athletes: BLD of >15% warrants priority unilateral loading for at least one mesocycle before bilateral peaking.
• Weightlifters and powerlifters: Tend toward facilitation; BLD monitoring serves as a readiness indicator — a sudden emergence of a deficit signals accumulated fatigue or technique breakdown.
• Combat sports (MMA, wrestling): BLD in rotational hip extension tasks correlates with takedown vulnerability; Bulgarian split squats and single-leg RDLs are the highest-return interventions.

A practical BLD correction protocol runs 8–12 weeks and is divided into three phases:

1. Assessment (Week 1): Measure BLD using bilateral CMJ vs. mean of left/right single-leg CMJ. Record 3 trials per condition; use median values. Classify: <5% deficit = minimal concern; 5–15% = moderate — schedule 1 unilateral priority session per week; >15% = significant — 2 unilateral priority sessions per week.
2. Unilateral emphasis phase (Weeks 2–9): Two unilateral lower-body sessions per week featuring Bulgarian split squats (4×5 at 70–80% estimated 1RM), single-leg RDLs (3×8), and single-leg box jumps (4×4). One bilateral session maintains strength base (back squat or hex-bar deadlift).
3. Bilateral integration phase (Weeks 10–12): Return to bilateral emphasis with frequency, now monitoring BLD monthly. Expect 30–50% deficit reduction in untrained athletes; 15–25% in trained.

Progressive overload within unilateral sessions: increase load 2.5–5% per week as long as technique remains clean and the weekly CMJ single-leg height does not drop more than 5% from the previous week's value. These velocity and height markers provide a more sensitive readiness signal than RPE alone for unilateral work.

Current research is moving in two directions. First, neuroimaging studies using transcranial magnetic stimulation (TMS) are beginning to localise the interhemispheric inhibitory pathways responsible for BLD and exploring whether cortical plasticity from unilateral training is the principal driver of correction. Second, asymmetry profiling is expanding beyond simple bilateral-vs-unilateral force comparisons to include velocity asymmetry at different percentages of 1RM — recognising that left-right differences can be load-dependent (greater at lower loads, smaller at high loads).

A third emerging direction is the relationship between BLD and injury history: athletes with prior ACL reconstruction consistently show elevated BLD (10–18% vs. 6–9% in matched controls) up to 24 months post-return-to-sport (Gokeler et al., 2021). This makes BLD monitoring a potentially useful return-to-sport screening tool, though sensitivity and specificity thresholds require further investigation.

For coaches without access to force plates, four practical field methods approximate BLD with acceptable reliability:

1. Jump-based BLD: Bilateral CMJ vs. mean single-leg CMJ. Measure jump height via contact mat, force plate, or calibrated IMU. ICC 0.84–0.91.
2. Isometric squat BLD: Bilateral squat hold at 90° knee flexion on two separate bathroom scales (or dual load cells) vs. single-leg hold. Reliable enough for directional tracking if the same equipment is used consistently.
3. Single-leg vs. bilateral leg press: Available in most commercial gyms. Use 5RM bilateral load, then test each leg separately with 60s rest. Sum unilateral 5RM loads and compare.
4. Sprint-derived asymmetry: Ground contact time asymmetry during 30m sprint (from dual-sensor timing gates or IMU) correlates moderately (r = 0.67) with force-plate BLD (Haugen et al., 2019).

Re-assess every 4 weeks minimum. A deficit that is not narrowing by 3–5 percentage points after 8 weeks of targeted unilateral work warrants a technique audit and potential referral for neuromuscular assessment.