ACL Rehabilitation Strength Testing: Protocols, Benchmarks, and Return-to-Sport Criteria

Anterior cruciate ligament (ACL) reconstruction is one of the most common orthopedic procedures, with over 200,000 surgeries performed annually in the United States alone. Despite advances in surgical technique, re-injury rates remain concerning — approximately 1 in 4 young athletes who return to cutting and pivoting sports will sustain a second ACL injury within two years.

The strongest predictor of re-injury? Insufficient quadriceps strength restoration. This guide provides evidence-based strength testing protocols and criteria for each phase of ACL rehabilitation.

The Quadriceps Strength Problem

After ACL reconstruction, quadriceps strength loss is profound and persistent:

• 2 weeks post-op: Quadriceps strength is typically 20–40% of the uninvolved side
• 3 months: 50–60% LSI (still a 40–50% deficit)
• 6 months: 70–80% LSI
• 9 months: 80–90% LSI
• 12 months: Many patients still have not achieved 90% LSI

Research shows that quadriceps LSI below 90% at the time of return to sport is associated with a 3x higher risk of re-injury. Yet many athletes are cleared based on time alone ("you're 9 months out, you're good to go") without objective strength testing.

Testing Equipment

You don't need an expensive isokinetic dynamometer to perform reliable ACL strength testing. A handheld dynamometer with belt fixation provides excellent reliability (ICC > 0.95) at a fraction of the cost.

For a complete ACL testing battery, we recommend:

• Handheld dynamometer: For isometric knee extension and flexion strength testing
• Digital goniometer: For precise ROM measurement, especially knee extension deficit monitoring
• Force plates: For jump testing (CMJ, single-leg hop) at later rehabilitation phases

Phase-by-Phase Strength Testing Protocol

Phase 1: Acute Post-Operative (Weeks 0–2)

Focus: ROM restoration, quad activation

ROM targets:

• Knee extension: 0° (full extension) — this is the #1 priority
• Knee flexion: 90° by end of week 2

Strength testing: Not appropriate at this stage. Focus on quad activation (quad sets, straight leg raises). Monitor for ability to perform a strong quad set without extension lag.

Key measurement: Use a digital goniometer to precisely measure extension deficit. Any extension deficit >5° at 2 weeks requires aggressive intervention.

Phase 2: Early Rehabilitation (Weeks 2–6)

Focus: ROM normalization, progressive strengthening

ROM targets:

• Knee extension: 0° (must be achieved and maintained)
• Knee flexion: 120° by week 4, full flexion by week 6

Strength testing: Begin isometric quad testing at week 4 if pain-free

Test	Position	Target LSI	Notes
Isometric knee extension	Seated, 60° flexion	Baseline measurement	Pain-free only; submaximal if needed
Isometric knee flexion	Prone, 30° flexion	Baseline measurement	Hamstring graft patients: defer to week 6

Phase 3: Strengthening Phase (Weeks 6–12)

Focus: Progressive strengthening, movement quality

Testing frequency: Every 4 weeks

Test	Position	6-Week Target	12-Week Target
Knee extension (quads)	Seated, 60° flexion	≥50% LSI	≥65% LSI
Knee flexion (hamstrings)	Prone, 30° flexion	≥60% LSI	≥75% LSI
Hip abduction	Side-lying	≥70% LSI	≥85% LSI
Hip extension	Prone	≥70% LSI	≥85% LSI

Clinical decision point at 12 weeks: If quad LSI is below 60%, investigate barriers (pain, swelling, inhibition, compliance) and consider adjusting the rehabilitation program before progressing.

Phase 4: Advanced Strengthening (Months 3–6)

Focus: Power development, functional progression

Testing frequency: Monthly

Test	4-Month Target	6-Month Target
Knee extension (quads)	≥75% LSI	≥80% LSI
Knee flexion (hamstrings)	≥80% LSI	≥85% LSI
H:Q ratio	≥0.55	≥0.60
Single-leg squat quality	Good control, no valgus	Equal to uninvolved

Add at 4–5 months (if quad LSI >70%):

• Bilateral CMJ on force plates — assess peak force symmetry
• Begin monitoring jump height progression

Phase 5: Return-to-Sport Preparation (Months 6–9+)

Focus: Sport-specific training, RTS testing

Test	7-Month Target	RTS Criterion
Knee extension (quads)	≥85% LSI	≥90% LSI (≥95% for elite)
Knee flexion (hamstrings)	≥85% LSI	≥90% LSI
H:Q ratio	≥0.60	≥0.60
CMJ height	≥80% LSI	≥90% LSI
Single-leg hop	≥80% LSI	≥90% LSI
Drop jump RSI	—	≥85% LSI
Knee extension ROM	0° (full)	0° (no deficit)
Knee flexion ROM	Within 5° of uninvolved	Within 5° of uninvolved
ACL-RSI score	—	≥70/100

Testing Protocol: Step-by-Step

Isometric Knee Extension Test (Quadriceps)

1. Position: Patient seated on a firm treatment table, hips and knees at 60° flexion, trunk upright
2. Fixation: Belt around the table and patient's thigh for stabilization. Dynamometer attached distally on the shin (5 cm above the ankle malleoli)
3. Warm-up: 2 submaximal contractions at 50% and 75% effort
4. Test: 3 maximal isometric contractions, hold for 5 seconds each
5. Rest: 30 seconds between contractions
6. Cue: "Kick out as hard as you possibly can! Hold it! Keep pushing!"
7. Record: Peak force from each trial. Calculate LSI using the highest value.
8. Test uninvolved side first, then involved side

LSI Calculation

LSI = (Involved limb peak force ÷ Uninvolved limb peak force) × 100

Example: Involved quad = 280 N, Uninvolved quad = 340 N → LSI = (280/340) × 100 = 82.4%

H:Q Ratio Calculation

H:Q ratio = Hamstring peak force ÷ Quadriceps peak force (on the same limb)

Example: Involved hamstring = 180 N, Involved quad = 280 N → H:Q = 180/280 = 0.64 (adequate)

Red Flags During ACL Rehabilitation

These findings during testing warrant investigation and potential program modification:

• Extension deficit >3° at any point after 6 weeks: Requires immediate attention (aggressive stretching, possible manual therapy)
• Quad LSI not improving between testing sessions: Investigate pain, effusion, fear-avoidance, compliance
• H:Q ratio <0.50: Isolated hamstring strengthening needed
• Increasing pain or effusion during the strengthening phase: Reduce load and reassess
• ACL-RSI score <56: Psychological readiness is a barrier — consider referral for sports psychology

Documentation Template

For each testing session, record:

Metric	Uninvolved	Involved	LSI	Target	Status
Quad peak force (N)	___	___	___%	≥90%	Pass / Fail
Hamstring peak force (N)	___	___	___%	≥90%	Pass / Fail
H:Q ratio (involved)	—	___	—	≥0.60	Pass / Fail
Knee extension ROM	___°	___°	Δ ___°	0° deficit	Pass / Fail
Knee flexion ROM	___°	___°	Δ ___°	≤5° deficit	Pass / Fail
CMJ height (cm)	___	___	___%	≥90%	Pass / Fail

Conclusion

ACL rehabilitation is not complete when the calendar says it is. It's complete when objective testing confirms that the athlete has restored sufficient strength, power, and movement quality to safely return to their sport. A handheld dynamometer, digital goniometer, and force plates give you the tools to make these decisions with confidence — protecting your athletes and your professional judgment.

Test early. Test often. Let the data guide the return.