Reactive Strength and ACL Rehab: What is it and why does it matter?
By Lisa Roscarel, September 2026
When recovering from ACL reconstruction, most athletes focus on rebuilding strength and stability. While these are essential, one major performance quality is often overlooked: reactive strength. Reactive strength plays a key role in returning to sport safely, confidently, and effectively.
What Is Reactive Strength?
Reactive strength is your body’s ability to absorb and then quickly release force, much like a spring.
Reactive strength is powered by the stretch-shortening cycle:
Stretch (Eccentric phase): The muscle lengthens under load and stores elastic energy (like when landing from a jump)
Transition (Amortisation phase): A brief transition between the eccentric phase and the concentric phase. The goal is to minimise the duration of this phase to utilise the stored elastic energy effectively.
Shortening (Concentric phase): The muscle contracts and releases that stored energy to jump or push off.
This process is crucial in sport because most movements happen quickly and repeatedly, including sprinting, jumping, and changing direction.
Why Is It Important After ACL Surgery?
Differences in reactive strength are among the most persistent functional deficits after an ACL reconstruction. Even when athletes appear strong in the gym, they may still struggle to use that strength at speed.
Reactive strength matters after ACL surgery for several reasons:
1. It directly impacts sport performance
Most sports involve rapid deceleration and acceleration. If reactive strength is reduced, athletes often struggle with:
quick land-and-go movements
Sprinting and jumping
change of direction
They may feel slower, less confident, and hesitant in game situations.
2. It reduces re-injury risk
Having good reactive strength improves
Safe landing mechanics
Knee control during unplanned movements
Confidence under fatigue
If the knee cannot efficiently absorb and re-use force, the body compensates. This often increases load through the hip, ankle/calf, or the opposite knee. These compensation patterns are strongly linked with a higher risk of a second ACL injury - either on the reconstructed side or the other leg.
3. Strength alone isn’t enough
Gym-based strength training (squats, deadlifts, leg press) is important, but reactive strength is different. It depends on:
rate of force development
tendon stiffness and elasticity
neuromuscular timing
coordination under speed and fatigue
All of these, together with strength, determine how well an athlete can absorb force and redirect it at speed. Strength is the foundation, but these are the qualities that carry it into sport.
Measuring Reactive Strength: RSI
A common way to assess reactive strength is the Reactive Strength Index (RSI), measured during a drop jump test:
RSI = Jump Height ÷ Ground Contact Time
A higher RSI indicates better ability to use the stretch-shortening cycle efficiently. But equally important is how the athlete produces the score - whether they’re favouring one limb, utilising a more hip dominant jump action, or spending more time on the ground.
Research has shown that:
Drop jump performance often reveals lingering asymmetries, even after return-to-sport clearance
Athletes tend to rely more on their hips, reducing knee and ankle contribution
These compensations increase the risk of re-injury and can limit performance
Key Principles for Developing Reactive Strength in ACL Rehab
To safely build reactive strength, athletes should focus on:
No pain or swelling – Exercises must be pain-free, with no next-day joint swelling.
Restore muscle strength – Especially quadriceps and calf strength, which contribute directly to jumping performance.
Improve tendon stiffness – Through exercises like isometric calf raise holds.
Minimise ground contact time – To promote faster, more reactive movement.
Train on the forefoot – Keeping heels off the ground improves knee and ankle coordination.
Use a straight knee in ankle-focused work – To isolate the calf-Achilles complex.
Volume before intensity – Build capacity first, then shift toward higher-demand tasks.
Train in multiple directions – Vertical, horizontal, and lateral.
Reactive strength is what allows you to recruit strength gains during rehab when competing in your sport. Without it, athletes may return to training but remain vulnerable in the exact situations where ACL injuries often occur: landing, cutting, and decelerating at speed.
If you’re recovering from ACL reconstruction and want expert guidance on building reactive strength and returning to sport safely, the team at iPerform in Adelaide’s North can help with evidence-based testing, strength programming, and return-to-play rehab. Call us on 8151 1347 or check out our physiotherapy page to learn more.
