What Is Reactive Strength — And Why It Matters After ACL Surgery by Mark Corry APA Physiotherapist

When recovering from ACL reconstruction (ACLR), most athletes focus on regaining strength and stability. But one often overlooked piece of the puzzle is reactive strength — a quality that plays a major role in returning to sport safely and effectively.

Physiotherapist Konstantinos Defteraios, from Aspetar Orthopaedic and Sports Medicine Hospital in Qatar, outlines just how important this quality is in modern ACL rehabilitation.

What Is Reactive Strength?

Reactive strength is your body’s ability to absorb and then quickly release force, much like a spring.

This ability is powered by the stretch-shortening cycle (SSC):

  • Stretch (Eccentric phase): The muscle lengthens under load (like when landing from a jump)

  • Store (Amortization phase): A brief transition where the energy is stored

  • Release (Concentric phase): The muscle contracts and releases that stored energy to jump or push off

This process helps athletes move explosively and efficiently — especially during sprinting, jumping, and changing direction.

Why Is It Important After ACL Surgery?

Even when strength and hop tests appear symmetrical, many athletes still show deficits in reactive strength — especially in single-leg tasks.

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

This is why focusing solely on strength or distance-based tests (like a hop for distance) might miss key movement deficits.

Measuring Reactive Strength: RSI

The Reactive Strength Index (RSI) is a valuable tool to assess plyometric ability. It’s calculated during drop jump tests as:

RSI = Jump Height ÷ Ground Contact Time

A higher RSI reflects better use of the SSC. But equally important is how the athlete produces the score — whether they’re favouring one limb, spending more time on the ground, or lacking explosiveness.

Total Physiotherapy assesses both double- and single-leg drop jump tests throughout the ACL rehab pathway using VALD Force-Decks to monitor this critical quality.

Key Principles for Developing Reactive Strength in ACL Rehab

Based on the Aspetar model, here are eight essential elements to consider when building reactive strength:

  1. No pain or swelling – Exercises must be completely pain-free, with no next-day joint swelling.

  2. Restore muscle strength – Especially quadriceps and calf strength, which contribute directly to jumping performance.

  3. Improve tendon stiffness – Through exercises like heavy calf raises to enhance force transfer.

  4. Minimize ground contact time – To promote faster, more reactive movement.

  5. Train on the forefoot – Keeping heels off the ground improves knee and ankle coordination.

  6. Use a straight knee in ankle-focused work – To isolate the calf-Achilles complex.

  7. Volume before intensity – Build capacity first, then shift toward higher-demand tasks.

  8. Train in multiple directions – Vertical for sprinting, horizontal for acceleration, and lateral for cutting and change of direction.

A Structured Approach: The Aspetar Curriculum

Aspetar has developed a systematic progression for reactive strength development post-ACLR. The program focuses on:

  • High-quality movement

  • Controlled loading

  • Sport-specific direction and task replication

Exercises progress from basic to advanced with careful attention to load, posture, and symmetry — all without upper body compensation (hands on hips).

Final Thoughts

Reactive strength is a foundational part of athletic performance — and a vital piece of successful ACL rehabilitation. It bridges the gap between basic strength and high-speed sport demands.

If you're in ACL rehab or preparing to return to sport, talk to your physio about whether your program is targeting this critical ability.

This blog post is based on the work of:
Konstantinos Defteraios PT
Physiotherapist
Aspetar Orthopaedic and Sports Medicine Hospital, Doha, Qatar

 

References

  1. Kotsifaki A, Van Rossom S, Whiteley R, et al. Single leg vertical jump performance identifies knee function deficits at return to sport after ACL reconstruction in male athletes. Br J Sports Med. 2022;56(9):490-498.

  2. King E, Richter C, Franklyn-Miller A, et al. Whole-body biomechanical differences between limbs exist 9 months after ACL reconstruction across jump/landing tasks. Scand J Med Sci Sports. 2018;28(12):2567-2578.

  3. Comyns TM, Flanagan EP, Fleming S, et al. Interday Reliability and Usefulness of a Reactive Strength Index Derived From 2 Maximal Rebound Jump Tests. Int J Sports Physiol Perform. 2019.

  4. Crotty NMN, Daniels KAJ, McFadden C, et al. Relationship Between Isokinetic Knee Strength and Single-Leg Drop Jump Performance 9 Months After ACL Reconstruction. Orthop J Sports Med. 2022;10(1):23259671211063800.

  5. Read PJ, Davies WT, Bishop C, et al. Residual Deficits in Reactive Strength After Anterior Cruciate Ligament Reconstruction in Soccer Players. J Athl Train. 2023;58(5):423-429.

  6. Read PJ, Pedley JS, Eirug I, et al. Impaired Stretch-Shortening Cycle Function Persists Despite Improvements in Reactive Strength After Anterior Cruciate Ligament Reconstruction. J Strength Cond Res. 2022;36(5):1238-1244.

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