Rate of Force Development and ACL Rehab: Why Producing Force Quickly Matters

By Lisa Roscarel, October 2026

In our recent blog on Reactive Strength and ACL Rehab (see here), we explained why the ability to absorb and rebound force like a spring is one of the most persistent deficits after an ACL reconstruction. In this article, we go one layer deeper and look at why that gap exists and the quality that sits underneath it: Rate of Force Development (RFD).

What is Rate of Force Development?

RFD is how quickly you can produce force. It’s usually looked at in the first 50 to 200 milliseconds of a muscle contraction. Reaching peak force often takes 300 milliseconds or more, but most sporting actions are over long before that making the early window what matters in sport.

Think of two athletes who can both squeeze a force plate with the same peak force. One reaches that peak in half a second. The other gets there in a fifth of a second. They have the same maximal strength but they are very different athletes, with the second athlete being much better at producing force quickly.

How RFD Influences Reactive Strength

Reactive strength is built on the stretch-shortening cycle: you land, the muscle and tendon absorb the load, and you rebound. As we covered in our reactive strength blog, the key is keeping the ground contact short. That’s why the Reactive Strength Index is calculated as jump height divided by ground contact time.

Now think about what happens during that contact. In a sprint stride or a drop jump, your foot may be on the ground for only around 100 to 250 milliseconds. In that tiny window, your leg has to absorb force, hold the position, and then push off again.

If your RFD is low, your muscles can’t build force fast enough to meet the ground, your contact time stretches out, and you lose the benefit of the stored elastic energy.

If your RFD is low, your push-off is weaker even if your peak strength is fine, you simply run out of time to use that strength, resulting in a lower jump height.

Longer contact time and lower jump height both pull your RSI down. In other words, a slow rate of force development shows up directly as poor reactive strength.

Studies of athletes who have returned to sport after an ACL injury have found that even when peak strength has recovered and horizontal hop scores look good, athletes often still show deficits in how quickly they generate and absorb force. These deficits can hang around for a long time, with between-limb differences in RFD reported up to four years after surgery, well beyond a typical 9 to 12 month return to play.

Working on RFD matters in ACL rehab because sport happens in a fraction of a second. Sprinting, cutting, landing and changing direction all demand fast force, and if you are unable to produce force quickly, performance suffers.

The Emphasis Model in Rehab

Traditional rehab works in sequence: settle pain and swelling, restore range of motion, then build strength, then add power, then speed and sport-specific work. It’s tidy, but it assumes the phases simply add onto each other and one aspect of rehab needs to be achieved before moving onto the next. This leaves the fast, high-intensity work until the very end, when there’s little time left to build it.

RFD takes time to build across the course of ACL rehabilitation and it’s one of the slowest qualities to come back. If training the ability to produce force at speed only starts once muscle hypertrophy, maximal strength, and full range of motion is achieved, you end up chasing RFD at the end when there’s very little time left.

Introducing RFD early alongside these other qualities, in a safe and stage-appropriate way, so it can build across the whole rehab journey is an effective strategy to reduce the need to play catch up at the end of your rehab. Instead of finishing one phase before starting the next, every key quality stays in the program from early on. What changes month to month is how much of the focus each one gets.

In the early weeks, the emphasis is on settling pain and swelling, restoring range of motion and switching the quadriceps back on. But strength work is already in the mix, at a level the knee can handle.

As the knee tolerates more, the emphasis moves to hypertrophy and maximal strength, with RFD training introduced early at a level that is safe and appropriate to do so, with earlier qualities that were the main focus now being maintained in the background.

From there, the emphasis shifts more toward ballistic and reactive work, such as jumping, braking and landing, with strength and hypertrophy still a big focus, and early rehab qualities still being managed alongside, just in smaller quantities.

In the final phase, the main focus becomes plyometrics, sprinting, accelerating, and other high-intensity actions on the field, while strength, RFD, and mobility are still being targeted too.

Final thoughts

RFD is one of the slowest qualities to come back after ACL surgery, so the biggest benefit of starting early is simply time. Introducing safe, stage-appropriate fast-force work in the first weeks and months means you’re building RFD from the start, rather than cramming it into the final stretch. That gives your reactive strength a stronger foundation, leaves less to fix at the end, and gives you more time to prepare for sprinting and game intensity.

If you’re starting ACL rehab and want to introduce RFD training into your rehab safely, the iPerform team in Adelaide’s north can help. Get in touch to book a physiotherapy assessment and talk through a plan built around your sport and your goals.