Force Plates and ACL Return to Sport Testing: Seeing What the Eye Can't

When an athlete is recovering from an ACL injury or ACL reconstruction, one of the biggest questions we eventually have to answer is:

Are they actually ready to return to their sport?

That's a much more complicated question than asking whether an athlete can run, jump, or complete a few exercises without pain.

Returning to sport requires an athlete to produce and absorb force, jump, land, accelerate, decelerate, change direction, and repeatedly perform these tasks under high levels of fatigue and pressure. An athlete can look "good" while still having meaningful deficits in how they produce and distribute force.

This is where force plates become incredibly useful.

At RPE Physical Therapy and Performance, force plates are one component of our comprehensive ACL rehabilitation and return-to-sport testing process. They don't replace strength testing, hop testing, movement analysis, or psychological readiness assessments. Instead, they give us another layer of information that is difficult to see with the naked eye.

What Do Force Plates Actually Measure?

A force plate is a highly sensitive platform that measures the forces an athlete applies to the ground.

Depending on the test, we can measure things like:

• Peak force: How much force an athlete can produce or absorb.

• Rate of force development: How quickly they can produce force.

• Impulse: How much force they produce over a period of time.

• Eccentric braking: How well they can absorb and control force.

• Concentric force: How well they can produce force to accelerate upward or forward.

• Jump height: How high an athlete jumps

• Reactive strength: How effectively they transition from absorbing force to producing force.

• Landing forces: How an athlete handles the forces created when returning to the ground.

• Limb asymmetries: How differently the involved and uninvolved legs contribute

  
  

This matters because two athletes can jump the exact same height using completely different strategies.

One athlete may use both legs equally. Another may unload their surgical leg and rely heavily on their non-surgical leg. Both athletes jumped the same height. Only one demonstrated symmetrical force production.

Why Does This Matter After ACL Reconstruction?

After ACL reconstruction, athletes commonly develop deficits in strength, power, and their ability to absorb force. Some are obvious. Others are much harder to see.

An athlete may be able to run without a limp, perform a single-leg squat, and complete a hop test. But that doesn't necessarily mean they are producing and absorbing force normally.

Research has shown that athletes following ACL reconstruction can demonstrate lasting deficits in eccentric and concentric force, impulse, landing forces, jump performance, and reactive strength.

A 2024 systematic review and meta-analysis found that reconstructed limbs demonstrated lower eccentric and concentric forces and impulses, as well as deficits in jump height and reactive strength across various jumping tasks. In other words, an athlete may be able to perform a movement without having fully restored the qualities required to perform it well. (PubMed)

The goal of ACL rehabilitation isn't simply to get someone to complete a movement.

It's to restore the strength, power, control, and efficiency required to perform that movement at the demands of their sport.

The Problem With Only Looking at Limb Symmetry

One of the most common metrics in ACL rehabilitation is the Limb Symmetry Index (LSI), which compares the surgical leg to the non-surgical leg.

This is useful, but it has limitations.

The non-surgical leg isn't necessarily a perfect benchmark. During months of reduced activity, both legs can get weaker. An athlete might therefore appear "symmetrical" simply because both legs have lost performance.

This is why ACL return-to-sport testing shouldn't rely on one number.

Force plates allow us to look at:

1. How the two legs compare to each other

2. How well each leg compares to peers of the same age/sex/sport

3. How well the athlete is performing overall

4. How the athlete is producing and absorbing force

   
   

For example, imagine an athlete who achieves 90% symmetry in a bilateral jump. That sounds pretty good!

But what if their overall jump height is still well below what would be expected for their sport? Or what if they can jump high but the muscles only contract slowly (the opposite of nearly any sport)? Or what if their peak force looks good but their ability to absorb force during landing is significantly reduced?

The athlete may have "passed" one metric while still having important deficits. A clinician would be unable to pick that up with the naked eye.

That's where force plates provide a more complete picture.

Force Plates and Return-to-Sport Testing

Return-to-sport testing shouldn't be a single pass-or-fail event. It should help answer a bigger question:

Does this athlete have the physical qualities necessary to handle the demands of their sport?

Force plates help us evaluate several of those qualities.

Can the athlete produce enough force?

Strength testing tells us whether an athlete has regained the ability to produce force. Force plates complement this by showing how that force is expressed during dynamic movements.

Can they produce force quickly?

Sprinting, jumping, and reacting to an opponent often require force to be produced in fractions of a second. Rate of force development and other time-dependent metrics help us evaluate this ability.

Can they absorb force?

Athletes don't just accelerate—they have to brake. Landing from a jump, stopping a sprint, and changing direction all require an athlete to absorb and control large amounts of force.

Can they effectively use both legs?

Athletes often unconsciously favor their non-surgical leg after ACL reconstruction. Force plates can reveal differences in how each leg contributes to a movement, even when the movement looks normal.

Can they repeatedly produce and absorb force?

Reactive strength is critical for athletes who repeatedly jump, sprint, and change direction. Research has shown that athletes can still demonstrate meaningful asymmetries in metrics such as concentric impulse and landing force even when they are considered ready to return to sport. (PubMed)

The takeaway is simple:

Passing a return-to-sport test doesn't necessarily mean every aspect of performance has been restored.

Force Plates Don't Just Test Rehabilitation—They Help Guide It

One of the biggest advantages of force plates is that they aren't only useful at the end of rehabilitation. We can use them throughout the process to help determine what an athlete is ready for and what they still need to improve.

If an athlete has a large deficit in eccentric force absorption, we may need to focus more on their ability to tolerate and control higher loads. If peak force is good but rate of force development is poor, we may emphasize faster force production. If strength is excellent but reactive strength remains poor, we may need to progress plyometric and elastic training rather than simply adding more traditional strength work.

This is the difference between rehabilitating based on a calendar and rehabilitating based on objective performance.

Instead of saying, "You're six months post-op, so it's time to start jumping," we can ask:

"What does your current performance tell us you're ready to do?"

Why Don't You See Force Plate Testing Everywhere?

Force plates have traditionally been used in research laboratories, universities, and high-level sports performance environments. They aren't typically part of a standard physical therapy visit.

This doesn't mean force plates are required for every successful ACL recovery. They aren't. But they can provide information that is difficult to obtain through traditional testing alone.

You can watch someone jump. You can measure how high they jump. You can perform a hop test. But force plates allow us to understand what happened between the moment an athlete touched the ground and the moment they left it.

That can reveal differences in force production, force absorption, timing, impulse, and movement strategy that may not be obvious from watching the athlete.

The Bigger Picture

At RPE Physical Therapy and Performance, we don't believe an athlete should be cleared for sport based on one test.

Not a force plate test.

Not a hop test.

Not a strength test.

Not simply because a certain amount of time has passed since surgery.

Return to sport is a complex decision that should consider strength, power, movement quality, sport demands, psychological readiness, training exposure, and the specific requirements of the athlete's sport.

Force plates are valuable because they add another piece to that puzzle.

They help us objectively measure how an athlete produces and absorbs force, identify deficits and asymmetries that may not be obvious, and track how those qualities change throughout rehabilitation.

Most importantly, they connect the two sides of ACL rehabilitation:

Testing and training.

The same technology we use to evaluate where an athlete is can help us determine where they need to go next.

Because the goal of ACL rehabilitation isn't simply to get an athlete to pass a test.

The goal is to build the physical qualities necessary to confidently handle the demands of their sport—and use objective data to help us know when they're ready.