Seeing the Whole Picture
How Professor John Cronin is redefining return-to-sport testing with Plantiga.
Researcher
Prof. John Cronin, PhD
Institution
Auckland University of Technology
Focus
ACL Return-to-Sport
Field
Human Perfomance/Strength & Conditioning
The question that no one could answer precisely
Every year, between 100,000 and 200,000 people in the United States rupture their anterior cruciate ligament. Every one of these people must go through months of gruelling rehab and the question that haunts every athlete, physio, and coach along the way: When is it actually safe to go back?
For Professor John Cronin, a researcher at Auckland University of Technology (AUT) with decades of work in strength, conditioning, and human performance, that question has never had a satisfying answer — at least not with the tools clinicians have traditionally used to answer it.
“The current standard measure for change of direction performance is completion time. But a total time tells you very little about movement strategy and as such can’t accurately characterize where the performance deficit actually is”
That gap between what a stopwatch reveals and what a clinician actually needs to know is what set JC and his team on the path to Plantiga.
Every year
100–200k
ACL ruptures in the United States alone
Leads to
Months
Of gruelling rehab — physical, psychological, and professional
Ending in
1 question
When is it actually safe to go back?
The problem hidden in plain sight
Change of direction is the most common cause of ACL ruptures. Non-contact ACL injuries almost always involve some combination of deceleration, jumping, cutting, or a sudden shift in direction. If you ask most returning athletes from ACL injuries, what they are most scared of, contact and changing direction are the most common answers. It follows logically that any meaningful return-to-sport (RTS) assessment should put change of direction (COD) testing front and center.
And it does. Tests like the Illinois Agility Test, the Pro-Agility, the T-Test, the L-Run, and the 5-0-5 have long been staples of physiotherapeutic practice. Clinicians use them to compare the injured limb against the uninjured one, looking for asymmetries that might signal that an athlete isn't ready.
But here's the problem that JC kept coming back to: COD isn't one thing, it's four things: acceleration, deceleration, the direction change itself, and re-acceleration. A total time measurement collapses all four into a single number and, in doing so, throws away most of the diagnostic signal.
Component 1
Acceleration
Explosive drive from the start. The initial motor output that shapes everything downstream.
Component 2 · Most critical
Deceleration
Horizontal braking capacity. Most closely associated with the ACL injury mechanism — and most likely to hide a deficit.
Component 3
COD time
The plant-and-pivot moment itself. The point of highest joint loading in the movement.
Component 4
Re-acceleration
Drive out of the turn. Often reveals residual neuromuscular inhibition that gross strength measures miss entirely.
“Imagine you test both legs and they come out roughly equivalent in total time. You might be ready to clear that athlete. But what if the deficit is hiding their decelerative capability specifically? That’s the exact motor quality most associated with the injury mechanism — and you’ve missed it completely.”
The implications aren't abstract. If you know an athlete is struggling with deceleration, the exercise prescription changes. You move toward targeted acyclic and cyclic horizontal braking work, eccentric loading, movements that specifically rebuild the athlete's capacity to absorb horizontal impact. Without that granular picture, you're guessing — or worse, you're clearing athletes who aren't truly ready.
Finding the right tool
The question then becomes a practical one: how do you get that level of granularity within the existing clinical workflow? Stopping mid-test to place force plates at a change-of-direction point isn't realistic. Motion capture labs aren't accessible for most practitioners. What clinicians need is something that travels with the athlete through the full movement, captures what matters, and delivers results that are actually actionable.
JC’s team conducted a systematic product review of inertial measurement unit (IMU) technologies — wearable sensors capable of capturing movement data in real-world conditions. The review, published in 2025 (Albarran et al.), evaluated the available options across a range of criteria relevant to applied clinical and research use.
Plantiga came out on top.
“After the product review, we found Plantiga to be the best sensors on the market to provide the information we needed. The pod-based system, worn in the shoe, was capable of capturing the variables that matter most in COD assessment: peak acceleration, peak deceleration, COD time, and peak re-acceleration — the four components that a total time score buries.”
From pilot to protocol
What followed was the kind of methodical, trust-building process that characterizes JC's lab. His team didn't simply adopt the technology and run — they worked directly with the Plantiga team to validate the measures, ensure accuracy, and develop a protocol that they could stand behind scientifically.
“We’ve been piloting the use of the technology and working with the Plantiga team to ensure we can accurately quantify the measures of interest”
That collaborative process reflects something important about how JC operates. At AUT, his research doesn't happen in isolation — it happens through a network of PhD candidates and master's students whose projects collectively advance a larger research agenda. The COD-RTS work is one thread in that broader tapestry, with students contributing to data collection, analysis, and the ongoing refinement of the testing protocols.
The next phase is underway: athletes are being tested across three sessions separated by seven days each, with the goal of establishing the reliability of the key variables. Because a measure is only as useful as it is repeatable — if peak deceleration scores fluctuate randomly from session to session, they can't serve as meaningful clinical benchmarks. JC's team is determined to know exactly what they can and cannot trust in the data before making clinical recommendations.
Research structure
Network of PhD & master's students
Each student project contributes to a larger research agenda — data collection, analysis, and ongoing protocol refinement.
Current phase
Reliability testing
Three sessions, seven days apart. Establishing whether key variables are repeatable enough to serve as clinical benchmarks.
What the metrics actually look like
Within a single COD task, Plantiga allows the team to isolate and benchmark each motor quality independently. Peak acceleration reflects explosive drive out of the start. Peak deceleration — arguably the most critical variable in the ACL context — quantifies the body’s horizontal braking ability. COD time isolates the plant-and-pivot moment itself, the point of highest joint loading. Peak re-acceleration captures how powerfully the athlete drives away after the turn, often revealing residual neuromuscular inhibition that gross strength measures miss entirely.
Critically, each of these variables is examined bilaterally. Interlimb asymmetries in acceleration, deceleration, COD time, and re-acceleration are where the diagnostic signal often lives — two legs that perform equally according to one measurement, total time, can tell a very different story when their movement profiles are placed side by side. Those asymmetries, not the aggregate score, are what inform targeted intervention.
JC’s team is also exploring a deeper layer of variables within each ground contact event: impact and push-off characteristics, ground contact time, and stride length — metrics that can reveal subtle compensatory patterns, protective bracing, or impact absorption deficits that persist long after an athlete appears clinically ready. Together, these variables build a movement signature precise enough to tell a clinician not just how an athlete performed, but exactly where, why, and on which side.
Phase 1
Acceleration
Peak acceleration — explosive drive out of the start
Phase 2 · Most critical
Deceleration
Peak deceleration — horizontal braking ability
Phase 3
Direction change
COD time — plant-and-pivot, highest joint loading
Phase 4
Re-acceleration
Peak re-acceleration — drive out of the turn
Each phase examined bilaterally
Two legs that look equal in total time can tell a very different story side by side. Interlimb asymmetries — not the aggregate score — are what inform targeted intervention.
L / R asymmetry is the diagnostic signalA bigger vision for return-to-sport
Change of direction is where the story starts for Cronin, but he's clear that it isn't where it ends. The ACL return-to-sport testing battery is multi-faceted — it encompasses strength assessments, jump testing, functional movement screening, and more. Across most of those domains, JC sees the same fundamental problem: clinicians are working with blunt instruments when they need precision tools.
“I am sure Plantiga will drive better diagnostic insights into the rehabilitation journey — and therefore better clinical, patient, and athlete outcomes.”
It's a statement grounded less in optimism than in evidence — or at least, in the evidence that's already emerging from his lab and the evidence that's coming. For JC, what Plantiga offers isn't just a better stopwatch. It's a window into the mechanics of movement that the rehabilitation field has been trying to see clearly for decades.
The athletes who rupture their ACL this year deserve more than a total time and a cleared-to-play stamp. They deserve to know — and their clinicians deserve to know — exactly where they are in the process of becoming whole again.
JC and his team at AUT are working to make that possible. Stay tuned.
Professor John Cronin, PhD, BEd, MA, Dip High Performance Coaching, DipPE, Dip Tchg, is a researcher at Auckland University of Technology. Learn more about his work at professorjohncronin.com.
References
Albarran, S., et al. (2025). Comparative evaluation of inertial measurement unit technologies for change of direction assessment. (Pending publication.)
Clark, N. C., et al. (2019). Return-to-sport testing after ACL reconstruction. Physical Therapy in Sport.
Jang, S. H., et al. (2014). Functional performance testing in athletes following ACL reconstruction. Journal of Physical Therapy Science.
Keays, S. L., et al. (2000). The association of knee joint position sense and force reproduction to return-to-sport following ACL reconstruction. Knee Surgery, Sports Traumatology, Arthroscopy.
Keays, S. L., et al. (2003). Factors involved in return to work and sport following ACL reconstruction. Journal of Orthopaedic & Sports Physical Therapy.
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King, E., et al. (2019). Comprehensive battery of return to sport tests reveals residual neuromuscular deficit in ACL reconstructed patients. Knee Surgery, Sports Traumatology, Arthroscopy.
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