Seeing the Whole Picture
How Professor John Cronin is redefining return-to-sport testing with Plantiga.
INSTITUTION
Auckland University of Technology
RESEARCHER
Prof. John Cronin, PhD
FIELD
Human Performance/Strength & Conditioning
FOCUS
ACL return-to-sport assessment
The question that no one could answer precisely
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?
For Professor John Cronin (JC), the question of “when is it safe to return-to-play?” has never had a satisfying answer — at least not with the tools clinicians have traditionally used to find it.
“The current standard measure for change of direction performance is completion time. But a total time tells you almost nothing useful. It 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.
The problem hidden in plain sight
Change of direction is the most common cause of ACL ruptures. Tests like the Illinois Agility Test, the Pro-Agility, the T-Test, and the 5-0-5 have long been staples of physiotherapeutic practice. But here's the problem that JC kept returning to: change of direction isn't one thing, it's four things.
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.
A total time score collapses all four into a single number — and in doing so, throws away most of the diagnostic signal.
“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 in their decelerative capability specifically? That’s the exact motor quality most associated with the injury mechanism — and you’ve missed it completely.”
Finding the right tool
JC's team conducted a systematic product review of inertial measurement unit technologies — wearable sensors capable of capturing movement data in real-world conditions. The review, published in 2025 (Albarran et al.), evaluated 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.”
WHY PLANTIGA FIT THE CLINICAL WORKFLOW
Pod-based, worn in the shoe — travels with the athlete through the full movement
Captures all four COD components independently, not just total time
Bilateral data — interlimb asymmetries are where the diagnostic signal lives
No lab setup required — works in any clinical or field environment
Delivers actionable results, not raw data that requires specialist interpretation
From pilot to protocol
JC's 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 they could stand behind scientifically.
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.
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.
If peak deceleration scores fluctuate randomly from session to session, they can't serve as meaningful clinical benchmarks. Cronin's team is determined to know exactly what they can and cannot trust in the data before making clinical recommendations.
A bigger vision for return-to-sport
Change of direction is where the story starts for JC — but not where it ends. The ACL return-to-sport testing battery is multi-faceted, encompassing strength assessments, jump testing, functional movement screening, and more. Across most of those domains, JC sees the same fundamental problem: clinicians working with blunt instruments when they need precision tools.
Together, the variables Plantiga captures build a movement signature precise enough to tell a clinician not just how an athlete performed, but exactly where, why, and on which side.
“I am sure Plantiga will drive better diagnostic insights into the rehabilitation journey — and therefore better clinical, patient, and athlete outcomes.”
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.
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.
King, E., et al. (2018). Biomechanical deficits persist 2 years after ACL reconstruction. Orthopaedic Journal of Sports Medicine.
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.
Kong, D. H., & Burns, J. (2012). Changes in agility performance following ACL reconstruction. Journal of Physical Therapy Science.
Kyritsis, P., et al. (2016). Likelihood of ACL graft rupture: Not meeting six clinical discharge criteria before return to sport is associated with a four times greater risk of rupture. British Journal of Sports Medicine, 50(15), 946–951.
Lephart, S. M., et al. (1992). Functional assessment of the anterior cruciate ligament deficient knee. Clinical Orthopaedics and Related Research.
Marques, J. B., et al. (2020). Change of direction deficit: A more specific and reliable measure of change of direction performance than total time. Journal of Human Kinetics.
Myer, G. D., et al. (2011). Tuck jump assessment for reducing anterior cruciate ligament injury risk. Athletic Training & Sports Health Care.
Pollard, C. D., et al. (2015). Two-dimensional kinematics during cutting maneuvers in female soccer players. Orthopaedic Journal of Sports Medicine.
Stearns, K. M., & Powers, C. M. (2013). Improvements in hip muscle performance result in increased use of a hip strategy during a double-leg squat. American Journal of Sports Medicine.
Tibone, J. E., & Antich, T. J. (1988). Electromyographic analysis of the anterior cruciate ligament–deficient knee. Clinical Orthopaedics and Related Research.
