The Playbook
Common Sports Injuries
What athletes face most — what they are, how they happen, and what evidence-based recovery looks like.
Common Sports Injuries — Featured Research
ACL Tears: What Actually Happens in 40 Milliseconds
An ACL can tear in about 40 milliseconds — you blink in roughly 100. By the time an athlete feels anything, it’s already over. Two research teams studied exactly what happens in that window, and they don’t fully agree.
40ms
Time from initial contact to ACL tear
Koga et al., Am J Sports Med, 2010 — 10 real ACL injuries on video
23°
Knee bend at initial contact — nearly straight
Koga et al., Am J Sports Med, 2010
+12°
Increase in knee valgus (cave) within 40ms
Koga et al., Am J Sports Med, 2010
3.2x
Body weight in force through the leg
Koga et al., Am J Sports Med, 2010
Why It Matters
Torn is torn — so why does it matter how it happened? ACL prevention programs are built on assumptions about the mechanism of injury. Get that mechanism wrong, and coaches end up training the wrong thing. Studying an injury that happens by accident, in a fraction of a second, in the middle of a game is difficult — two research teams solved that problem in two completely different ways.
Study 1 — The Video
Koga et al., 2010
Collected 10 real ACL injuries caught on video and used model-based image-matching — fitting a 3-D skeleton model to each frame to measure real-time joint angles from actual game footage.
What they found:
A stiff, nearly straight leg hits the ground hard. The knee caves inward (valgus) roughly 12° within 40ms, coupled with internal rotation of the shin. Damage is done before the athlete can ever react. This finding shaped two decades of prevention programs built around limiting knee cave.
Study 2 — The Strain
Foody et al., 2023
Video can’t show what the ligament actually feels. Foody et al. used MRI to build each person’s exact knee anatomy, then high-speed biplanar radiography during single-leg jumps to calculate real-time ACL strain.
What they found:
ACL strain peaked when the knee was straight — in both the sagittal (front-to-back) and coronal (side-to-side) planes. There was no significant relationship between how much the knee caved in and how much the ACL was strained.
So Which Is Right?
This is a real disagreement — but both papers are good, they’re just answering slightly different questions. Koga watched injuries actually happen, and valgus was there every time, consistent with other similar studies. Foody measured strain in healthy people doing safe jumps, and knee cave wasn’t driving it. One possibility: the knee caves after the ligament is already failing — Foody’s lab jumps never reached the extreme loads associated with real injury.
It’s not settled, but the two studies don’t entirely disagree either. Koga found athletes at 23° of knee bend — nearly straight — at the moment of injury. Foody found peak ACL strain occurred with a straight knee in the sagittal plane. Different methods, different conclusions about knee cave, but the same finding about extension: a straight knee is a dangerous knee.
Clinical Takeaway
You can’t react in 40 milliseconds — but you can train the landing position before you ever need it.
Land bent: hips back, knees bent to absorb the load. Not stiff-legged.
Train it until it’s instinct — there’s no time for conscious correction at game speed.
Limitations
Koga: only 10 injuries, all in women’s handball and basketball; video analysis carries real measurement error, though similar trends appear in other studies. Foody: healthy volunteers doing controlled jumps, not actual injuries — safe landings don’t represent injury-level loads. Neither study proves causation, and the broader ACL literature is far more comprehensive than these two papers alone.
References
- Koga H, Nakamae A, Shima Y, et al. Mechanisms for Noncontact Anterior Cruciate Ligament Injuries: Knee Joint Kinematics in 10 Injury Situations From Female Team Handball and Basketball. American Journal of Sports Medicine. 2010.
- Foody JN, Bradley PX, Spritzer CE, et al. Elevated in Vivo ACL Strain Is Associated With a Straight Knee in Both the Sagittal and the Coronal Planes. American Journal of Sports Medicine. 2023.
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For educational purposes only — not a substitute for medical advice.