How Physical Therapy Took a Guy Named Kermit and Distorted Knee Rehab for 60 Years

The strange journey from Karl Klein's research to “never let your knees go past your toes”

Unless you've been living under a rock for the last few years, you've probably seen the relentlessly enthusiastic "Knees Over Toes Guy" somewhere on your social media feed. He's usually sprinting backward, dragging a sled, or folding himself into positions that would have caused a 1980s physical therapist’s head to explode.

What's funny is that many younger PTs I talk to were never formally taught the old "don't let your knee go over your toes" rule. Yet despite the evidence, despite the biomechanics, and despite the fact that humans have been climbing stairs successfully for a few hundred thousand years, I still hear the phrase in clinics today!

So where did this confusion come from? How did a cautious interpretation of a few studies become one of the most stubborn myths in rehabilitation? And why, in 2026, are we still arguing about whether knees should be allowed to do knee things?

The answer begins with a researcher named Karl K. Klein—and a profession that became far more attached to a rule than to the evidence behind it.

Physical therapy has a remarkable talent for taking a nuanced scientific observation, stripping it of context, repeating it for a few decades, and eventually teaching it as if it descended from a mountain carved into stone tablets.

The "knees over toes" myth may be one of the best examples.

And somehow, a guy named Karl K. Klein—yes, Kermit—became one of the unintended villains of modern knee rehabilitation.

The irony?

Karl Klein never actually said that knees should never go past the toes.

Yet for decades, clinicians, trainers, coaches, and patients behaved as though he did.

The Original Sin

In the 1960s, Karl Klein published research examining the effects of deep squatting on the knee. His work suggested that deep squatting could increase ligamentous laxity and potentially place stress on knee structures (Klein, 1961; Klein, 1962).

At the time, this wasn't an unreasonable question.

Researchers were trying to understand:

• How much load knees could tolerate

• Whether deep squatting damaged ligaments

• How to protect injured athletes

The problem wasn't the question.

The problem was what happened next.

Subsequent researchers raised substantial methodological concerns about Klein's conclusions. Measurements of ligament laxity were questioned, testing procedures lacked modern biomechanical rigor, and later investigations failed to support the idea that deep squatting inherently damaged healthy knees.

Yet the message survived.

Not because it was correct.

Because it was simple.

The Birth of a Rehab Zombie

Somewhere along the way:

"Certain populations may need caution with deep loaded knee flexion"

became:

"Deep squats are bad."

Which became:

"Don't let the knees go past the toes."

Which became:

"Normal human movement is dangerous."

And thus another rehabilitation zombie was born.

The evidence changed.

The teaching didn't.

The Problem With Looking at Force Without Adaptation

Many early biomechanical arguments focused on a real observation:

As knee flexion increases:

• Patellofemoral compression increases

• Quadriceps force requirements increase

• Tendon loading increases

All true (Escamilla, 2001).

But this is where rehabilitation repeatedly makes the same mistake.

Force is not injury.

If it were:

• Running would be dangerous.

• Stairs would be dangerous.

• Standing from a chair would be dangerous.

• Landing from a jump would be dangerous.

The human body adapts to load.

That's literally what rehabilitation is supposed to accomplish.

A tendon isn't harmed because it experiences force.

A tendon becomes stronger because it experiences force.

Modern Evidence Doesn't Support the Myth

Today we understand far more about knee biomechanics than we did when Klein published his work.

Research demonstrates that:

• Healthy knees regularly tolerate forward tibial translation.

• Squatting with unrestricted knee motion is a normal movement strategy.

• Restricting forward knee travel often shifts stress to the hips and lumbar spine.

• Deep squatting is not associated with increased rates of knee osteoarthritis in healthy populations (Escamilla, 2001; Hartmann et al., 2013).

In fact, when Fry and colleagues experimentally restricted knee travel during squatting, they found a dramatic increase in hip and trunk moments despite only modest reductions in knee torque (Fry et al., 2003).

In other words:

Saving the knee often meant sacrificing the back.

A trade many clinicians unknowingly accepted.

The Problem Was Never the Knee

The funny thing is that the knee itself was rarely the limiting factor.

Restricting forward knee travel simply moved the demands elsewhere.

When Fry and colleagues limited anterior knee movement during squatting, hip torque increased by more than 1,000% compared to unrestricted squatting (Fry et al., 2003).

The body still had to get the job done—it simply found a different way to do it.

Unfortunately, that often meant more stress on the hips, pelvis, and lumbar spine.

Rehabilitation didn't eliminate force.

It just relocated it.

Humans Do This All Day Long

Humans allow their knees to travel past their toes thousands of times per day.

Every staircase, every hill, every squat to a low chair, every athletic movement requires some degree of forward tibial translation.

If knees-over-toes were inherently dangerous, the waiting room outside every orthopedic clinic would be full of people injured by stairs.

Normal movement isn't pathology.

Loss of capacity is.

When Forward Knee Travel Actually Helps

Clinically, allowing progressive forward knee travel can be beneficial in:

Patellar Tendinopathy

Progressive tendon loading improves tissue capacity and often reduces symptoms in patients with patellar tendinopathy (Malliaras et al., 2013).

Patellofemoral Pain

Many patients improve when quadriceps strength and knee-flexion tolerance are restored rather than avoided. Exercise therapy remains one of the most effective interventions for patellofemoral pain (Thomeé, 1997; Crossley et al., 2016).

ACL Rehabilitation

Sport requires forward knee translation. Pretending otherwise creates athletes who are strong in the clinic and vulnerable on the field. Modern ACL rehabilitation emphasizes progressive exposure to sport-specific loading rather than avoidance (Ardern et al., 2016).

Post-Operative Knees

Patients frequently develop fear-based movement patterns that persist long after tissue healing has occurred.

Sometimes the painful knee isn't weak because it bends.

It's painful because it stopped bending.

The Rule We Should Have Been Teaching All Along

Not:

"Don't let your knees go past your toes."

But:

"Earn your knee flexion."

Progressively.

Gradually.

Intelligently.

Just like every other physical quality.

The goal of rehabilitation is not avoidance.

The goal is capacity.

Clinical Reality

Karl Klein asked a reasonable scientific question.

Physical therapy turned it into a commandment.

For sixty years, an entire profession treated forward knee travel as if it were a pathology instead of a normal human movement.

The lesson isn't that Klein was wrong.

The lesson is that rehabilitation becomes dangerous whenever we confuse:

• caution with prohibition,

• biomechanics with biology,

• and force with injury.

As sports scientist Greg Lehman likes to say:

"Load is neither good nor bad. It depends on whether you're prepared for it."

Karl Klein didn't break knee rehab.

We did.

A profession took a cautious observation, turned it into a commandment, and spent sixty years teaching people to fear a normal human movement.

The knee was never the problem.

The dogma was.

-the Pissed-off PT- comment, dislike, unsubscribe-

References

Klein KK.
The deep squat exercise as utilized in weight training for athletes and its effect on the ligaments of the knee. Journal of the Association for Physical and Mental Rehabilitation. 1961;15:6–11.

Klein KK.
Further studies on the effects of deep squatting on the knee joint. Journal of the Association for Physical and Mental Rehabilitation. 1962.

Meyers EJ.
Effect of selected exercise variables on knee stability. Research Quarterly. 1971;42(4):411–422.

Chandler TJ, Wilson GD, Stone MH.
The effect of the squat exercise on knee stability. Medicine & Science in Sports & Exercise. 1989;21(3):299–303.

Fry AC, Smith JC, Schilling BK.
Effect of knee position on hip and knee torques during the barbell squat. Journal of Strength and Conditioning Research. 2003;17(4):629–633.

Hartmann H, Wirth K, Klusemann M.
Analysis of the load on the knee joint and vertebral column with changes in squatting depth and weight load. Sports Medicine. 2013;43(10):993–1008.

Hartmann H, Wirth K, Klusemann M, Dalic J, Matuschek C, Schmidtbleicher D.
Influence of squatting depth on jumping performance. Journal of Strength and Conditioning Research. 2012;26(12):3243–3261.

Escamilla RF.
Knee biomechanics of the dynamic squat exercise. Medicine & Science in Sports & Exercise. 2001;33(1):127–141.

Escamilla RF, Fleisig GS, Zheng N, Barrentine SW, Wilk KE, Andrews JR.
Biomechanics of the knee during closed kinetic chain and open kinetic chain exercises. Medicine & Science in Sports & Exercise. 1998;30(4):556–569.

Thomeé R.
A comprehensive treatment approach for patellofemoral pain syndrome in young women. Physical Therapy. 1997;77(12):1690–1703.

Additional References Worth Adding

• Ardern CL, Glasgow P, Schneiders A, et al. 2016.

• Crossley KM, van Middelkoop M, Callaghan MJ, et al. 2016.

• Malliaras P, Cook J, Purdam C, Rio E. 2013.

• Escamilla RF. 2001.

• Fry AC, Smith JC, Schilling BK. 2003.

• Hartmann H, Wirth K, Klusemann M. 2013.