Dr. Keith Baar — Professor of molecular exercise physiology at UC Davis who runs a strength physiology lab studying tendon, ligament and connective-tissue adaptation; helped discover mTOR's role in muscle growth and co-founded a tendon-loading device company called Sinew.
The gist
Keith Baar argues that strength is not just bigger muscles but the ability to transfer force through tendons and connective tissue, and that these tissues need only about 10 minutes of optimized loading with a six-to-eight-hour refractory period to maximally adapt. He explains that tendon scars form from 'stress shielding' (removing tension), which is why immobilization in a boot or RICE can make injuries worse. The core insight is that the eccentric-loading dogma actually worked because of low velocity, and that isometric holds (especially slow-onset 'overcoming' isometrics) maximize the healing signal while minimizing the 'jerk' that causes injury. He debunks orthobiologics like BPC-157 and PRP for tendons, details collagen-plus-vitamin-C timing, and covers estrogen/testosterone effects on tendon stiffness, ketogenic diets and longevity, and using load instead of anti-inflammatories.
Big reveals
- In Kjaer's study, patients who loaded an injury at day two after injury returned to sport 25% faster than those who waited nine days to load.
- Eccentric loading for tendons was a 'false positive' — it worked because of slow velocity, not the eccentric motion itself; heavy strength training has the identical effect, and zero velocity (isometric) gives the biggest bang for the buck.
- A boot or RICE is a 'mechanical stress shielder' that removes the tension tendons need to heal, so the standard orthopedic treatment can actually make the injury worse and cause scarring.
- BPC-157 has no direct effect on tendon or ligament cells — engineered human ligaments treated with it showed no change in mechanics, collagen, or strength.
- Women are four to eight times more likely to rupture their ACL because estrogen periodically inhibits a collagen cross-linking enzyme, lowering tendon stiffness during parts of the menstrual cycle.
- Physical therapist Rob Whitley told a room of orthopedic surgeons that PRP, stem cells and prolotherapy show no benefit in randomized controlled trials — only getting load through the tissue works.
- Mice on a ketogenic diet lived 13% longer than controls, an effect comparable to rapamycin, and old keto-fed animals had strength and brain function at the level of young animals.
Things worth remembering
- Connective-tissue cells need only about 10 minutes of loading (e.g. 10s on, 50s off) to get the full adaptation signal, then a six-to-eight-hour refractory period before responding again — matching bone's response to as few as 40 stimuli.
- Tendon stress relaxation drops exponentially; by 30 seconds you reach about 85% of the bottom, and waiting two minutes only adds about 15% more — hence 30-second holds for injured tendons.
- Three days in a 3D-printed cast cost a mouse tendon 15–20% of its collagen and made it roughly 30% mechanically weaker — a larger loss than the muscle itself.
- Fluoroquinolone antibiotics like ciprofloxacin raise Achilles rupture rate about 3.5-fold, while AT-1 (angiotensin) receptor blocking 'sartan' blood-pressure drugs taken by 15+ million Americans raise it 7.6-fold.
- On keto, no carbohydrate forces reliance on mitochondria, and lowered mTOR triggers 'mitophagy' that culls weak mitochondria — but keto severely harms bone mass, which is why epileptic kids must eventually cycle off it.
- Using a resorbable suture in Achilles repair allowed native tissue to recover and produced better collagen-synthesis markers than non-resorbable, with no difference in tendon length — contradicting surgeons' fears.
- Testosterone activates lysyl oxidase and decreases collagen, creating stiff but brittle tendons — explaining why steroid-using baseball players' performance would suddenly 'drop off a cliff' from tendon ruptures.
- Rupturing your ACL makes you 50% more likely to have a heart attack, driven by the habitual decrease in activity that follows the injury.
- A 2017 study found ~15g gelatin plus ~200–250mg vitamin C taken an hour before brief exercise doubled blood markers of new collagen formation versus control.
- The 'refractory period' for tendons is why Emil Abrahamsson's brother, with many injuries, did isometric hangs twice a day (six-plus hours apart) and got his hands healthy.