Stress Shielding, Early Loading and Low-Jerk Isometrics After Injury

After injury, load travels around the compliant scar through healthy tissue, so the injured region is stress shielded and never gets the signal to regenerate. Work by Hayashi showed unloading a healthy tendon produces scar-like tissue, and Keith's mouse casting studies found 20% less tendon collagen after three days that did not recover after reloading. Kjaer's group found starting rehabilitation two days after injury, rather than nine, returned people to activity 25% faster. A sustained hold lets the healthy tissue relax, and about 80 to 90% of this stress relaxation happens within 30 seconds, sending load through the scar. Jerk, the rate of change of acceleration, is what damages tissue, so force should build over three to five seconds. In a rat model, isometric loading drove tendon-like gene expression while dynamic loading drove cartilage-like expression. A Nijmegen trial in 75 athletes found starting with isometrics beat eccentric loading for patellar tendinopathy. The protocol is four 30-second holds with about two minutes of rest, at little or no pain, starting at long muscle-tendon length.