Viscoelasticity

Biologic tissues are viscoelastic. That means their stretchiness changes depending on how hard they are stretched.  We can take advantage of this characteristic when we are rehabilitating a stiff joint. This becomes very important with certain medical problems. Specifically: total knee replacement and frozen shoulder. This concept is generally helpful in orthopedic rehabilitation and I take advantage of it whenever applicable. 

Think of silly putty.  When slowly stretched it can be drawn out into a long strand, but when pulled aggressively it will snap and break in two.  This is an extreme example of viscoelasticity. 

Your tissues are similar.  While extreme force stretching can cause tissue to tear, this is generally far beyond any amount of stretching a patient can do, even with a physical therapist. A manipulation under anesthesia is a maneuver performed by a surgeon to rapidly regain motion in a particular joint that has become stiff. Tissues tear, and inflammation results. This is the most extreme example of a high force, low duration stretch. It is best to avoid this type of intervention if possible. It is preferable for a patient to spend the time necessary to recover joint range of motion using a long duration, low force stretch. It will result in less inflammation and less pain. 

Shoulders and knees commonly become stiff. Total knee rehabilitation requires stretching to regain range of motion after surgery. Stretching is required to speed up the recovery of a frozen shoulder. When attempting to regain range of motion patients are often told to stretch for 10-15 seconds and then relax. Over and over. Sometimes this is effective. Sometimes it is not. There is significant genetic variation with regard to tissue strength and inflammatory response, and significant psychological variation with regard to pain tolerance, and ability to relax while stretching. When a patient has trouble regaining range of motion I try to focus them on long duration, low force stretching. This tends to create less inflammation and is more likely to allow a patient to relax the muscles while stretching. Relaxing is very important because any muscle resistance will prevent gains in range of motion. 

Force vs. duration stretching. A longer stretch done with less force will create less inflammation.

This sketch depicts how I think about stretching. A high force, brief stretch is more likely to cause inflammation. A gentle prolonged stretch is less likely to create an inflammatory response. The "amount of stretch"  or the total area under the curve depicted by the hash marks could be identical, but my experience suggests the long duration, low force stretching will give a superior result. 

​How do I know this?

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When I was a resident, I developed a frozen shoulder and used long duration, low force stretching to cure myself. I have subsequently recommended this technique to countless patients who presented with frozen shoulders that had failed to improve after many weeks of standard physical therapy.  Although occasionally surgical intervention was necessary, the vast majority progressed using this technique and never needed surgery. This technique has become my standard recommendation following total knee replacement and to rehabilitate a frozen shoulder, and has minimized the need for manipulation.