Dosage is one of the most important factors to consider when prescribing an exercise. This decision is often made based off level of acuity, tissue type, anatomical location, patient age, and more. In school, one learns the general principles of exercise prescription, but what is often neglected is WHY you prescribe in certain ranges. "Mechanotherapy: how physical therapists' prescription of exercise promotes tissue repair" is a 2009 article published in the Journal of Sports Medicine that elaborates on this topic.
Physiologically, what stimulates tissue repair of articular cartilage, muscles and tendons is a term called Mechanotransduction. In this article Mechanotransduction is defined as "the process by which the body converts mechanical loading into cellular responses." This can be thought of in clinical terms as what is occurring at the histological level to allow one to prescribe a certain exercise dosage without increasing the risk of injury.
Mechanotransduction can be broken down into 3 phases:
1) Mechanocoupling: This is the physical load cells undergo while in repair. The physical load is transferred into chemical signals which stimulate cellular changes.
2) Cell-Cell Communication: When one cell is stimulated, other cells in the area (whether directly stimulated by the initial mechanical stimulus or not) will undergo a cellular response.
3) Effector Response: When a cell is mechanically stimulated through compression, distraction, etc., several processes will occur intrinsically to allow change to occur.
Now knowing each of the 3 stages, you must think about them in relation to the Type of Tissue involved to understand how that tissue heals.
When a tendon is trying to heal, there is up-regulation of insulin-like growth factor, other growth factors, and cytokines which allows for cellular proliferation and tissue remodeling. Because healing is occur at the cellular level, too much stress OR too little stress on the tendon tissue could cause an alteration in the up-regulation, not allowing the tendon to rehabilitate optimally. The research up-to-date shows that tendons responds positively to "controlled loading." Research focusing on the type and intensity of controlled loading (eccentrics, assisted, resisted) is still ongoing.
The authors Khan and Scott state that "muscle offers one of the best opportunities to exploit and study the effects of mechanotherapy" because of how muscle tissue responds to loading. We know there is an overload of mechanogrowth factor (MGF) released when load is induced on the muscle force. This in turn causes muscle cell hypertrophy due to a cell-to-cell communication with nearby satellite cells. At this point, the research shows that early loading after a brief immobilization period is essential for minimizing atrophy and restoring normal cellular structure of the muscles.
Articular cartilage is comprised of a large population of mechanosensitive cells. It is hypothesized that by repetitively stimulating the articular cartilage with a low load/high repetition exercise dosage, better outcomes will result. One study assessing full thickness cartilage defects following periosteal transplantation demonstrated that individuals who used continuous passive motion (low load/high repetitions) had greater outcomes than those who did not receive this intervention. As with all things, research is ongoing.
When assessing bone healing, osteocytes are the primary mechanosensors. A recent study looking at individuals following a distal radius fracture had stronger bone growth if they received intermittent compression as an adjunct to the standard of care (compression & gripping exercises). The pneumatic compression allowed for extra stimulation of the bone cells and an increased healing rate.
We know parts of this article are dense, but understanding what is occurring at the cellular level can greatly change your viewpoint of how various tissues heal. Through each of these tissues we can see that Mechanotherapy plays a unique role in healing of different tissues types. The healing of osteocytes differs from that of chondrocytes which differs from myocytes. It is fundamental to understand these differences in order give appropriate doses during exercise - just as it is important to know the tissue type you should be treating following your examination.
As a general rule of thumb: Articular Cartilage: Low Load, High Repetition; ~15% 1 RM; Thousands of repetitions.
Tendon: Controlled Loading; consider eccentric exercises, but do not overload the tissue. Muscle: allow for a brief period of immobilization to restore homeostasis following injury. Bone: Based on location of the fracture, consider adding compression to your treatment to improve rate of bone growth and decrease healing time.
Khan and Scott. (2009) "Mechanotherapy: how physical therapists' prescription of exercise promotes tissue repair."
British Journal of Sports Medicine. 2009; 43: 247-251. Web. 5 Dec. 2013.