Why make a passively shortened muscle shorter by actively contracting it?
I’m so glad you asked. First of all, how much a muscle contracts is a major determinant of its size and rate of cellular regeneration. Decrease muscle activity and the rate of protein degradation will accelerate, resulting in muscle atrophy. Increase muscle activity and boost the production of new proteins.
Second, you never want to stretch a muscle when it’s cold. Concentric muscle contractions before static stretching minimize stretch-induced force deficits. In plain-speak, marching in place prior to stretching prevents the muscle from weakening after a static stretch.
Lastly, to keep a muscle healthy, its nerve must supply both impulses (contraction instructions) and certain chemical factors. A muscle that is deprived of electrical or nervous stimulation will degenerate. On the other hand, an active muscle can increase short-term blood supply by telling its capillaries to open and implement longer term changes by causing the proliferation of capillaries. This is why muscles also need to warm up prior to an extensive workout, and particularly before “cardiovascular” work. If the heart rate goes from resting to aerobic, the heart pumps a much larger quantity of blood – that needs to go somewhere. If the capillaries in the muscles are not open to receive the additional blood flow, it’s like turning on a fire hose that’s connected to a cocktail straw. This is not kind to blood vessels. And guess what happens to blood pressure if the muscles are cold and heart rate goes up?
If you’ve been guilty of inflicting long passive stretches or hundred-yard dashes on cold muscles, relax. Muscles can heal themselves and grow stronger with a sensible and healthy practice like Yoga Tune Up. Active muscles that are kept at their optimal lengths move more blood, oxygen and nutrients – and increase one’s basal metabolic rate. Better cellular regeneration and burn more calories while you sleep. The body is pretty spectacular.
May all beings be healthy,
P.S. For all you fitness nerds: I urge you to walk optimally to grab a copy of Prime Mover by Steven Vogel. It’s a fascinating and entertaining introduction to the muscle in the context of biomechanics.
 Whaaaaaaat?!?! Stretching weakens muscles? Yes, under certain conditions. Static stretching of cold muscles has been shown to damage muscle fibers. Eccentric stretching under heavy load conditions has also been shown to damage muscle fibers. And overstretching the muscle-tendon complex can increase joint instability and increase the risk of injury. Anthony D. Kay & Anthony J. Blazevich, Concentric Muscle Contractions Before Static Stretching Minimize, But Do Not Remove, Stretch-Induced Force Deficits, Journal of Applied Physiology, 108: 637–645 (2010).
 Steven Vogel, Prime Mover, A Natural History of Muscle (2001)