Activation of trigger points may be caused by a number of factors, including acute or chronic muscle overload, activation by other trigger points (key/satellite, primary/secondary), disease, psychological distress (via systemic [inflammation](http://en.wikipedia.org/wiki/Inflammation)), [homeostatic](http://en.wikipedia.org/wiki/Homeostasis) imbalances, direct trauma to the region, accident trauma (such as a car accident which stresses many muscles and causes instant trigger points) [radiculopathy](http://en.wikipedia.org/wiki/Radiculopathy), infections and health issues such as [smoking](http://en.wikipedia.org/wiki/Smoking). Trigger points form only in muscles. They form as a local contraction in a small number of muscle fibers in a larger muscle or muscle bundle. These in turn can pull on tendons and ligaments associated with the muscle and can cause pain deep within a joint where there are no muscles. When muscle fibers contract, they use biochemical energy, and depletion of these biochemicals leads to accumulation of fatigue toxins such as lactic acid. The tightened muscle fibers constrict capillaries and prevent them from carrying off the fatigue toxins to the body's recycling system (liver and kidneys)\[_[citation needed](http://en.wikipedia.org/wiki/Wikipedia:Citation_needed)_\] . The buildup of these toxins in a muscle bundle or muscle feels like a tight muscle—a slippery elongate bundle. When trigger points are present in muscles there is often pain and weakness in the associated structures. These pain patterns in muscles follow specific nerve pathways and have been readily mapped to allow for identification of the causative pain factor. Many trigger points have pain patterns that overlap, and some create reciprocal cyclic relationships that need to be treated extensively to remove them.