Muscle contraction is the tightening, shortening, or lengthening of muscles when you do some activity. It can happen when you hold or pick up something, or when you stretch or exercise with weights.

Muscle weakness may be due to a chronic condition or an infection. Conditions that can weaken muscles include Addison’s disease, anemia, diabetes, fibromyalgia, and more. Muscle weakness is a lack of ...

Human movement is reciprocal in nature: Opposing muscle groups must coordinate for proper movement. Muscle length and strength between opposing muscle groups need to be in balance for normal movement ...