In humans, the heart is the first functional organ to develop, starting to beat by four weeks after conception. During the development, the heart grows an intricate and complex network of muscle ...

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.

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 ...