In medical terms rigidity is often defined as increased resistance to the passive movement of a limb and the inability to achieve complete muscle relaxation. Normally, when muscles contract, the opposing muscles relax. The contracting muscles are called the agonists and the relaxing muscles the antagonists. For example, when a person touches their mouth, the flexor muscles (agonists) of the elbow joint contract, whilst the extensor muscles (antagonists) relax. As a result, the elbow joint achieves full range of movement at a good speed and the hand reaches the mouth. In rigidity however, this harmonious interplay of contraction of the agonists and simultaneous relaxation of the antagonists is disrupted.
