The ability to know accurately the positions and movements of one's skeletal joints.
Kinesthesis refers to sensory input that occurs within the body. Postural and movement information are communicated via sensory systems by tension and compression of muscles in the body. Even when the body remains stationary, the kinesthetic sense can monitor its position. Humans possess three specialized types of neurons responsive to touch and stretching that help keep track of body movement and position. The first class, called Pacinian corpuscles, lies in the deep subcutaneous fatty tissue and responds to pressure. The second class of neurons surrounds the internal organs, and the third class is associated with muscles, tendons, and joints. These neurons work in concert with one another and with cortical neurons as the body moves.
The ability to assess the weight of an object is another function of kinesthesia. When an individual picks up an object, the tension in his/her muscles generates signals that are used to adjust posture. This sense does not operate in isolation from other senses. For example, the size-weight illusion results in a mismatch between how heavy an object looks and how heavy the muscles "think" it should be. In general, larger objects are judged as being heavier than smaller objects of the same weight.
The kinesthetic sense does not mediate equilibrium, or sense of balance. Balance involves different sensory pathways and originates in large part within the inner ear.
Bartenieff, Irmgard. Body Movement: Coping with the Environment. New York: Gordon and Breach Science Publishers, 1980.
Moving Parts (videorecording). Princeton, NJ: Films for the Humanities, 1985.