Kinesiology is broadly defined as the scientific study of human movement that encompasses anatomical, physiological, and mechanical concepts. Although most kinesiological analysis in clinical practice is qualitative in nature (i.e., one's knowledge of functional anatomy and physiology forms the basis for movement analysis), quantitative approaches to describing motion characteristics (kinematics) and identifying the causes of movement (kinetics) will also be considered in this section. These latter approaches to evaluating human movement are further expanded in the field of biomechanics.
Types of Muscle Contractions
When excitation-contraction coupling occurs in skeletal muscle, actinomyosin is formed. A muscle contraction is defined as the formation of the cross-bridges between actin and myosin contractile proteins, resulting in the actin filaments being pulled toward the middle of the sarcomere. Depending on the relationship of the external load (resistance) to the tension generated, there may or may not be sliding of the actin filaments with a resultant shortening of the distance between z lines. The following contraction types are based on whether there is shortening of the whole muscle when excitation-contraction coupling occurs:
concentric contraction: There is a shortening of the sarcomeres, leading to a decrease in the length of the whole muscle. In practice, this means that joint rotation occurs and the limb segment moves in the direction of the muscle tension vector.
isometric contraction: There is a shortening of the sarcomeres to take up any slack in the muscle or connective tissue, and there is no observable change in the length of the whole muscle. In practice, this means that no joint rotation occurs and the limb segment does not move.
eccentric contraction: There is a lengthening of the sarcomeres leading to an increase in the length of the whole muscle. In practice this means that joint rotation occurs and the limb segment moves in the direction opposite to the muscle tension vector.
Force Relationships in Different Types of Muscle Contractions
The magnitude of force produced by a muscle is related to the length of the muscle relative to its resting state as described in the figure on the following page.
TABLE 14.1Three Types of Contractions Relative to Muscle Length |Favorite Table|Download (.pdf) TABLE 14.1 Three Types of Contractions Relative to Muscle Length
|Type of Contraction ||Muscle Length ||Forces ||Function ||Mechanical Work |
|Concentric ||Shortened ||Mm > Mr ||Acceleration ||Positive (W = F [+D]) |
|Isometric ||Same ||Mm = Mr ||Fixation ||Zero (no change in length) |
|Eccentric ||Lengthened ||Mm < Mr ||Deceleration ||Negative (W = F...|