Muscular strength: isotonic, isometric, isokinetic

Muscular strength is the force that  a muscle or muscle group can exert against a resistance in one maximal effort. There are four types of muscular contraction: isotonic, isometric, eccentric and isokinetic.

With isotonic contractions (muscle shortening while lifting a constant load), the tension developed the range of motion is related to (1) the length of the muscle fibers, (2) the angle of pull of the muscle on the bony skeleton, and (3) the speed of shortening. As a result, the tension developed during the lifting of a constant load varies over the full range of joint motion with the muscle stressed maximally only at its weakest point in the range.  This contrast to an isokinetic contraction in which the tension developed by the muscle as sit shortens at constant speed is maximal at all joint angles.

An isometric contraction is one in which tension is developed but there is no change in the external length of the muscle. An eccentric contraction refers to the lengthening of a muscle during contraction.

Local muscular endurance is usually defined as the ability of a muscle group to perform repeated contractions (either isotonic, isokinetic, or eccentric) against a load or to sustain a contraction (isometric) for an extended period of time. However, muscular endruance may also be defined as the opposite of muscular fatigue.

Physiological changes that accompany increased strength are as follow:

1. Hypertrophy-- an increase in the size of the muscle due to an increased size muscle fibers (mainly fast twitch) and myofibrals, an increased total amount of protein, and increased number of capillaries, and increased amounts of connective tendinous, and ligamentous tissues.

2. Biochemical changes-- including increased concentrations of creatine, PC, ATP and glycogen and decreased volume of mitochondria, but only small changes in anaerobic and aerobic enzyme activities.

3. Adaptations within the nervous system including changes in recruitment pattern and synchronization of motor units.

The physiological principle on which strength and endurance development depends is called the overload principle. It states that strength and endurance increase only when a muscle performs at its maximal capacity. With weight training programs, the resistance against which the muscle works should be increased periodically as gains in strength are made.  This is the principle of progressive-resistance exercises or PRE.

Weight training is specific in that gains in strength and muscular endurance improve skill performance to the greatest extent when the training program consists of exercise that include the muscle groups and simulate the movement patterns used during the skill. Also, the strength training is specific to the joint angle at which the muscle is trained (isometrics) and to the type of contraction used.