A second technique of resistance training is perhaps the most commonly used and most popular technique for improving muscular strength. Progressive resistive exercise training uses exercises that strengthen muscles through a contraction that overcomes some fixed resistance such as with dumbbells, barbells, or various weight machines. Progressive resistive exercise uses isotonic contractions in which force is generated while the muscle is changing in length. Isotonic contractions may be either concentric or eccentric. If an athlete is performing a biceps curl, to lift the weight from the starting position the biceps muscle must contract and shorten in length. This shortening contraction is referred to as a concentric or positive contraction, If the biceps muscle does not remain contracted when the weight is being lowered, gravity will cause the weight to simply fall back to the starting position. Thus to control the weight as it is being lowered, the biceps muscle must continue to contract while at the same time gradually lengthening. A contraction in which the muscle is lengthening while still applying force is called an eccentric or negative contraction.
It is possible to generate greater amounts of force against resistance with an eccentric contraction than with a concentric contraction. This may be explained by the fact that eccentric contractions require a much lower level of motor unit activity to achieve a certain force than do concentric contractions. Because fewer motor units are firing to produce a specific force, additional motor units may be recruited to generate increased force. In addition, oxygen utilization is much lower during eccentric exercise than in comparable concentric exercise. Thus eccentric contractions are less resistant to fatigue than are concentric contractions. The mechanical efficiency of eccentric exercise may be several times higher than that of concentric exercise.
Various types of exercise equipment can be used with progressive resistive exercise, including free weights (barbells and dumbbells) or exercise machines such as Universal, Nautilus, Cybex, Eagle, and Body Master. Dumbbells and barbells require the use of iron plates of varying weights that can be easily changed by adding or subtracting equal amounts of weight to both sides of the bar. The exercise machines have a stack of weights that are lifted through a series of levers or pulleys. The stack of weights slides up and down on a pair of bars that restrict the movement to only one plane.
There are advantages and disadvantages to both free weights and machines. The exercise machines are relatively safe to use in comparison with free weights. It is also a simple process to increase or decrease the weight by moving a single weight key with the exercise machines, although changes can generally be made only in increments of 10-15pounds. With free weights, iron plates must be added or removed from each side of barbell. Regardless of which type of equipment is used, the same principles of isotonic training may be applied. In progressive resistive exercise it is essential to incorporate both concentric and eccentric contractions. Research has clearly demonstrated that the muscle should be overloaded and fatigued both concentrically and eccentrically for the greatest strength improvement to occur.
