This type of resistance training is normally performed against an immovable object such as a wall, a barbell, or a weight machine loaded beyond an individuals maximal concentric strength.
Isometrics can also be performed by having a weak muscle group contract against a strong muscle group. Isometrics came to attention of the American public in the early 1950's, when Steinhaus introduced the work of two Germans (Hettinger and Muller).
They concluded that gains in strength of 5% per week were produced by one daily two-thirds maximal isometric action 6s in duration. Gains in strength of this magnitude with such little training time and effort seemed unbelievable.
Review of subsequent studies demonstrates that isometric training leads to static strength gains but that the gains are substantially less than 5% per week.
Increase in strength from isometric training are related to the number of muscle actions performed, the duration of the muscle actions, whether the muscle action is maximal or sub maximal, and the frequency of training So the research conducted by scientist has shown a slight beneficial effect to isometric training in some conditions.
ISOTONIC
Isotonics is traditionally defined as a muscular contraction in which the muscle exerts a constant tension. The execution of free weight exercises and exercises on various weight training machines, though usually considered isotonic, is not really isotonic in definition.
The force exerted by a muscle in performance of such exercises is not constant, but rather varies with the mechanical advantage of the joint involved in the movement and the length of the muscle at a particular point in the movement.
A more workable term for resistance training exercise in which the external resistance or weight does not change and both a lifting (concentric) and lowering (eccentric) phase occur during each repetition is dynamic constant external resistance (DCER) training.
DCER implies that the weight or resistance being lifted is constant and held constant. When using free weights or machines, the external resistance or weight lifted is chosen.
Thus, DCER better describes this type of resistance training than the old term isotonic.
VARIABLE RESISTANCE TRAINING
Variable resistance equipment operates through a lever arm, cam, or pulley arrangement. Its purpose is to alter the resistance in an attempt to match the increases and decreases in strength throughout the exercises range of motion.
Proponents of variable resistance machines believe that by increasing and decreasing the resistance to match the exercises strength curve, the muscle is forced to contract near maximally gains in strength.
There are three major types of strength curves: ascending, descending, and bell-shaped. In an exercise with an ascending strength curve, it is possible to lift more weight if only the first half or first quarter of a repetition is performed.
Such an exercise is upright rowing. An exercise in which it is possible to lift more resistance if only the middle portion of the range of motion is performed has a bell-shaped strength curve. Elbow curls have a bell-shaped strength curve.
Additionally, because of variations in limb length, in the point of attachment of the muscles tendons to the bones, and in body size, it is hard to conceive of one mechanical arrangement that would match all individuals strength curves for a particular exercise...
ISOKINETICS
Isokinetics refers to a muscular action performed at constant angular limb velocity. Unlike other types of resistance training, there is no specified resistance to meet, rather, the velocity of movement is controlled.
The resistance offered by the Isokinetic machine cannot be accelerated; any force applied against the equipment results in an equal reaction force.
The reaction force mirrors the force applied to the equipment throughout the range of movement of an exercise, making it theoretically possible for the muscle's) to exert a continual, maximal force through the movements full range of motion.
The majority of equipment found in the gym allow for only concentric actions.
Advocates of Isokinetic training believe that the ability to exert maximal force throughout the range of motion leads to optimal strength increases.
Other advantages are minimal muscle and joint soreness, making difficult training sessions more tolerable.
ECCENTRIC TRAINING
Eccentric training (also known as negative resistance) refers to a muscular action in which the muscle lengthens in a controlled manner.
This type of muscle action occurs in daily activities such as walking. During normal DCER training, when the weight is being lifted, the muscles shorten or perform a concentric action.
When the weight is lowered, the same muscles that lifted the weight are active and lengthen in a controlled manner, or perform an eccentric action.
If the muscles did not perform an eccentric action when the weight was lowered, the weight would fall.
Eccentric action is a great way to maximize on a fat burning stage where increase stress, and longer repetitions could greatly produce a much cut ripped look.
Remember to use lighter weight when performing these movements, because the weight can be hard to control on the negative phase.
PLY METRICS
The term stretch-shortening cycle exercise is starting to replace the term ply metrics and describes this type of resistance exercise more accurately. The stretch-shortening cycle refers to natural part of most movements.
When the sequence of eccentric to concentric actions is performed quickly, the muscle is stretched slightly prior to the concentric action.
Thus the term stretch-shortening cycle describes what happens: The muscle is stretched slightly and then shortens. The slight stretching stores elastic energy.
The addition of the elastic energy to the force of a normal concentric action is one of the reasons commonly given to explain why a more forcible concentric action results after a stretch-shortening cycle.
The other common explanation for the more forcible concentric action is a reflex that results in quicker recruitment of muscle fibers or recruitment of more muscle fibers involved in the movement.
In more athletic sports revolving in running or jumping the stretch-shortening training is better than a concentric action.
A new Isometric training device has been redesigned and made more effective than ever before. Check out the Bullworker Bully Xtreme
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