[Summary: The addition of bands or chains to free weights permits adjustment of the resistance to the muscles' load-bearing capacity throughout an exercise movement. This broad biomechanical correspondence allows exercises to be performed explosively over their full range, effectively conditioning the body for actual sporting and athletic activities. The development of sophisticated mechanisms employing the same principle has important implications for sport-specific strength development.]
There are two main types of variable resistance exercise equipment:
Close biomechanical correspondence
Manufacturers of cam-driven machines claim to vary their resistance to closely match the torque curves of natural joint movements. In other words they assert a close biomechanical correspondence (CBC) between resistance and muscular capacity. However, given the variation between individuals in relative limb lengths, muscle attachment points, genetically endowed strength relativities between various muscles, etc., it is doubtful that such claims of accurate correspondence between load and load-bearing capacity are justifiable.
CBC machines are basically designed for single-joint movement of isolated muscle groups performed at a measured pace. Their very limited applicability to sports training is highlighted by Zatsiorsky's comment: "The important limitation of many strength machines is that they are designed to train muscles, not movement."
Broad biomechanical correspondence
The other type of variable resistance apparatus does not attempt to achieve any precise correspondence between resistance and muscular capacity.Rather the rationale for their use is that substantial benefits are achievable from load variance so long as the changing load-bearing capacity of the muscles involved is approximated. Bands and chains are examples of apparatus that rely only on such broad biomechanical correspondence (BBC).
In operation heavy rubber bands or steel chains are attached to either end of a loaded barbell and anchored to the floor or other fixed points. This enables a progressive increase in resistance for exercises such as squats and bench presses. A distinguishing feature of these exercises is that they are heavy load and involve multi-joint or whole limb movements.
The deceleration problem with free-weight exercises
Explosive strength is fundamental to many sporting or athletic activities but free weights are defective in building explosive strength. In the squat or bench press, for example, various studies have shown the bar decelerating for much of the final section of the range of motion. In the deceleration phase there is significantly decreased motor unit recruitment, velocity of movement and power production. In addition, when free weight movements are performed forcefully, antagonistic muscle action takes place to slow down and halt the limb to avoid soft tissue rupture or joint dislocation.
Conditioning the muscles for deceleration during the final stage of an exercise movement is counterproductive if the objective is to enhance ballistic-type sporting actions like throwing or jumping. This is also true where there is an inertia-dissipating or energy-absorbing mass to be moved, as in tackle engagements in football. Similar dynamics apply at the line of scrimmage in American football or in a rugby union scrum. In each of these cases the appropriate simulation is an acceleration through the whole range of limb movement.
Adding BBC characteristics to free weights enables exercises to be performed explosively or ballistically with the progressively increasing resistance providing a braking effect. Peak power occurs near the extreme points of angular motion.
A new generation of BBC machines
Recently MyoQuip have introduced a new system of lever and fulcrum technology that achieves the same basic effect as bands and chains but permits the development of sophisticated mechanisms with a high degree of specificity to particular sporting activities.
Because of their primitivity the use of bands and chains has largely been restricted to the power-lifting community. A major limitation of the equipment has been the difficulty in incrementally changing load. The fixed load component can be readily altered by adding or removing weight plates, but there is no way of making minor adjustments to the variable element provided by bands or chains.
With MyoQuip's machines, incremental load changes are effected simply by adding or removing weight plates, and the rate at which the load changes during a movement can be altered by choosing a different pin setting.
The MyoQuip technology also permits considerable flexibility in the orientation of effort. For example, the ScrumTruk machine is operated in the horizontal rather than the vertical plane, while the HipneeFlex, which is used to develop the leg flexor muscles, is configured for decreasing instead of increasing resistance.
The further development of machines delivering full-range muscle activation in either extension or flexion across multiple joints is likely to have important implications for strength training for sport.
bands
chains
biomechanical
multi-joint
explosive strength
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