Variable resistance training (VRT) is a method of strength development where the load applied to the muscles changes throughout the range of motion of an exercise. Unlike standard weightlifting, which provides a static load from start to finish, this training style applies a dynamic degree of force to the target muscle group. VRT is an approach to strength building that works with the body’s natural mechanics. This method is designed to provide a more consistent challenge to the muscles, leading to more efficient and effective workouts.
Defining the Mechanism of Variable Resistance
The human body’s capacity to produce force is not uniform across a movement’s full range of motion. This phenomenon is described by the “strength curve,” which illustrates how muscle strength naturally varies based on joint angles and leverage. For many common exercises, such as the bench press or squat, strength follows an ascending curve, meaning an individual is weakest at the start of the movement and strongest toward the end, or “lockout” position.
Traditional weightlifting is limited by the weakest point in this strength curve, often called the “sticking point,” where the lift is most likely to fail. Variable resistance training addresses this by specifically altering the external resistance to better match the user’s natural strength fluctuations. By adjusting the load, VRT ensures the muscle is under near-maximal voluntary contraction throughout the entire repetition.
This dynamic loading effectively eliminates the sticking point by reducing resistance in the weakest range and increasing it in the strongest range. This mechanism forces the muscle fibers to be recruited and engaged over a broader range of motion than they would be with a static weight. The resulting continuous, high level of tension acts as a powerful stimulus for both strength adaptations and muscle size gains.
Tools Used to Create Variable Resistance
The principle of variable resistance is implemented through several distinct tools that dynamically adjust the load.
Resistance bands are the most common and accessible form of VRT, applying the variable load based on their elastic properties. As the band is stretched further during the concentric (lifting) phase of an exercise, the tension it exerts on the muscle increases proportionally.
Chains are another popular tool, typically draped over the ends of a barbell during a lift like a squat or bench press. At the bottom of the movement, a significant portion of the chain rests on the floor, reducing the total weight being lifted. As the lifter pushes the weight up, more links lift off the ground, causing a progressive and steady increase in the overall resistance.
Specialized gym equipment also employs this mechanism, often through cam systems or digital pneumatic resistance systems. Cam machines use an irregularly shaped pulley to alter the lever arm and resistance throughout the movement. Modern digital systems can automatically adjust the resistance hundreds of times per second, offering the most precise match to a user’s strength curve.
Comparison to Constant Load Training
Constant Load Training (CLT), which involves standard barbells, dumbbells, or fixed-weight machines, provides the same gravitational force against the muscle throughout the entire exercise. The primary limitation of CLT is that the weight a person can successfully lift is dictated entirely by their weakest point in the range of motion. This means that once the lifter moves past the sticking point, the load is no longer sufficient to maximally stimulate the muscle in its strongest biomechanical position.
This limitation results in a significant portion of the repetition being under-stimulated, which is an inefficient use of training time for strength and size gains. Variable resistance training directly solves this problem by allowing for “supra-maximal” loading in the strongest ranges of motion. The resistance is high enough to challenge the muscle when it has the best leverage, but low enough in the weaker range to prevent premature failure.
The difference in how the muscle experiences tension is significant; VRT maximizes the time under tension at an optimal load, forcing higher motor unit recruitment. In contrast, CLT’s constant load means the muscle is only truly challenged for a brief moment at the sticking point. By varying the resistance, VRT allows a person to generate more force in the final portion of the lift, which is a major factor in improving maximum strength and overcoming training plateaus.