If the first rep of an exercise feels like the most difficult of the entire set, you're not alone. However, there's good news: you can preload your muscles to avoid this phenomenon.
Preloading occurs when a muscle is stimulated to contract in order to resist or control a weight before the actual exercise begins. For example, consider a Bicep Curl. When holding the dumbbell in a start position, your biceps are firing and prepared to lift the weight, and you are able to apply near maximal force when beginning the Curl. To feel how neglecting to preload your muscles limits your strength, try performing a Bicep Curl by lifting a dumbbell directly off a bench.
When a muscle must work against weight—whether it's contracting or immobile—the nervous system fires to activate the muscle. This causes cross-bridges within each muscle fiber to form, pre-setting to shorten the fiber when called upon to contract the muscle. Preloading the muscle allows more time for it to form cross-bridges for optimal strength.
The key here is that if you don't preload your muscles before beginning a set, they will not be fully prepared to apply maximal strength during an exercise. This reduces the challenge to the muscle, potentially sabotaging strength gains.
Most free weight exercises incorporate a degree of preloading, so they are good choices for developing your strength. On the other hand, machines and cables are designed to begin exercises with zero resistance, throwing you into a potential trap of neglecting to preload before your set. To avoid the trap, apply some force to the machine or cable and lift the weight off the stack slightly before carrying on with your exercise. Also, do not lower the weight all the way to the stack during the exercise. This will ensure your muscles remain activated and prepared for the following repetition.
This technique should also be applied to exercises that involve pulling weight from the ground, such as the Deadlift or Power Clean. Instead of immediately performing the exercise from the resting position, assume your start position and apply force to the bar before exploding into the movement.
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