The advantages of performing a high velocity prestretch before an explosive concentric action is well known in the world of sports. This type of movement, known as the stretchshorten cycle (SSC) is the natural way our muscles work in most tasks requiring a high force production of a ballistic nature (throwing, jumping, etc.) as well as in locomotion tasks (walking, running, hopping, etc.). In muscle contractions preceding the concentric phase (the lifting portion in our case) by a short and forceful stretch can significantly increase the amount of force produced. This is due to:
1. The potentiating effect of the myotatic stretch-reflex: When a musculotendinous structure (a muscle and its tendons) is forcefully stretched, there is the onset of a "stretch reflex" governed by the activation of the muscle spindles. Muscle spindles are small fibers that run parallel to your muscle fibers and when they are stretched beyond a certain point they initiate the myotatic stretch reflex that helps the body to shorten. This is a protective mechanism designed to protect the musculotendinous structures against tears caused by excessive stretching.
2. The elastic component of the musculotendinous structure: The muscles, fascias and tendons are elastic by nature (more or less depending on the structure) and just like a rubber band; if they are stretched they will tend to shorten powerfully in return. This characteristic of the musculotendinous structures can also contribute to an increase in force production.
3. The increase in motor-units activation: Walshe et al. (1998) have stated that prestretching a muscle prior to a concentric phase promotes a higher active muscle state. They also found that stretch-induced movements the forceful stretch could potentiate the capacity of the contractile elements of the muscle.
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The use of dumbbells gives you a much more comprehensive strengthening effect because the workout engages your stabilizer muscles, in addition to the muscle you may be pin-pointing. Without all of the belts and artificial stabilizers of a machine, you also engage your core muscles, which are your body's natural stabilizers.