To get the most out of your time at the gym and make sure that your repetitions are effective, consider the following advice. If you are in good health, this is a great breathing technique that has long been used in martial arts and was labeled “Breath Strength” by Bruce Lee (Tsatsouline, 2004). Fill the stomach with a lot of air before starting the repetition and then hold that air in the stomach through at least ½ to ¾ of the concentric contraction (when the muscle is being shortened, such as the way up on a squat or the way down on a lat pull-down). This will keep the core tight for higher performance; keeping this region of the body contracted keeps the low back safe from injury.
If you perform the eccentric phase (lengthening of the muscle such as on the way down in a squat or on the way up in a lat pull-down) quickly, then you risk losing technique. This also makes the weight harder to reverse and push or pull the other direction. Look at a squat – if you go down fast, you bottom out with your back out of place and the weight is harder to reverse and come back up. So you don’t have to go slow on the eccentric phase, just stay under control for safety and performance.
The properly performed concentric phase (shortening of the muscle, such as coming up in the squat or down in the lat pull-down) is a huge myth buster. Research has shown that performing the weight slowly through the concentric phase has no benefit (Ferenhall, et al, 2001). If you perform this slowly, valuable energy will be wasted, decreasing the amount of weight performed. In this scenario, less weight = less calories burned, less bone density, less muscle strengthening, and less of everything.
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Fernhall, B., Finkelstein, L. H., Keeler, L. K., Miller, W. Early-Phase Adaptations of Traditional-Speed Versus Superslow Resistance Training on Strength and Aerobic Capacity in Sedentary Individuals. The Journal of Strength and Conditioning Research. 309-314. August, 2001. Volume 15, Issue 3.
Tsatsouline, Pavel. (2004). The Naked Warrior. St. Paul, MN. Dragon Door Publications, Inc.