Muscles need stimulation to grow. If you don’t expose your muscles to stress, there’s no driving force for muscles to grow. Muscle tissue is remarkably adaptable. If you’re confined to bed rest for a long period of time due to illness, your muscles will atrophy and become smaller and weaker. That’s because muscles don’t need the same degree of strength and mass when they’re not bearing weight. Your body knows how to adapt to almost any situation! Muscles develop when they’re stressed and when they have the proper building blocks, amino acids, to grow. Research shows three main factors work together to create muscle growth. Here’s what these three factors are.
Muscles need to be exposed to tension to grow. Tension comes from overloading the muscle. When you subject a muscle to more overload than it’s accustomed to, it sends a signal to cells called satellite cells. Once signaled, these satellite cells begin to proliferate and fuse with existing muscle fibers to form new contractile proteins. The increase in contractile proteins makes each individual muscle fiber larger and your muscles stronger and more defined.
Where does this muscle tension come from? You create muscle tension during the concentric phase of a movement when you contract or shorten the muscle during a resistance exercise and during the eccentric phase when you lengthen the muscle in a controlled manner. As you lower the weight, the muscle you’re working maintains tension to resist the pull of gravity. Both the concentric and eccentric phase of muscle contraction exposes muscles to tension. In fact, the eccentric portion of the movement places more tension on the muscle than the concentric or shortening phase.
When you lift a heavier weight or use a greater resistance, you expose your muscles to greater amounts of tension than when you lift lighter weights. So, heavy resistance equals more tension and stress on the muscle you’re working. You need a certain level of tension, at least 65% of one-rep max, to stimulate muscle growth. Some studies suggest lifting lighter weights with high reps to failure also produces enough stress on the muscle to stimulate growth.
Muscle tension is one factor that promotes muscle hypertrophy but it’s not the only one. If muscle tension were the only impetus for muscle growth, powerlifters would have bigger muscles than bodybuilders due to the heavy weights they lift. Heavier weights create more muscle tension. In most cases, bodybuilders have more muscle hypertrophy than powerlifters.
Another factor that stimulates satellite cells and causes them to create larger fibers is muscle damage. When you work your muscles against resistance, it creates microscopic tears in the muscle fibers. This cause a short-term inflammatory response as white blood cells called macrophages move into the area to remove the damage and “clean things up.” This stimulates the release of growth factors. These growth factors “wake up” the sleeping satellite cells and encourage them to merge with existing muscle fibers to increase their size and strength. This inflammatory response partially explains why you feel sore after a hard work out. Some experts believe muscles can grow in size without damage. It’s possible. If you think about it, your muscles continue to grow even when you no longer become sore after a workout. Muscle damage may not be an absolute requirement for muscle hypertrophy.
Why do bodybuilders have more muscle development than powerlifters even though they lift heavier weights? Powerlifters lift very heavy weights but they do a low number of reps since their main goal is to build strength. Bodybuilders lift lighter weights, thereby exposing their muscles to less tension, but they do more repetitions and sets. This recruits a greater number of muscle fibers. It also increases the production of free radicals, stimulates growth factors that cause muscle growth and boosts production of hormones that promote muscle hypertrophy. As free radical production increases and lactic acid builds up, muscle cells swell. Cellular swelling and build-up of metabolites promote muscle growth. Some research suggests metabolic stress may be more important than muscle tension for turning on muscle hypertrophy.
Maximizing Muscle Growth
To maximize muscle growth, strike a balance between muscle tension and metabolic stress. If you lift a very heavy weight you can only lift for a few reps, you won’t create enough metabolic stress to boost muscle growth, although you should become stronger over time. Using a weight you can complete 6 to 10 reps creates muscle tension AND metabolic stress. This combination is best for promoting muscle growth.
Research shows doing a moderate number of repetitions as opposed to a lower number of reps maximizes release of anabolic hormones like testosterone and growth hormone. In addition, doing a greater total volume maximizes growth hormone release.
Another way to manipulate muscle tension and metabolic stress is to manipulate the rest interval between sets. A long rest period (2 to 3 minutes) maximizes muscle recovery so you can lift heavier. Long rest periods are ideal for building strength but not for hypertrophy since metabolic stress is minimal. Moderate rest periods (60 to 90 seconds) create more metabolic stress but still allow you to use a weight heavy enough to exert adequate tension on the muscle.
Training to muscle failure activates more muscle units and increases metabolic stress but doing it consistently can lead to overtraining. Short rest periods of 30 seconds create metabolic stress but limit the amount of tension you put on the muscle since the muscles haven’t completely recovered from the previous set. This can limit your gains. Short rest periods work best for increasing muscle endurance and burning fat.
The Bottom Line
You’ll maximize muscle growth by using a resistance that’s 65% of your one-rep max or greater and using moderate periods of rest (60-90 seconds) between sets. Increasing your training volume and doing some sets to failure will also promote greater muscle growth – but don’t overtrain. Alternate between microcycles where you lift to failure with ones where you don’t.
Journal of Strength and Conditioning Research. Vol. 24, No. 10. Oct. 2010.
Sports Med DOI 10.1007/s40279-013-0017-1 “Potential Mechanisms for a Role of Metabolic Stress in Hypertrophic Adaptations to Resistance Training”
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