Muscles grow in response to overload while lack of stimulation causes them to contract in size. Your body is too smart to carry around excess muscle tissue that you’re not putting to good use. Hide your weights in the back of the closet and sit on the couch for a few months and you’ll see your own muscles shrink in size, not to mention become weaker. Have you ever broken an arm or a leg and had it in a cast for a while? Once the cast came off, you were probably surprised by how much strength and size you had lost in that limb. It’s an example of “use or lose it.”
After losing muscle tissue and strength, either through detraining or injury, you have the challenging task of regaining it. Fortunately, it’s easier the second time around to regain strength due to a phenomenon called muscle memory. That’s because you’ve forged new pathways between mind and muscle that aren’t completely lost when you stop training for a while. You just have to reactivate those pathways by training consistently for a few weeks.
When your goal is to build muscle, your muscles need to be stimulated and challenged. In fact, three main factors contribute to muscle growth. Let’s look at each one.
Muscle Damage and Muscle Growth
When you overload a muscle with more resistance than it’s accustomed to, it creates microscopic tears in muscle fibers. While you might think of damage as a bad thing, in this case, controlled damage serves as a stimulus for growth. In response to injury, immune cells rush into the area to break down the injured tissue and patch things up, creating an inflammatory response. Immune cells called to the scene produce molecules called cytokines that activate growth factors that aid in muscle growth. They also turn on satellite cells that fuse with existing muscle fibers to increase their size.
Of course, you might also feel sore for a few days if you’re just starting out or pushed your muscles hard, due to the phenomenon called delayed-onset muscle soreness or DOMS. When you experience DOMS, you’re feeling the effects of those overzealous inflammatory cells that rush to the scene of injury and release factors that cause pain and localized swelling. Although muscle damage is an important stimulus for muscle growth, just because you no longer feel sore doesn’t mean you’re not making progress. Due to the “repeated bout effect,” your body becomes more resistant over time to repeated muscle damage.
Metabolic Stress and Muscle Growth
Metabolic stress, characterized by the build-up of muscle breakdown products like lactate and a drop in pH, is another driving force for muscle growth, although it’s not really clear how metabolic stress facilitates muscle hypertrophy. One theory says that build-up of waste products like lactate draws water in muscle cells and this, in turn, stimulates muscle protein synthesis. Another theory is that metabolic stress stimulates growth-promoting factors called myokines. Metabolic stress may also activate anabolic hormones like growth hormone and testosterone that enhance muscle growth.
To create the most metabolic stress for muscle growth, go for more training volume. Lighten the weight slightly and increase the number of reps. Another way to maximize metabolic stress is to reduce the rest period between sets.
Muscle Tension and Muscle Growth
The total amount of tension a muscle is exposed to is another factor that impacts muscle growth. Time under tension is the total amount of time a muscle acts against a force. Let’s say you did an overhead press and the concentric phase took 2 seconds, the eccentric phase took 2 seconds and you did 8 reps. The total time under tension would be 32 seconds. When you’re trying to hypertrophy muscle, time under tension is more important than when your main goal is to build strength. For strength gains, a heavy resistance, around 90% of your one-rep max or greater is ideal, but because of the heavier weight you’re using, the muscles won’t spend as much time under tension due to more rapid fatigue. A weight that you can only lift a few times won’t create enough metabolic stress or time under tension to maximize growth.
For hypertrophy gains, a somewhat lighter weight than you’d use for strength building and more reps increases the total time under tension and is more effective for stimulating growth. A reasonable time under tension per set for hypertrophy gains would be around 32 to 40 seconds. This would correspond to 8 to 10 reps using a moderately heavy weight. Increasing the time under tension, even more, could lead to even greater gains in muscle size. This is the basis for super-slow training where you reduce the speed of the concentric and eccentric movements to increase the length of time the muscle is under tension.
Why is tension important for muscle development? The more time a muscle spends under strain, the more motor units you recruit and the more muscle fibers you activate. A study published in the Journal of Physiology showed that more time under tension led to greater muscle protein synthesis 24 to 30 hours after a workout.
The Bottom Line
A combination of muscle damage, time under tension, and metabolic stress work together to maximize hypertrophy gains. Unlike strength training, where some of the strength adaptations you make come from more efficient recruitment of motor units, hypertrophy gains are maximized by more metabolic stress and time under tension.
Despite the fact that volume is important, a certain threshold stimulus or mechanical stress is required for hypertrophy gains. You won’t build muscle using very light weights. The threshold, based on research, is around 60 to 65% of one-rep max.
To maximize time under tension and metabolic stress, slow training would seem to be one approach, but your muscles will eventually adapt to this type of training and your gains will slow, so you need to vary factors like the rep speed, rest periods, volume and resistance to maximize growth over time. Muscles need different types of stimulation to keep growing. Make sure you’re giving it to them by adding variety or periodizing your workouts.
The Max Muscle Plan. Brad Schoenfeld. Human Kinetics Bookstore.
Bret Contreras. “Training for Maximum Muscle Growth Explained”
J Physiol. 2012 Jan 15; 590(Pt 2): 351-362. Published online 2011 Nov 21. doi: 10.1113/jphysiol.2011.221200.
Sports Med. DOI 10.1007/s40279-013-0017-1.
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