Muscle definition doesn’t come for free, and for many people, especially women, it doesn’t come easily. Muscles have a way of stubbornly remaining the same unless you stress them in a manner they’re not accustomed to. That’s what weight training is all about.
As you know, we don’t all hypertrophy at the same rate or to the same degree. Women have a more difficult time building muscle due to hormonal factors, mainly a lower level of testosterone. Age and genetics are other factors that influence how much and how quickly you build muscle. Once you’ve been lifting for a while, the gains also slow and you have to stress your muscles in a different way to keep the gains coming.
When you get down to it, muscles grow by one of three mechanisms. Brett Contreras discusses these mechanisms in his article entitled, The Mechanisms of Muscular Hypertrophy. Let’s look at each of these mechanisms of muscle hypertrophy in more detail.
Muscle Hypertrophy: Muscle Damage
Weight training causes microscopic damage to fibers of the muscles that you’re working. This damage causes an inflammatory response but also activates cells called satellite cells, located along the outside of muscle fibers. These specialized cells begin dividing rapidly and fusing with existing muscle fibers. The satellite cells then donate their own nuclei to the damaged muscle fibers to help the ailing fibers rebuild. With the donated nuclei, the damaged muscle fibers can more efficiently build new contractile elements. As a result, the muscle cells hypertrophy and can contract with greater contractile force.
The type of muscle contraction that causes the most muscle damage and probably also elicits the most muscle growth are eccentric contractions. As you may know, there are three types of muscle contractions – eccentric, concentric, and isometric. Concentric contractions are those where the muscle shortens against resistance. When you do a biceps curl, the concentric portion is bringing your hands toward your chest. In contrast, eccentric contractions are where the muscle lengthens against resistance. An example would be running downhill or the downward movement of a biceps curl. In the case of biceps curls, you brake as you lower the weight to keep the weight from dropping too quickly. An isometric contraction is where the muscle generates force but the joint doesn’t move. An example is holding a plank.
Research shows eccentric contractions create more muscle fiber damage than concentric ones. You can take advantage of this benefit by emphasizing the eccentric portion of an exercise. To do this, you slow the movement down. For example, a standard rep would be 3-0-2-0. The eccentric, or lowering phase, is 3 seconds, no pause at the bottom of the movement, 2 seconds for the concentric movement, and no pause at the top. You can create more muscle damage, and more growth, by increasing the length of the eccentric phase. For example, 4-0-2-0 or even 5-0-2-0. You can even include a pause at the top and bottom of the movement, for example, 4-1-2-1.
Muscle Hypertrophy: Mechanical Tension
Mechanical tension refers to the amount of tension muscle fibers develop in response to a stimulus. When you lift a weight through its full range-of-motion, you place the muscle under passive and active tension. Active tension is where you’re actively contracting the muscle while passive tension is where the muscle is passively elongated. Muscle tension is necessary to achieve muscle growth but there are different degrees. A large amount of muscle tension would correspond with lifting a very heavy weight, yet you can’t maintain that high degree of mechanical tension very long. For hypertrophy, it’s best to place a moderate amount of mechanical tension on the muscle and increase the time the muscle stays under tension.
You control the time a muscle spends under mechanical tension by the rep speed. If you use a fast tempo, the muscle spends less time under tension than if you slow the tempo down. When you increase the speed of each rep, you can use more weight but you reduce the amount of time the muscle spends under tension. Fast rep speeds and short time under tension is effective for building power. For muscle size and strength, more time under tension using a lighter load provides a stronger stimulus for muscle hypertrophy.
One way trainers use time under tension to spark muscle growth is to choose a set period of time to keep the muscle under tension. For example, if you do 10 reps of biceps curls and it only takes you 3 seconds to complete each rep, your muscles were only under tension for 30 seconds. To increase time under tension to maximize hypertrophy, you slow the tempo down to 4 seconds per rep for a total of 40 seconds of tension.
One thing to remember is mechanical tension equals muscle growth but you have to balance it with another mechanism by which muscle grow – metabolic stress.
Muscle Hypertrophy: Metabolic Stress
You create metabolic stress when you train in a fashion that forces your muscles to use anaerobic pathways, pathways that don’t use oxygen. The best way to do this is to do a moderate to a high number of reps without resting for long periods of time between sets. If you choose a weight that’s a high percentage of your one-rep max where you can only do a few reps, it doesn’t create a lot of metabolic stress – but higher reps will. That burning sensation you get in your muscle is a sign that your muscles are experiencing metabolic stress.
How does metabolic stress lead to growth? With metabolic stress, lactate builds up and muscle cells begin to swell. Metabolic stress also stimulates the release of growth factors called myokines that promote muscle hypertrophy. Another way in which metabolic stress may induce growth is by fatiguing the slow-twitch fibers faster. Once the slow-twitch fibers fatigue, the bigger, fast-twitch fibers come out in force. Remember, muscle fibers are recruited according to size. You start by recruiting slow-twitch fibers. As they fatigue, the fast-twitch fibers are increasingly brought into play. The faster you fatigue the slow-twitchers, the more quickly you can recruit the fast-twitch fibers.
Another way to create more metabolic stress to promote growth is to do two or more sets, using moderate weight, with a brief rest in-between. For example, do a set of an exercise to fatigue. Rest for 10 seconds and do more reps to fatigue.
The Bottom Line
You can manipulate these three mechanisms of muscle growth by how you structure your routine. If you choose a heavy weight (a high percentage of your one-rep max), you maximize mechanical tension but time under tension and metabolic stress will be reduced since you can’t do as many reps with such a heavy load. When you use a lighter weight and a higher number of reps, you create more metabolic stress and time under tension, which favors hypertrophy. In terms of muscle damage, slowing the tempo eccentric portion of the exercise creates more muscle damage and, theoretically, greater growth.
How do you balance it all? The best way to optimize your gains and reduce the risk of overtraining is to periodize your workouts so that you take advantage of these mechanisms. This allows you to use different resistances and tempos during certain training cycles. Ultimately, you’ll get the most benefit by training over a variety of rep ranges and tempos.
Brecontreras.com. “Training for Maximum Muscle Growth Explained”
J Strength Cond Res24(10): 2857–2872, 2010.
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