We weight train to get stronger and to force our muscles to grow. That’s important since we wage a relentless battle with muscle loss after the age of 30. Loss of muscle tissue continues throughout each decade of life, averaging about 4% per decade. This loss of muscle adds up over time and increases the risk of sarcopenia and frailty. Just as importantly, muscle tissue is linked with better metabolic health. Having more muscle tissue improves how your body handles glucose and also helps you avoid a drop in resting metabolic rate that goes along with aging.
Fortunately, weight training slows the age-related loss of muscle tissue. Of course, when we train, we want to maximize the results we get from our workouts. To get muscles to grow, we need to stress them beyond what they’re accustomed to – to the point that they sustain microscopic tears. This type of microdamage elicits a chain of reactions that ultimately lead to increased muscle protein synthesis and increases in muscle size and strength.
Mechanical Tension: Making Muscles Grow
What is the key to getting muscles to grow? One stimulus for muscle growth is mechanical tension. In fact, some experts believe that high mechanical tension is the primary stimulus for muscle hypertrophy. The other two components are muscle damage and metabolic stress.
What is mechanical tension anyway? Mechanical tension is the force you place on your muscles under when you contract them against resistance. Muscles have receptors called mechanosensors that sense how much force the muscle is under and the length of time it sustains that tension. Once these sensors detect tension, it sets off a cascade of chemical reactions that ultimately cause the muscle to grow.
The force a muscle is exposed to is one factor. For example, studies show that eccentric contractions stimulate mechanosensors more than concentric or isometric contraction, whereas passive tension elicits the least. Passive tension is where you stretch a muscle without allowing it to contract. When you do a full repetition of an exercise, for example, like bicep curls, the muscles experience both active and passive tension as the muscles are activated but also stretched.
There’s also the issue of time under tension. This refers to how long the muscle sustains tension. When you lift or work your muscles against resistance using a slow tempo, the muscle stays under tension longer. The thought is that more time under tension, up to a point, provides a greater stimulus for muscle growth. Some experts believe you need 30 to 60 seconds of time under tension for size gains but that 60 to 90 seconds is optimal.
Let’s say you’re doing triceps kickbacks and you complete 10 reps with the concentric portion taking 3 seconds and the eccentric portion 3 seconds, the total time under tension would be 60 seconds. That would be in the range that’s optimal for muscle hypertrophy.
But, if you’re using a slower tempo and keeping a muscle under tension longer, the muscle won’t be able to handle as much weight without fatiguing. So, when you do slow reps and hold the tension for a greater period of time, you’ll need to use a lighter weight and lift at a lower percentage of your one-rep max. As such, focusing exclusively on time-under-tension isn’t necessarily the best approach for strength gains. When you’re trying to build maximal strength, lifting at a high percentage of one-rep max is the most expedient way to train.
What about Metabolic Stress?
It’s also important to remember that a long time under tension creates more metabolic stress, another stimulus for muscle growth. Metabolic stress is triggered by metabolites that accumulate in the muscle during intense exercise. These metabolites include lactate, phosphate, and hydrogen ions. Studies suggest that metabolite build-up also signals the muscle to grow. Metabolites typically accumulate in the muscle during exercise that lasts between 20 seconds and 2 minutes as they are by-products of glycolytic pathways within the muscle cell. Muscle cells predominantly use glycolytic pathways for exercise that lasts at least 20 seconds but less than 2 minutes.
In contrast, when you lift a weight at a high percentage of your one-rep max to build strength, muscle cells mainly use stored ATP and the phosphagen system for ATP (the energy currency that drives contractions) as opposed to the glycolytic cycle, so these by-products don’t build up and metabolic stress is minimized. So, metabolic stress and time under tension are strategies for gains in muscle size but are less effective for building strength.
Does more time under tension equate to more muscle growth? A study published in the Journal of Physiology found that increasing time under tension was associated with greater muscle protein synthesis. Participants who performed a leg extension exercise using a lighter load (30% of maximal strength) with a slow tempo (6 seconds concentric and 6 seconds eccentric) to fatigue experienced a greater increase in muscle protein synthesis than those who did the same exercise using a 1 second concentric and 1 second eccentric. Interestingly, the increase in protein synthesis was delayed. Normally, you’d expect to see a boost in protein synthesis within 6 hours of training but, in the study, the increase happened at 24 to 30 hours after the workout.
The Bottom Line
Keeping your muscles under tension longer isn’t the most efficient way to build strength, but it boosts muscle protein synthesis and helps you gain muscle size. If you use a slower tempo, focus more on reducing the tempo of the eccentric portion of the rep as this is when the greatest muscle damage occurs. Greater damage helps maximize muscle growth. Be prepared for more muscle soreness as well. Slow, eccentric training is associated with greater delayed-onset muscle soreness or DOMS.
Science and Development of Muscle Hypertrophy. Brad Schoenfeld. Human Kinetics. 2016.
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