Mechanical tension

Mechanical tension is the primary stimulus for muscular hypertrophy. In fact, it is probably the only stimulus for hypertrophy. 

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#1. Why is mechanical tension a stimulus for hypertrophy?

There are two types of mechanical tension: active and passive. Each stimulates a different adaptation. Active mechanical tension (caused by muscular contractions) cause the addition of myofibrils leading to an increase in muscle fiber cross-sectional area. Passive mechanical tension (caused by stretch) causes the addition of sarcomeres in series leading to an increase in muscle fiber length (read more).

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We know that passive mechanical tension can cause an increase in sarcomeres in series without any muscle activation because stretching denervated muscles causes them to grow. Denervated muscle fibers are not able to receive electrical signals from the brain and so cannot be activated. Additionally, they cannot experience any metabolite accumulation due to the supply of energy to the crossbridge cycle. Neither can they experience muscle damage due to the influx and accumulation of calcium ions. For this reason, mechanical tension is the only possible mechanism that can stimulate growth.

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We cannot show that active mechanical tension is able to cause an increase in myofibrils in parallel without any muscle activation because muscle activation is necessary for active mechanical tension to be generated. Nevertheless, we can show that muscle fibers do not grow when they are activated but crossbridge cycling is blocked from happening.

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#2. Why is muscle fiber activation NOT a stimulus for hypertrophy?

Some researchers and commentators have suggested that achieving a high level of motor unit recruitment can stimulate hypertrophy. However, this hypothesis can be dismissed easily, because very high levels of motor unit recruitment can be attained during high-velocity movements. Indeed, research has shown that maximal voluntary activation (the gold standard measurement for motor unit recruitment) is maximal whenever effort is maximal and is not affected by load or velocity.

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Fast movement training programs do not produce hypertrophy. Yet, as shown above, when the fast movements are performed with maximal efforts, they involve high levels of motor unit recruitment. This tells us that high levels of motor unit recruitment cannot alone cause muscle growth.

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Recently, this observation was tested directly. Researchers used a rodent model to assess the differences in hypertrophy caused by two different training programs that involved the exact same dosage of muscle activation but different levels of mechanical tension. The study showed that muscle activation alone was not sufficient to cause hypertrophy. Only muscle activation that occurred together with substantial mechanical tension was enough to bring about hypertrophy.

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When muscle fibers shorten slowly, they produce very high forces due to the greater number of attached myosin motors. When muscle fibers shorten quickly, they produce very low forces due to the smaller number of attached myosin motors. The number of attached myosin motors is determined by the shortening velocity of the muscle fiber because the detachment rate of each myosin motor increases with increasing velocity. For this reason, each myosin remains attached to actin for a shorter period of time as shortening velocity increases.

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#3. How does light load strength training produce hypertrophy?

Some researchers have argued that mechanical tension cannot be the sole mechanism that causes hypertrophy because light load strength training causes the same amount of muscle growth as more moderate loads. This objection fails to understand that muscle force is not the same as muscle fiber mechanical tension. As explained above, the active mechanical tension experienced by muscle fibers is largely determined by the force-velocity relationship and so is independent of load (read more).

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#4. Why is muscle damage NOT a stimulus for hypertrophy?

Some researchers have suggested that causing muscle damage can stimulate hypertrophy. Yet, this hypothesis has now largely been discarded (read more).

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#5. Why is metabolic stress NOT a stimulus for hypertrophy?

Some researchers have suggested that metabolite accumulation can stimulate hypertrophy. Yet, this hypothesis has similarly been discarded (read more).

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Conclusions

Mechanical tension is likely the only stimulus to cause muscle hypertrophy. There are two types of mechanical tension: active and passive. We know that passive mechanical tension causes the addition of sarcomeres in the absence of muscle activation because stretching denervated muscle fibers causes them to increase in length (albeit not in cross-sectional area). In contrast, it is more difficult to separate out the effects of active mechanical tension and muscle activation because activation is needed to form actin-myosin crossbridges. Even so, muscle fibers do not grow properly if they are activated while crossbridge formation is experimentally blocked. Since crossbridge formation is what produces active mechanical tension, this indicates that active mechanical tension and not muscle activation is what stimulates the addition of myofibrils leading to increases in muscle fiber cross-sectional area. Also, human and animal studies have shown that hypertrophy does not occur after training with high-velocity movements even when they involve maximum levels of motor unit recruitment and hence high levels of muscle activation. Since we know that the amount of crossbridge force (and therefore the amount of active mechanical tension)​is determined by the force-velocity relationship, the fact that only slow movements can stimulate muscle growth also strongly implicates active mechanical tension as the driver of increases in muscle fiber size.​