Mechanical tension is currently the only known stimulus for muscular hypertrophy. Even so, there are two types of mechanical tension: [1] active mechanical tension (which we know is produced by the creation of actin-myosin crossbridges) and [2] passive mechanical tension (which we know is produced by the elongation of the stiff segment of titin). We can see how these two types of mechanical tension contribute to hypertrophy by studying contraction modes and strength curves.
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#1. Eccentric training tends to cause large increases in fascicle length but small increases in muscle cross-sectional area and pennation angle, while concentric training tends to produce the opposite result
When the number of reps and relative loading are both equated, concentric-only and eccentric-only strength training programs tend to produce the same increases in whole muscle volume but the relative contributions of the muscle fiber length increases (as indicated by fascicle length increases) and the increases in muscle fiber cross-sectional area (as indicated by pennation angle increases) do differ.
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These differences can be very pronounced, with very large increases in pennation angle being observed after concentric-only training (alongside small increases in fascicle length) and extremely large increases in fascicle length being observed after eccentric-only training (alongside very small increases in pennation angle).
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The differences between contraction modes can sometimes be so substantial that the eccentric-only training program causes increases in muscle fascicle length but decreases in pennation angle, while the concentric-only training program causes increases in pennation angle but decreases in muscle fascicle length.
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Indeed, this finding is actually quite common after Nordic curl strength training.
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Overall, these results indicate that [1] concentric contractions (which only involve active mechanical tension) only stimulate increases in muscle fiber diameter and do not produce any increases in muscle fiber length. This in turn tells us that passive mechanical tension is the necessary stimulus for causing muscle fibers to increase in length. The results also indicate that [2] eccentric contractions (which involve both passive and active mechanical tension) cause smaller-than-expected increases in muscle fiber diameter when passive mechanical tension is present and stimulating increases in muscle fiber length. It is possible that this means that the creation of passive mechanical tension diverts some of the hypertrophy stimulus away from myofibrillar addition and uses it to produce sarcomerogenesis instead.
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#2. Eccentric training tends to cause larger increases in fascicle length and larger reductions in pennation angle when maximum muscle length is longer
In line with the above results, eccentric-only training can be further manipulated to involve higher levels of passive mechanical tension alongside similar levels of active mechanical tension by performing the contractions to different maximum muscle lengths but through exactly the same range of motion. When eccentric contractions are performed to longer maximum muscle lengths (which causes a larger amount of passive mechanical tension to be generated), there are larger increases in muscle fascicle length and larger decreases in pennation angle.
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#3. Eccentric training tends to cause larger increases in distal region muscle size and concentric training tends to cause larger increases in middle region muscle size
Alongside the greater increases in muscle fascicle length and the smaller increases in pennation angle, eccentric-only training tends to cause greater increases in distal region muscle size and smaller increases in middle/proximal region muscle size, when compared to concentric-only training. Yet, these effects are independent of one another, suggesting that we cannot always attribute increases in distal region muscle size to increases in muscle fascicle length.
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#5. Full range of motion during normal strength training tends to cause larger increases in muscle fascicle length compared to a partial range of motion but similar increases in pennation angle
Research has shown that when the reps and relative loading are both equated, long maximum muscle length strength training exercises cause more hypertrophy than short maximum muscle length strength training exercises. Obviously, this is different from eccentric-only and concentric-only strength training because the addition of passive mechanical tension with the long maximum muscle length has a net positive effect on muscle growth rather than a net zero effect.
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Nevertheless, strength training with a long maximum muscle length also causes greater increases in fascicle length alongside similar increases in pennation angle, which is similar to the observations regarding contraction mode. This suggests that the extra hypertrophy that is stimulated by the long maximum muscle length exercise can be attributed entirely to the increases in muscle fascicle length (which is logical if we think that the primary signaling function of passive mechanical tension is to stimulate an increase in muscle fiber length).
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Finally, it is worth noting that even when long maximum muscle length fails to produce more hypertrophy, it still causes greater increases in muscle fascicle length, while causing smaller increases in pennation angle and physiological cross-sectional area.
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#6. Full range of motion during normal strength training tends to cause larger increases in distal region muscle size, while partial range of motion training tends to cause larger increases in proximal region muscle size
Alongside the greater increases in fascicle length, strength training with a long maximum muscle length tends to cause greater increases in distal region muscle size when compared to strength training with a short maximum muscle length.
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Indeed, normal strength training with a short maximum muscle length can actually fail to cause any meaningful increases in distal region muscle size whatsoever.
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Nevertheless, we again cannot equate increases in distal region muscle size with increases in muscle fascicle length. Indeed, strength training with long maximum muscle length exercises can cause increases in distal region muscle size even though no increases in muscle fascicle length are stimulated.
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Conclusions
Mechanical tension is currently the only known stimulus for muscular hypertrophy. There are two types of mechanical tension: [1] active tension (which is generated by actin-myosin crossbridges) and [2] passive tension (which is produced when the stiff segment of titin elongates). We can study the adaptations produced by active and passive tension by comparing the adaptations after training with [1] eccentric-only and concentric-only contractions, and [2] strength training exercises with long and short maximum muscle lengths. Concentric-only contractions only involve active tension and predominantly stimulate increases in muscle fiber diameter (assessed by pennation angle) and rarely in muscle fiber length (assessed by fascicle length). Eccentric-only contractions involve both active and passive tension and stimulate increases in muscle fiber diameter and muscle fiber length. This tells us that passive tension is likely necessary to increase muscle fiber length. Curiously, eccentric-only contractions stimulate smaller-than-expected increases in muscle fiber diameter for the amount of active tension they produce. This suggests that when passive tension dosages are very high, myofibrillar synthesis rate elevations are diverted towards sarcomerogenesis instead of towards myofibrillar addition, in much the same way as muscle damage inhibits the production of hypertrophy when it is excessively high. Normal strength training with a short maximum muscle length involves less passive tension than normal strength training with a long maximum muscle length and yet it causes similar increases in muscle fiber diameter (assessed by pennation angle) alongside smaller increases in muscle fiber length (assessed by fascicle length). This suggests that the additional muscle growth produced by normal strength training with a long maximum muscle length can be accounted for by sarcomerogenesis.
