Why is there passive tension during the concentric phase of normal strength training exercises?
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Hypertrophy is produced when muscle fibers experience mechanical tension and their mechanoreceptors detect this tension and commence signaling processes that cause an increase in muscle protein synthesis rates. This then causes either an increase in the number of myofibrils in parallel or an increase in the number of sarcomeres in series. The mechanical tension can be created either actively (by the formation of crossbridges) or it can be created passively (by the stretching of titin). It is often assumed that titin only provides passive force during the eccentric phase, but this is not actually true. Let’s take a quick look at how titin produces passive forces in the concentric phase.
How does titin create passive tension while muscle fibers are lengthening?
Titin molecules comprise three segments (a compliant segment, a bridge segment, and a stiff segment). These segments are arranged in series. When the compliant segment is elongated, this produces minimal resistance, but when the stiff segment of titin is stretched, it produces a high passive force, much like a strong elastic band. Since the segments are arranged in series, the compliant segment elongates in preference to the stiff segment, unless it is already fully lengthened or unless it is prevented from lengthening.
When a muscle fiber is lengthened passively, its titin molecules elongate over the majority of the joint angle range of motion by lengthening the compliant segments. Obviously, this does not produce very much force. Only towards the end of the joint angle range of motion do the compliant segments run out of capacity to elongate. At this point, the stiff segments must instead begin to elongate. Consequently, static stretching only creates passive forces inside muscle fibers when they are stretched to very long lengths. And, although static stretching is not usually considered as a method for producing hypertrophy, these passive forces do still cause hypertrophy in both humans and animal models, because titin also functions as a mechanoreceptor and detects the tension that it itself generates.
When a muscle fiber is lengthened actively (which is commonly called an eccentric contraction), the compliant segments of titin cannot lengthen. This is because the presence of calcium ions causes the bridge segment of each titin molecule to attach to the nearest actin myofilament. Consequently, only the stiff segment of titin can now elongate as the muscle fiber is lengthened. Obviously, this produces a very high force. Indeed, eccentric contractions create far larger passive forces inside muscle fibers in comparison to static stretching, because the stiff segment of titin is stretched over the whole joint angle range of motion, rather than just for a small distance at the end of the joint angle range of motion. Indeed, this is why eccentric contractions can achieve higher muscle forces than concentric or isometric contractions. In eccentric contractions, there is a high level of passive tension provided by stretching the stiff segment as well as the active tension provided by the formation of crossbridges. In concentric and isometric contractions, there is typically only active tension provided by the formation of crossbridges.
N.B. In this analysis, we are talking about muscle fibers and not muscles. Muscle behavior is more complex, because it is made up of a large number of muscle fibers, some of which are activated and some of which are not. During the lowering phase of a normal strength training movement, the muscle fibers of low-threshold motor units (and some of the smaller high-threshold motor units) are activated, while the muscle fibers of most of the high-threshold motor units are not activated. Obviously, titin will behave differently depending on whether the muscle fiber is activated or not. Therefore, the muscle fibers of the low-threshold motor units (and also some of the smaller high-threshold motor units) will experience high passive tension all the way through the entire lowering phase, because they are performing eccentric contractions, while the muscle fibers of most of the high-threshold motor units will experience only small amounts of passive tension, because they are essentially only being passively stretched.
How does titin create passive tension while muscle fibers are shortening?
The stiff segment of titin functions exactly like an elastic band. Elastic bands produce passive forces while they are being elongated (indeed, they always try to return to their starting length). However, elastic bands also produce passive forces when they are held static at a long length, and they continue producing passive forces while they are being shortened back to their starting length. In fact, the only time they stop producing a passive force is when they have actually returned to their starting length.
Consequently, we can identify three scenarios, depending on what happens immediately before a concentric contraction.
Firstly, when a muscle fiber is lengthened passively to a short or even to a moderately-short length, only the compliant segments of its titin molecules are lengthened. The stiff segment never lengthens. This means that when the muscle fiber is activated and begins shortening to produce a concentric phase, no passive force is produced. In practice, this means that concentric-only partial range of motion exercises will involve zero passive force production (and this will very likely lead to hypertrophy occurring exclusively by means of myofibrillar addition).
Secondly, whenever a muscle fiber is lengthened passively to a long or even a moderately-long length, the stiff segments of its titin molecules will be lengthened slightly, because the compliant segment will have reached its maximal length such that the stiff segment has to lengthen. This means that when the muscle fiber is activated and subsequently begins shortening to produce a concentric phase, a small passive force is produced. In practice, this means that concentric-only full range of motion exercises will always involve some small amount of passive force production (and this will likely lead to hypertrophy occurring mainly from myofibrillar addition, but potentially also with some sarcomerogenesis as well).
Thirdly, whenever a muscle fiber is lengthened actively in an eccentric contraction, the stiff segments of its titin molecules will be lengthened substantially, because the compliant segments will be fixed in place. This means that when the muscle fiber subsequently begins shortening to produce a concentric phase, a large passive force is produced (and this is called the residual force enhancement effect in exercise science). In practice, this means that stretch-shortening cycle exercises will always involve a meaningful amount of passive force production in the concentric phase as well as in the preceding eccentric phase (and together these passive forces will likely lead to hypertrophy occurring from both myofibrillar addition as well as from sarcomerogenesis).
N.B. Whenever a muscle fiber is deactivated between an eccentric contraction and a subsequent concentric contraction, calcium ions are removed from the cytoplasm. Thus, the bridge segment of titin detaches from the nearest actin myofilament. Consequently, the compliant segment of titin is freed to move and it immediately increases to a long length, while the stiff segment returns to its starting length. This stops passive tension from being experienced in the subsequent concentric contraction (and is one of the reasons why pauses between eccentric and concentric phases makes the concentric phase harder).
What determines the amount of passive tension produced when muscle fibers are shortened?
The amount of passive tension generated in the concentric phase of normal strength training exercises (which typically involve stretch-shortening cycles and therefore can be modeled by scenario three in the above section) depends on three factors as follows:
(1) When more muscle fibers are activated in the preceding lowering phase, more muscle fibers will experience eccentric contractions rather than passive elongations, and therefore more stiff segments of titin will be elongated to very long lengths, thereby producing very high forces. These forces will then be available in the subsequent lifting phase. In practice, this means that heavy loads will lead to higher passive forces than lighter loads even when training to failure, since the level of activation in the lowering phase is higher (indeed, the peripheral fatigue produced by lifting light loads does not promote greatly increased activation in the lowering phase, because it is metabolite-related fatigue and so mainly affects muscle fiber shortening velocity).
(2) When muscle fibers are stretched to longer lengths or over longer ranges of motion in lowering phase, their stiff segments of titin will be elongated to longer lengths, thereby producing higher forces. These higher forces will then be available in the subsequent lifting phase. In practice, this means that greater passive tension in concentric phases will be achieved whenever we are training with larger ranges of motion or to longer maximum muscle lengths.
(3) When muscle fibers are stretched during exercises with flatter strength curves, the number of muscle fibers (and therefore the number of stiff segments of titin) that experience active lengthening for the entirety of the exercise range of motion is increased.
What does this mean?
Muscle fibers produce and experience passive forces in the concentric phases of strength training movements, particularly when those movements are done with normal stretch-shortening cycles (as in, with eccentric phases followed immediately by concentric phases). More muscle fibers will experience those passive forces in the concentric phase when the previous lowering phase involves a higher level of muscle activation (as when using heavier loads). And, the passive forces in the concentric phase will be higher when the muscle fibers are activated and stretched in the previous lowering phase for longer ranges of motion or to longer maximum muscle lengths. And, the passive forces in the concentric phase will be higher when using flatter strength curves, because this makes sure that the muscle fibers are actually active throughout the entire preceding eccentric phase and not just at the end.