Why does the number of sarcomeres in series differ between muscles (and why does this matter)?
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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 (MPS) rates. When the mechanoreceptors detect passive tension, they direct most of this increase in MPS rates to increase the number of sarcomeres in series (this is known as “sarcomerogenesis”). In contrast, when the the mechanoreceptors detect active tension, they tend to direct most of this increase in MPS rates to increase in the number of myofibrils in parallel.
The amount of passive tension that is produced by a muscle fiber during either a passive, static stretch or during a muscular contraction is determined by the extent to which the stiff segments of titin molecules inside the muscle fiber are stretched. And this means that the number of sarcomeres that comprise the muscle fiber is a key determinant of the amount of passive tension that can be generated (and therefore the amount of sarcomerogenesis that can be stimulated). Let’s take a close look at why this is the case.
How does the number of sarcomeres in series affect passive tension?
Introduction
We can understand how the number of sarcomeres in series affects the amount of passive tension (and therefore the amount of longitudinal hypertrophy, or sarcomerogenesis) by comparing the effects of muscle lengthening during static stretching in muscle fibers with few sarcomeres in series and in muscle fibers with many sarcomeres in series.
Muscle fibers with few sarcomeres in series
When a muscle fiber contains a relatively small number of sarcomeres, each sarcomere will already be somewhat stretched when the muscle is sitting at a relatively short length. Consequently, when the muscle is stretched, it quickly reaches a long sarcomere length. After this point, the compliant segment of titin can no longer extend, so the stiff segment must extend instead. This starts to create passive tension, which then stimulates sarcomerogenesis (it is worth noting that this point tends to correspond to what is described as the “descending limb” of the active length-tension relationship). This means that [1] the muscle fiber will begin to experience passive tension long before the muscle itself is lengthened, and [2] the muscle fiber will produce a very large amount of passive tension when the muscle does finally reach its maximum length.
Muscle fibers with many sarcomeres in series
When a muscle fiber contains a very large number of sarcomeres, each sarcomere will be very short when the muscle is sitting at a relatively short length. Consequently, even when the muscle is stretched, it takes a long time to reach a long sarcomere length. Indeed, in some cases, it may not actually reach a long sarcomere length. This means that the compliant segment of titin extends for most of the time while the muscle fiber is lengthening, such that the stiff segment rarely extends very far. This means that very little passive tension is ever generated, which means that little sarcomerogenesis occurs. (in other words, the sarcomeres of the muscle fiber do not work on the descending limb of the length-tension relationship). This means that [1] the muscle fiber will rarely experience much passive tension, even when the muscle itself is lengthened, and [2] the muscle fiber will create minimal passive tension when the muscle does finally reach its maximum length.
When does the number of sarcomeres in series differ between muscles?
Introduction
Several studies have shown that the number of sarcomeres in series differs between muscles, such that the amount of passive tension (and therefore the amount of sarcomerogenesis) varies between those muscles, even when they are stretched to their maximum length. We can observe this by looking at the length-tension relationships.
If the muscle fibers of a muscle display only ascending limb and plateau phases of the active length-tension relationship, they will likely not display very much passive tension even when the muscle itself is stretched to its maximum length. Therefore, they will not respond to stretched position exercises by increasing to a greater extent in muscle size. Conversely, if the muscle fibers of a muscle display only plateau and descending limb phases of the active length-tension relationship, they will likely display a large amount of passive tension even before the muscle itself is stretched to its maximum length. Therefore, they will respond very strongly to stretched position exercises by increasing to a greater extent in muscle size.
Differences between muscle groups
Different muscle groups display different length-tension relationships, and this is reflected in their tendency to produce extra hypertrophy when using full range of motion (ROM) exercises (compared to partial ROM exercises), when using exercises with peak forces in the stretched position (compared to exercises with peak forces in the contracted position), or even when using eccentric exercises (compared to concentric exercises). For example, the quadriceps display the plateau and descending limb phases of the active length-tension relationship, and therefore tend to grow much better after using either full ROM exercises or exercises with peak forces in the stretched position. Conversely, the triceps display the ascending limb and plateau phases of the active length-tension relationship, and therefore tend to grow similarly (or even slightly better) after using full or partial ROM exercises or exercises with peak forces in the stretched or contracted positions.
Differences between muscles within a group
Different muscles within a group also display different length-tension relationships, and this is sometimes (but not always) reflected in their tendency to produce extra hypertrophy when using exercises that involve high levels of passive tension. For example, the vastus medialis displays an active length-tension relationship that involves reaching the descending limb much earlier than the other quadriceps. Thus, it displays passive tension earlier, and a larger amount of passive tension at maximum muscle lengths. For this reason, it also displays more hypertrophy after eccentric training compared to the other quadriceps muscles. And since the internal moment arm lengths of the three single-joint quadriceps are very similar, we might also expect the vastus medialis to grow more after any normal strength training exercise with full ROMs or with peak forces in the stretched position. Similarly, the brachioradialis displays an active length-tension relationship that involves reaching the descending limb much earlier than the other elbow flexors. Thus, it could display passive tension earlier, and it could achieve a larger amount of passive tension at maximum muscle lengths. For this reason, it would probably display more hypertrophy after eccentric training compared to the other elbow flexor muscles (indeed, the brachioradialis does fatigue more than the biceps brachii during eccentric contractions). Nevertheless, since the internal moment arm length of the brachioradialis is shorter than that of the biceps brachii at long muscle lengths (such that it is not trained very effectively at long lengths), we might not expect the brachioradialis to grow more after any normal strength training exercise with full ROMs or with peak forces in the stretched position!
Why does the number of sarcomeres in series differ between muscles?
To understand why the number of sarcomeres in series might differ between muscles, we can look at where the differences exist. Unfortunately, these observations do not give a complete answer, but they do provide several factors that likely contribute.
Firstly, we know that the quadriceps operate mainly on the descending limb of the length-tension relationship, while the triceps brachii (and biceps brachii) operate mainly on the ascending limb and plateau phase. It is noteworthy that the quadriceps tend to sit at rest in the contracted position, while the biceps brachii tend to rest in the lengthened position, which might suggest that resting muscle length is important for determining the number of sarcomeres. Indeed, early immobilization studies showed that resting at long muscle lengths tends to increase the number of sarcomeres in series, while resting at short muscle lengths tends to decrease the number of sarcomeres in series. Nevertheless, this cannot be the full picture, because the triceps brachii also sit at rest in the contracted position, and this muscle does not operate on the descending limb.
Secondly, we know that the brachioradialis operates further down on the descending limb of the length-tension relationship compared to the other elbow flexors. It is noteworthy that the brachioradialis has much worse leverage than the other elbow flexors in the stretched position (full elbow extension). Therefore, it will receive a smaller amount of central motor command when the elbow is extended and the muscle group is stretched, due to the principle of neuromechanical matching. For this reason, it will not be stimulated to increase its sarcomeres in series to the same extent as the other muscles within the elbow flexor group. Nevertheless, this again cannot be the full picture, because the vastus medialis does not receive a different amount of central motor command from the other single-joint quadriceps when the knee is flexed and the muscle group is stretched.
What does this mean in practice?
Muscle fibers produce and experience passive forces during static stretches as well as in strength training movements. The magnitude of those passive forces (and therefore the amount of longitudinal hypertrophy or sarcomerogenesis that is stimulated) is determined by the number of sarcomeres in series, because this determines [1] how early in the lengthening phase of the contraction the stiff segment of titin starts to stretch, and [2] how far at the end of the lengthening phase of the contraction the stiff segment of titin stretches. The number of sarcomeres in series can vary both between muscle groups and also between muscles of the same group, and the research literature can therefore tell us how useful full range of motion or stretched position exercises are likely to be for training any particular muscle. It is not clear exactly why the number of sarcomeres in series varies between muscles, but it may be related to the customary resting length of the muscle and the extent to which the muscle is actually activated at long muscle lengths (in accordance with the principle of neuromechanical matching).
