The Biceps Length-Tension Relationship

In recent years, the scientific literature is increasingly finding that achieving a stretch of a muscle during an exercise is powerful for building muscle.

However, some people have proposed that not every muscle will grow more when exposed to a stretch during an exercise. More precisely, they believe that the working sarcomere length of a muscle can inform us of whether that muscle will grow more when exposed to stretch.

The biceps is one examples of these muscles. Some people believe that the biceps’ working sarcomere length suggests this muscle shouldn’t grow more when exposed to stretch.

In this article, I’m going to share my views on whether the data on the bicep’s working sarcomere length is truly strong enough for us to believe this muscle shouldn’t grow more when exposed to a stretch during an exercise.

Firstly though, an understanding of the length-tension relationship is needed. Feel free to gloss over this part if you’re already well aware of it.

Understanding The Length-Tension Relationship

Within muscles are sarcomeres, and sarcomeres are the force generating units of muscles.

Sarcomeres have the ability to generate active tension and passive tension. Note, tension is analogous to force in this case.

Active Tension

Active tension occurs when the myosin head extends from the myosin filament and pulls the actin filament towards the M-line, shortening the length of the sarcomere.

This action produces force that is partly transmitted longitudinally, but mainly laterally to the extracellular matrix (surrounding muscle fibers). These forces end up pulling on the tendon, resulting in muscle contraction.

Passive Tension

Passive tension is the force produced during stretch. Think of a rubber band, when you stretch a rubber band, it produces passive tension to try and resist that stretch. When you let go of the rubber band, that passive tension will snap it back into its original shape.

Titin is a protein within the sarcomere that produces passive tension when the sarcomere is stretched.

The Length-Tension Relationship

Now that we hopefully understand active and passive tension from the sarcomere, we can introduce the length-tension relationship.

In this relationship, the length of a sarcomere is on the x axis, while the amount of tension (analogous to force) produced by a sarcomere is on the y-axis.

We can see there are two components to it. Active tension is blue, and passive tension in green.

With passive tension, we can see it sharply increases at longer sarcomere lengths. This makes sense, since passive tenson is force from the stretch of titin, we’re going to want the sarcomere to be stretched for titin to be stretched.

Note, in all of the length tension relationship graphs shown in this article, passive tension begins to develop at a sarcomere length of 2.5μm. This would likely not be the case with all muscles. evidence indicates the passive tension curve is variable (much more so than the active tension curve), so keep this in mind.

With the active tension component of the length-tension relationship, there are generally 5 distinct components to it.

1) Excessively short sarcomere lengths result in little to no force production. At this length, the opposite actin filaments overlap with one another to some degree. This overlap prevents many of the myosin heads from extending and pulling on the correct actin filament. As a result, little force is produced.

2) Increasing the length of the sarcomere slightly removes some of the overlap between the opposite actin filaments, allowing some of the myosin heads to extend and pull on the correct actin filament. Therefore, the sarcomere can generate some force.

3) Increasing the sarcomere length slightly again removes any opposing actin filament overlap. Consequently, all the myosin heads can extend and pull the actin filament towards the M-line. This particular region of the length-tension relationship is known as the optimal length, as it is the length that allows the highest active tension. As a note, the optimal length isn’t one particular length, it is a region (2.6-2.8μm in humans), the small plateau region on the graph demonstrates this.

4) Further increasing the length of the sarcomere results in the actin filaments moving away from the myosin filament. This means that some myosin heads are not overlapping with the actin filament, meaning they cannot extend and pull the actin filament. Because fewer myosin heads can contribute to force here, active force production is less.

5) At this point, there is no overlap between the myosin heads and actin filaments, thus no myosin head can extend and pull the actin filament. Accordingly, no more active tension can be produced.

Passive Tension & Muscle Hypertrophy

It turns out within and around your muscle fibers are mechanosensors that are able to “detect” both active and passive tension, and convert this tension into a hypertrophy signal.

Therefore, to optimize the hypertrophy signal, it is favorable to have both active and passive tension being involved in an exercise.

The Working Sarcomere Lengths of the Biceps

It may be logical to assume that when the whole muscle is fully shortened (like at the top of a biceps curl), all the sarcomeres within the biceps are fully shortened. Likewise, when the muscle is stretched (like at the bottom of a biceps curl), all the sarcomeres within the biceps are stretched.

Although this is a very reasonable assumption, it isn’t actually always the case.

Different muscles have their sarcomeres organized in different ways that directly impact the length their sarcomeres can work through during any movement.

With the biceps, this study by Murrary et al. indicates when the elbow is flexed to 120 degrees (near the biceps being fully shortened), the sarcomeres of the biceps short and long head are at the very start of the length tension-relationship. However, when the elbow is more straightened out to 20 degrees of elbow flexion (when the biceps are stretched to a degree), the sarcomeres of the biceps short and long head are at the middle area of the length-tension relationship (near the plateau region of the active tension component).

In other words, despite the biceps being stretched to a degree when the elbows are flexed to 20 degrees, its sarcomeres are not stretched fully.

Therefore, some people have used this data to suggest since the sarcomeres are not stretched (presumably to the point where signficant passive tension can occur from titin), this means the biceps should not grow more when exposed to stretch during an exercise.

However, I think there are some important limitations people are missing if they make this conclusion.

Limitations with the Working Sarcomere Length Data

1. It’s Important to know that the research measuring the working sarcomere length of the biceps did so during isometric contractions (static contractions). This is a notable point because people commonly train the biceps with dynamic exercises that involve eccentric contractions (when the muscle is lengthening while producing force), and it seems that during eccentric contractions, titin is might actually be capable of producing passive tension earlier and to a greater magnitude. Additionally, recall how I noted earlier the precise sarcomere length at which passive tension is generated is variable between muscles. What this implies is we can’t rule out the possibilty that although the sarcomeres of the biceps may only reach the middle portion of the length-tension relationship, titin may still be generating some decent passive tension during biceps exercises.
2. This working sarcomere length data also ignores the potential interaction at other joints. Remember it only looked at the sarcomere length of the biceps during elbow flexion (isometrically) at 120 and 20 degrees of elbow flexion. However, we know that the biceps originate from the scapula, and will actually be stretched more when the shoulders are extended. What this means is that if we lengthen the biceps with simultaneous shoulder extension (such as done during an incline curl or cable face away curl), perhaps the sarcomeres reach an even greater length, permitting passive tension from titin.
3. Perhaps most importantly (in my view), the research evaluating the working sarcomere length of the biceps only look at a small proportion of sarcomeres relative to the overall muscle. This is problematic as it’s likely not all the sarcomeres within a single muscle go through identical lengths during a movement . In other words, when a whole muscle is stretched, it’s possible for some of its sarcomeres to be stretched quite a bit and other sarcomeres to not be stretched as much. This is termed sarcomere non-uniformity. Therefore, it is very possible (if not almost certainly the case) that if we looked at all the sarcomeres within the biceps, we’d discover various sarcomeres do indeed get signficantly stretched.
4. It’s also worth pointing out that titin isn’t the only source of passive tension. Although it’s possible titin is the predominant source of passive tension in the human body, the extracellular matrix surrounding muscle fibers is also capable of generating passive tension, and this extracellular matrix generated passive tension may also be capable of signaling hypertrophy too.
5. Finally, we still do not know everything about muscle hypertrophy. It is very possible that there are other reasons as to why stretch during an exercise could be beneficial for muscle hypertrophy.

Due to these range of very strong limitations, I simply do not believe we can use the current research on the working sarcomere lengths of the biceps to definitively conclude this muscle should not grow more when exposed to a stretch during an exercise.