When is strength training at long muscle lengths beneficial?
If you enjoy this article, you will like my second book (see on Amazon).
Strength coaches and bodybuilders often make use of exercises with peak forces in stretched positions and/or full ranges of motion (ROMs) with the goal of maximizing hypertrophy for a muscle that works at a specific joint. While the use of a full ROM was historically often ascribed quasi-magical properties that were mysteriously (but with no obvious basis in physiology) connected to the distance through which the muscle fibers contracted, it is now typically assumed that the use of a full ROM and/or a strength curve that requires peak forces in a stretched position causes greater hypertrophy as a result of greater passive tension.
But is this greater passive tension the only mechanism producing greater hypertrophy as a result of peak forces in a stretched position, or are there other mechanisms? To answer that question, let’s take a look at how the strength curve of an exercise can affect the hypertrophic adaptations that result after strength training.
What does the strength curve do?
Every strength training exercise has a strength curve. The strength curve describes how the difficulty of the exercise changes over the exercise ROM. This difficulty is affected by two main external factors: [1] the way in which the external moment arm length of the external resistance changes over the exercise ROM, and [2] the way in which the external resistance itself changes over the exercise ROM. However, it is also affected by several internal factors that determine the ability of the muscle to produce joint torque at different joint angles, including: [1] the way in which the internal moment arm length of the muscle changes over the joint angle ROM, and [2] the way in which the active and passive length-tension relationships affect muscle force production over the joint angle ROM.
How can the strength curve affect adaptations?
Physiologically, there are three ways in which the strength curve can affect hypertrophic adaptations, owing to [1] the principle of neuromechanical matching, [2] the passive length-tension relationship, and [3] the active length-tension relationship. We can summarize these as follows:
- Activation — the strength curve can affect which muscle is receiving the majority of the central motor command (and consequently achieves the highest level of muscle activation, and therefore the most hypertrophy). The majority of the central motor command is sent to the muscle (or region of a muscle) that has the longest internal moment arm length at the point in the exercise ROM where peak forces are required, in accordance with the principle of neuromechanical matching.
- Passive tension — the strength curve can affect how much passive tension the fibers of a muscle experience. When an exercise requires peak forces in stretched positions, this allows greater passive tension to be produced and experienced. But wait, there is more! Since activation affects the amount of passive tension that can be produced (because titin exerts greater passive tension in an active muscle fiber than in an inactive muscle fiber), when a muscle has poor leverage in the stretched position and is therefore not strongly activated at this joint angle due to the principle of neuromechanical matching, it will not be as positively affected by a strength curve that involves peak forces in stretched positions.
- Active tension — the strength curve can affect the active tension the fibers of a muscle experience. When an exercise requires peak forces at moderate muscle lengths, this allows greater active tension to be produced because the sarcomeres are working on the plateau of the active length-tension relationship. When an exercise requires peak forces at short muscle lengths, this often (but not always) produces less active tension because the sarcomeres are working on the ascending limb. Similarly, when an exercise requires peak forces at long muscle lengths, this often (but not always) produces less active tension because the sarcomeres are working on the descending limb. However! Since the dosage of active tension is limited predominantly by the time to task failure, and since the time to task failure is greatly influenced by the usage of ATP, and since working on the plateau of the active length-tension relationship uses ATP more rapidly than working on the ascending or descending limbs, this benefit is actually quite limited in its effect, and only applies when comparing the plateau region with points extremely far down the ascending or descending limbs, when magnitudes of active mechanical tension are far too small to ever stimulate hypertrophy,
Let’s now look at each of these three mechanisms a bit more closely, along with a couple of worked examples.
[1] How does the strength curve affect the activation of the muscle being trained?
What is the physiology?
Strength curves determine which point in the exercise ROM requires the greatest force. This point in the exercise ROM will correspond to a specific joint angle, and one muscle (or region of a muscle) in the group of muscles that act at that joint will have the longest internal moment arm length at that specific joint angle. This muscle (or region of a muscle) will then be the one that receives the highest level of central motor command during that exercise (even at muscular failure) because of the principle of neuromechanical matching, which states that muscles acting at a joint are activated in proportion to their leverage for producing the joint torque in question. This principle most likely exists because the central nervous system is trying to maximize joint torque for a given level of muscle force (and is therefore minimizing both the perceived level of effort and also the ATP used for the movement). Obviously, achieving the highest level of central motor command will then correspond to that muscle also achieving the highest level of motor unit recruitment. Consequently, more of its muscle fibers will be activated and this will then lead to it experiencing more total hypertrophy.
What is an example?
The back squat involves a peak force at the start of the concentric (lifting) phase because the external moment arm lengths of the barbell on the hip and knee joints are longest at this point. This point in the exercise ROM corresponds to a relatively flexed hip joint angle in the sagittal plane. Thus, while there are three hip extensor groups (the gluteus maximus, adductor magnus, and two-joint hamstrings), we might expect there to be one muscle that achieves higher levels of muscle activation than the other two because of differences in their relative leverages at this high degree of hip flexion. Indeed, since the adductor magnus has the longest internal moment arm length at this point, it is therefore most strongly activated of the three muscle groups. Consequently, the back squat is mainly an adductor magnus exercise at the hip, at least when weight is used as external resistance (changing the external resistance to accommodating resistance in the form of bands or chains would switch the strength curve around such that the most difficult part of the exercise ROM corresponded to a more extended hip joint angle, where the gluteus maximus would then be the primary hip extensor).
[2] How does the strength curve affect the passive force produced by the muscle being trained?
What is the physiology?
Strength curves that involve peak forces in stretched positions provide an opportunity for greater levels of passive tension to be produced (although, as I explained recently, this does not guarantee that greater levels of passive tension will be experienced by all muscle groups, if their sarcomere lengths do not reach the descending limb of the length-tension relationship). Obviously, this greater passive tension causes more hypertrophy to be stimulated, as this creates greater overall mechanical tension. Nevertheless, the way in which passive tension is generated during strength training is dependent on the trained muscle being active (since titin behaves differently when the muscle fiber is active than when it is inactive). Consequently, the principle of neuromechanical matching (and therefore the extent to which the muscle is activated) also impacts the ability of the muscle to experience passive tension. When a muscle at a joint is not strongly activated because it has a short internal moment arm length compared to other muscles working at the same joint, it will not experience as much passive tension as we might expect, despite apparently being “trained at a longer muscle length” in the exercise.
N.B. It is important to note that when strength training with exercises that involve peak forces at long muscle lengths, the nature of the muscle growth is slightly different because it is longitudinal rather than transverse hypertrophy (and, as I also explained recently, these two types of muscle growth have different long-term training implications). Moreover, the appearance of the muscle growth will also differ slightly, because longitudinal hypertrophy will present mainly as distal region muscle growth, while transverse hypertrophy will present mainly as proximal and/or middle region muscle growth.
What is an example?
The back squat involves a peak force at the start of the concentric (lifting) phase because the external moment arm lengths of the barbell on the hip and knee joints are the longest at this point. This point in the exercise ROM corresponds to a relatively flexed hip joint, which means that the gluteus maximus is apparently being trained at a long muscle length by this exercise. Thus, whatever gluteus maximus muscle fibers are activated at the start of the concentric (lifting) phase of the back squat will certainly experience high passive forces (since the gluteus maximus very likely does have sarcomeres that reach well down the descending limb of the length-tension relationship at long muscle lengths) and this has the potential to cause much greater hypertrophy *of these muscle fibers* than would be possible with the same number of reps with exercises that involve peak forces being required at short muscle lengths. Yet, the number of muscle fibers that are active will be lower than we might expect because both the adductor magnus and the two-joint hamstrings have better leverage for hip extension than the gluteus maximus at flexed hip joint angles, due to the principle of neuromechanical matching. Therefore, our ability to cause the muscle fibers of the gluteus maximus to experience passive tension is somewhat limited.
[3] How does the strength curve affect the active force of the muscle being trained?
What is the physiology?
Strength curves that involve peak forces in contracted or moderate length positions (that correspond to the sarcomeres of the muscle working on the plateau region of the length-tension relationship) provide an opportunity for greater magnitudes of active tension to be produced. We might expect that this will then cause more hypertrophy to occur. Yet, the greater magnitude of active tension at this muscle length occurs because of greater crossbridge formation (and therefore greater ATP usage), which means that the rate of metabolite accumulation is more rapid and so peripheral fatigue builds more quickly. Consequently, muscular failure occurs more quickly and so the total active force-time integral for the activated muscle fibers of high-threshold motor units (which is a very good indicator of the true dosage of active mechanical tension experienced by a muscle during exercise) is probably not substantially different when working on the plateau region of the active length-tension relationship compared to when working on the ascending or descending limbs. Thus, with one important exception, overall hypertrophy caused by active mechanical tension is unlikely to be different as a result of working on the plateau region compared to when working on the ascending or descending limbs of the active length-tension relationship, assuming that muscle activation is not different (and muscle activation is again largely determined by the principle of neuromechanical matching).
Although the total active force-time integral for the activated muscle fibers of high-threshold motor units is probably not that different when doing [A] strength training sets to failure with exercises that involve peak forces on the plateau region of the active length-tension relationship compared to when performing [B] strength training sets to failure with exercises that involve peak forces on either the ascending or descending limbs, the nature of the hypertrophy stimulus is such that there is threshold of mechanical tension that must be exceeded for muscle fibers to be stimulated to grow. Alone, a certain force-time integral does not guarantee that a muscle fiber will experience hypertrophy. The magnitude of the mechanical tension must also be sufficiently high. Consequently, it is very likely that hypertrophy is not stimulated by active mechanical tension when muscles work at the start of the ascending limb of the active length-tension relationship (or indeed at the very end of the descending limb, although this is much less common). Indeed, this situation is called “active insufficiency” since the muscle fiber can be activated but the extremely short muscle sarcomere length makes it impossible for a meaningful amount of force to be produced. For this reason, it is important to note that some muscles may grow better at long muscle lengths due to their being able to reach sufficiently high levels of active mechanical tension only at these long muscle lengths (and not due to the passive tension produced).
N.B. It is important to note that when strength training with exercises that involve peak forces at short muscle lengths, the nature of the muscle growth is going to be exclusively transverse hypertrophy rather than a mixture of both transverse and longitudinal hypertrophy (and, as I explained recently, these two types of muscle growth have different long-term training implications). Also, the appearance of the muscle growth will also differ slightly, because longitudinal hypertrophy will present mainly as distal region muscle growth, while transverse hypertrophy will present mainly as proximal and/or middle region muscle growth.
What is an example?
During calf raise variations, the soleus and gastrocnemius both contribute to plantar-flexion torque. But, while the soleus probably works across the ascending limb, plateau and descending limb of the active length-tension relationship, the gastrocnemius seems to work mainly on the ascending limb. By working mainly on the ascending limb, it seems quite likely that this muscle enters into active insufficiency extremely easily (which is why seated calf raises are often employed to target the soleus). More importantly, the muscle likely benefits from peak forces in stretched positions not in fact because it has the ability to experience passive tension in these positions, but rather because those are the positions necessary for the required threshold of active mechanical tension to be reached, such that hypertrophy is stimulated.
What does this mean in practice?
We can train muscles at different muscle lengths by choosing exercises that have different strength curves. Physiologically, there are three ways in which the strength curve can affect adaptations, by means of [1] the principle of neuromechanical matching, [2] the passive length-tension relationship, and [3] the active length-tension relationship. Since the the principle of neuromechanical matching determines the extent to which a muscle is activated at a joint and since both passive and active forces depend upon muscle activation, the principle of neuromechanical matching is by far the most important. Therefore, if a muscle has poor leverage for producing joint torque at a given point in an exercise range of motion, it will not be trained effectively, regardless of whether it displays [A] the potential for passive tension by sarcomeres that work on the descending limb of the length-tension relationship, or [B] an optimal sarcomere length for active force production on the plateau of the length-tension relationship.
If you enjoy this article, you will like my second book (see on Amazon).
