Exercise strength curves describe the way in which the difficulty of a movement changes over its range of motion.
Strength curves can be flat, in which case the exercise is similarly difficult over the whole range of motion, or varying, in which case the exercise is more difficult at one point than at others.
Although we tend to think of an exercise strength curve as being a single relationship, it is actually determined by two underlying relationships: (1) the way in which our capacity for producing force changes across the joint angles used in an exercise, and (2) the way that the requirement for force production changes over the exercise range of motion.
So how do these relationships affect hypertrophy?
What determines our capacity for producing a turning force at any given joint angle?
When we rotate our limb segments about a joint, we do so by producing a turning force, which is called a joint torque.
By testing our isometric strength at multiple joint angles, we can record a joint torque-angle curve that describes how our capacity for producing a turning force at a joint changes as the limb segment rotates.
There are four main factors that determine the shape of a joint torque-angle curve:
Internal moment arm length — each muscle has a certain leverage on its joint to rotate it in any given direction, and this leverage depends upon its size and its anatomical position relative to the joint center. Since a muscle moves and changes shape as the joint rotates, its leverage changes, and so the amount of muscle force required to produce a given joint torque changes. When the muscle has good leverage, the joint produces a high torque for a given muscle force. When the same muscle has poor leverage, the joint produces a small torque for the same muscle force.
Muscle activation — in order to produce force, a muscle must receive a signal from the central nervous system. The size of this signal determines the voluntary activation, which is the proportion of the total number of motor units that are recruited. Voluntary activation changes with joint angle, which means that voluntary and involuntary joint torque-angle curves can differ from one another. In other words, a muscle may be capable of producing more force at a given joint angle than it actually does, because voluntary activation is reduced at that point.
Length-tension relationship — each muscle contains fascicles, which are bundles of muscle fibers. Muscle fibers change their capacity to produce force depending on their length, because they are made up of long strings of sarcomeres. Each sarcomere produces force according to the amount of overlap between the actin and myosin crossbridges, and according to the amount of stretch of its passive elements. This combination of factors tends to make a fascicle produce a high level of force when it is at a moderate length (when there is maximum overlap between the actin and myosin myofilaments of each sarcomere) and also at a long length (when the passive elements are very stretched).
Antagonist muscle activation — during some multi-joint exercises, the behavior of two-joint muscles at neighboring joints can lead to changes in the muscle activation of antagonist (opposing) muscles. For example, in the squat, the activation of the hamstrings seems to increase with increasing depth, when lifting heavy weights, and this will increase the quadriceps muscle force that is required to produce any given knee extension joint torque.
While each of these factors can have a meaningful effect on our ability to produce a turning force at a joint, the internal moment arm is the factor that varies the most widely, and so it has the greatest effect on our capacity to produce a joint torque at any given joint angle.
What determines the requirement for a turning force at any given joint angle?
While joint torque-angle curves describe the way in which our capacity for producing a joint torque changes with joint angle, external resistance curves describe the required joint torque at each joint angle, or at a point in an exercise range of motion.
External resistance curves are determined by two main factors:
External moment arm length — the joint torque that we must exert to lift a weight is at least equal to the turning force that the weight exerts on the joint. This turning force is determined partly by the size of the weight, and partly by the leverage of the weight on the joint. When we lift a barbell or dumbbell, this leverage is determined by the horizontal distance between the weight and the joint. This is why the biceps curl is hardest in the middle of the movement, when the forearm is horizontal and the weight is furthest from the elbow. It is also why the squat is hardest near to the bottom of the movement, when the hip and knee joints are furthest in a horizontal direction from the barbell.
External resistance type — the object providing the external resistance affects the amount of force that is required at each point in the movement. For example, when we lift a weight, we must overcome the forces of both gravity and inertia. Gravity is constant at all points during the lift, but we only need to overcome inertia when we are accelerating the weight, which is at the start. In contrast, towards the end of the lift, we can allow the weight to decelerate, and this reduces the required force. This means that anytime we lift a weight, the force required is increased at the beginning and decreased at the end, compared to a completely constant resistance. In contrast, if we use elastic resistance for strength training, we must overcome the tension exerted as the material is deformed, and this tension increases progressively with increasing length. Therefore, any exercise is easy at the start, and becomes much harder towards the end.
Unless you often switch between weights and elastic resistance in your training, the factor that differs most between exercise variations is the external moment arm length. Even so, external moment arm length and external resistance type can each have large effects on the required joint torque at any point in the exercise range of motion.
What are the different types of strength curves?
Strength curves of most of the common exercises used in the gym tend to follow one or other of the following broad patterns:
Flat — if a strength curve is flat, then this means that we experience the exercise to be similarly difficult throughout its range of motion. Some machines are designed to have fairly flat strength curves, and we can also make some free weights exercises have flatter strength curves by adding elastic resistance to the barbell.
Bell-shaped — if a strength curve is bell-shaped, then this means that we experience the exercise as most difficult in the middle. The bell shape can be fairly flat, or it can be quite pronounced. The standing biceps curl is the classic example of a bell-shaped strength curve, since it is hardest when the forearm is parallel to the floor.
Linear ascending — if a strength curve is linear ascending, then this means that we experience the exercise as most difficult at the start of the lifting (concentric) phase. The barbell squat is the most commonly-used example of a linear ascending strength curve, since it is hardest at the bottom of the movement.
Linear descending — if a strength curve is linear descending, then this means that we experience the exercise as most difficult at the end of the lifting (concentric) phase. The best example of this type of strength curve is the dumbbell lateral raise, which is so difficult at the top that it is nearly impossible to pause in that position, unless a very light weight is used.
In the examples listed above, the main factor that contributes to the exercise strength curve is the external moment arm length, but for the purposes of understanding the effects on hypertrophy, we need to be aware of how both the capacity and the requirement to produce force are altered.
How can exercise strength curves affect hypertrophy? (active and passive elements)
When measuring the effects of a single exercise variation, performed in the same way, the number of stimulating reps performed by a muscle in a workout determines the hypertrophic stimulus.
The number of stimulating reps is probably the best way of representing training volume (and therefore time under tension). Training volume seems to have a dose-reponse relationship with muscle growth, although the exact shape of that relationship is still unclear.
However, not all reps produce the same hypertrophic effects, even when they are stimulating for the muscle.
Differences between stimulating reps can arise when the active or passive elements of a muscle fiber contribute proportionally more or less to the overall force that is exerted. The active elements of a muscle fiber are the contractile parts, which produce force when the central nervous system sends a signal to the motor unit that controls it. The passive elements of muscle fiber are the elastic parts, which are stretched when the fiber is lengthened.
For example, exercises can be performed as a lifting (concentric) only variation, or a lowering (eccentric) only variation, and they can be performed through different full or partial ranges of motion. In each case, the exercise can be performed so that it produces the same number of stimulating reps, but the effect on the type muscle growth will differ, although the overall change in muscle volume seems to be similar.
When we perform an exercise as a lifting (concentric) only variation, or through a partial range of motion, the mechanical tension inside the muscle fibers is experienced primarily by the active (contractile) elements. This seems to trigger hypertrophy through increases in muscle fiber diameter. When do an exercise as a lowering (eccentric) only variation, or through a full range of motion, the mechanical tension inside the muscle fibers is experienced by both passive (elastic) and active (contractile) elements. This seems to trigger hypertrophy through increases in muscle fiber length.
In the same way, exercises with different strength curves display different forces when the muscles are in lengthened and shortened positions, and this affects how much force is produced by either the passive (elastic) or active (contractile) elements. Exercises with flat, bell-shaped, or linear descending strength curves will place comparatively little load on the passive elements, while exercises with linear ascending strength curves will place a much greater load on the passive elements.
How can exercise strength curves affect hypertrophy? (stretch-mediated hypertrophy)
When we alter the contribution of the active and passive elements to muscle force, the main effect is on the type of muscle fiber growth. When the active elements are predominantly stimulated, the muscle fiber increases in diameter, but when the passive elements are predominantly stimulated, the muscle fiber increases in length.
However, in some cases, reaching a more stretched position in an exercise, and thereby loading the passive elements to a greater extent, also causes greater total hypertrophy, and not just greater increases in fiber length.
This is called “stretch-mediated hypertrophy.”
Stretch-mediated hypertrophy seems to happen when the muscle is working predominantly on the descending limb of the length-tension relationship in the exercise, such that the passive elements contribute substantially to total force production for the majority of the lift.
We cannot assume that stretch-mediated hypertrophy will always occur, regardless of the muscle being trained. Its effects seem to be limited to those muscles that display specific length-tension relationships.
Since exercises with different strength curves display different forces when the muscles are in lengthened and shortened positions, we can use strength curves to trigger stretch-mediated hypertrophy in those muscles that are responsive to it. Exercises with flat, bell-shaped, or linear descending strength curves are not suitable, since they place comparatively little load on the passive elements, but exercises with linear ascending strength curves are likely to be quite effective, because they place a much greater load on the passive elements.
How can exercise strength curves affect hypertrophy? (regional hypertrophy)
We tend to think of muscles as fairly uniform objects that contract in the same way regardless of the direction of force, or the joint angle at which force is produced.
In fact, muscles are often made up of multiple regions, which each function somewhat independently of one another. This subdivision of a muscle into regions probably happens arises due to differences in the leverage that each region of the muscle has on the joint when moving it in different directions, or at different joint angles. The variety in other aspects of muscle regions, such as architecture and fiber type, are probably features that arise subsequently to this original factor.
Consequently, exercises tend to stimulate more muscle growth in one region than another. This may occur for a number of reasons, but is most likely to happen because of differences in the activation of the muscle from one joint angle to another, and the subsequently greater deformations of the fibers in one region compared to in others.
Regional muscle growth differs between lifting (concentric) only and eccentric (only) strength training, between exercises that involve larger and smaller ranges of motion, and it also differs between exercises that display different strength curves by using either constant loads or accommodating resistance.
How can exercise strength curves affect hypertrophy? (proportional muscle contribution)
When there is more than one muscle acting at a joint, the effects of the exercise strength curve become more complicated, because their relative internal moment arm lengths determine which of the muscles contributes the most force at any given joint angle.
For example, at the hip joint, the hamstrings, gluteus maximus, and adductor magnus are all prime movers that contribute to the hip extension torque. However, their internal moment arms change with joint angle, and in different ways.
The internal moment arm length of the gluteus maximus increases as the hip approaches full extension, the internal moment arm length of the adductor magnus increases as the hip flexes, and the internal moment arm length of the hamstrings displays a shallow, bell-shaped curve. This means that when the squat is performed with a partial range of motion, the gluteus maximus is a larger contributor to hip extension torque, but when the squat is performed with a full range of motion, the adductor magnus contributes to a greater extent. We might therefore expect the same effects from altering the strength curve in the squat. The adductor magnus should be more active when conventional free weights are used, but the gluteus maximus should be more active when accommodating resistance is added.
Indeed, we already know that switching between constant load weights and accommodating resistance during knee extension training causes a shift in the amount of hypertrophy that occurs in the two-joint rectus femoris, but not in the single-joint quadriceps.
What does this mean in practice?
Strength curves are complex, so it is important to figure out simple guidelines to inform strength training practice when training for maximum increases in muscle size. As set out above, there are (at least) four ways in which the strength curve of an exercise can affect the resulting muscle growth that occurs after a workout, as follows:
Proportional contribution of the active and passive elements — exercises with linear ascending strength curves (like the barbell squat) likely to load the passive elements more than exercises with flat, bell-shaped, or linear descending strength curves. Such exercises will involve proportionally greater increases in fiber length than in fiber diameter, relative to exercises with other strength curves, which will likely affect the resulting changes in shape of the muscles.
Stretch-mediated hypertrophy — exercises with linear ascending strength curves (like the barbell squat) likely to load the passive elements more than exercises with flat, bell-shaped, or linear descending strength curves. For some muscles that operate primarily on the descending limb of the length-tension curve, this can cause stretch-mediated hypertrophy, which will lead to greater overall increases in muscle volume compared to other strength curves.
Regional muscle growth — exercises with different strength curves will likely produce different amounts of muscle growth in each functional subdivision or region of a muscle.
Proportional involvement of muscles — exercises with different strength curves will likely produce different amounts of muscle growth in each muscle that acts as a prime mover at a joint, because of variations in the internal moment arm lengths with joint angle.
In practice, this means that where a muscle is operating primarily on the descending limb of the length-tension relationship (like the quadriceps), selecting an exercise that has a linear ascending strength curve (like the barbell squat) is probably going to be slightly more effective for overall hypertrophy than an exercise with a different strength curve.
Additionally, it means that a variety of exercises, with different strength curves, are likely optimal for maximum muscle growth, as each exercise (and each strength curve) will produce a different stimulus, either because of their different effects on fiber length or fiber diameter, their different effects on each region of a muscle, or their different effects on the various prime mover muscles at a joint.
What is the takeaway?
Exercise strength curves describe the way in which the difficulty of a movement changes over its range of motion. Strength curves can be flat, such that the exercise is similarly difficult over the whole range of motion, or varying, such that the exercise is more difficult at one point than at others.
An exercise strength curve is determined by way in which our capacity for producing force changes across the joint angles used in an exercise, and the way that the requirement for force production changes over the exercise range of motion. This means that both internal (relating to the body) and external (relating to the exercise) factors affect the effects of a strength curve.
Strength curves affect hypertrophy because they (1) alter the proportional contribution of the active and passive elements, (2) affect how much stretch-mediated hypertrophy can occur in susceptible muscles, (3) determine which region of a muscle is most stimulated by an exercise, and (4) alter the proportional involvement of each muscle at a joint. Overall, this suggests that a variety of exercise strength curves are optimal for maximum muscle growth.