How can we identify the best way to train each muscle?

In the bodybuilding community, almost everyone has an opinion about which exercises (and ways of training) are best for each muscle group. Much of the time, these opinions are formed based on how the muscle feels when training using an exercise (or training technique), what famous bodybuilders are known to have done, or simply tradition.

While such information should not be discounted, there is also a wealth of knowledge to be found by trawling through the research literature. But what information should we look for? Many strength coaches and researchers use muscle activation to help them identify the best exercises, although this is not without its challenges. Other fitness professionals prefer to focus on the anatomy of the muscles to draw their conclusions.

However, few systems are available that (1) identify all the possible sources of information, and (2) show how each of these sources of information can be used to select exercises and determine other training variables, for each muscle. The following article sets out my current system.

#1. Anatomy

What it is: Gross anatomy describes the locations of the attachments of the muscle to the skeleton, in addition to a few other things such as innervation and muscle architecture. The attachment closest to the center of the body is usually known as the origin, while the attachment furthest from the center of the body is usually known as the insertion. Attachments can be located on various sides of a bone, which means that the muscle often rotates a limb segment while performing its more fundamental joint action.

Why it matters: It helps us figure out suitable exercises, and how we might alter them to target different muscles within a group. Even if we only learn the approximate locations of the origins and insertions, we can see whether the muscle crosses a single joint or two joints, and which directions the joint will move when the muscle contracts. This shows us the basic category of suitable exercises. Additionally, if we know which side of the bones the origins and insertions lie, we can predict whether lower limb joint rotation will have a positive or negative effect on training that muscle. This allows us to alter exercises in ways that enhance their effects for a muscle. This is most commonly relevant if we want to emphasize one muscle within a group (rotating the foot inwards or outwards during leg curls can be used to target either the medial or lateral hamstrings).

#2. Regional anatomy

What it is: Regional anatomy describes the way in which a muscle divides into several internal regions. Some muscles do not divide very strongly into separate regions (like the biceps brachii), while other muscles clearly subdivide into multiple compartments (like the latissimus dorsi).

Why it matters: It helps us see whether we are definitely going to need multiple exercises to train the muscle. When a muscle subdivides into several internal regions, these regions usually serve different purposes, particularly if the muscle is capable of multiple joint actions (although it can occur for other reasons too). Often one region will contribute to a greater extent during one of these joint actions, while another will contribute more during another (the rectus femoris subdivides into proximal and distal regions, and one region contributes more during its hip flexion action, while the other contributes more during its knee extension action).

#3. Internal moment arm lengths

What it is: The internal moment arm lengths of a muscle determine its leverage on the joint, and therefore its contribution to a joint moment, relative to other agonist muscles. When several muscles work at a joint, the one that has greatest leverage at any given point in the exercise range of motion will be the one that contributes most at that same point. Similarly, when a muscle has multiple regions, the region with the best greatest leverage at any given point in the exercise range of motion will be the one that contributes most at that same point.

Why it matters: It allows us to see where the peak force in an exercise joint range of motion needs to be, if we want to target one muscle within a group (or one region of a muscle). We can alter the point where peak force occurs in two key ways. Firstly, we can select the exercise according to its external moment arm lengths (the squat and the hip thrust both train the gluteus maximus, but the point in the hip joint range of motion where peak force occurs is very different). Secondly, we can alter the external resistance type (squats with weight and squats against heavy elastic resistance both train the gluteus maximus, but the point in the hip joint range of motion where peak force occurs is very different).

#4. Working sarcomere lengths

What it is: Working sarcomere lengths describe the lengths of the sarcomeres inside the fibers of muscle over its joint angle range of motion. Each muscle fiber is made up of a string of sarcomeres. Each sarcomere contains overlapping actin and myosin myofilaments, which attach to form crossbridges, in addition to titin molecules, which stretch to produce passive forces. The length of each sarcomere in a muscle increases as the muscle is stretched, and decreases as the muscle shortens. Depending on the number of sarcomeres in the muscle fibers of a muscle (and on other factors), the length of any sarcomere inside the muscle at each end of the joint action range of motion can vary. Some muscles (like the quadriceps) contain sarcomeres that are already very stretched even when they are only partway through their full joint angle range of motion. Other muscles (such as the biceps) contain sarcomeres that are not stretched even when they reach their full joint angle range of motion. Thus, sarcomere length (and not muscle length) determines (1) the extent to which actin and myosin myofilament overlap, and (2) the extent to which titin molecules are stretched.

Why it matters: It allows us to see if the muscle can experience (1) active insufficiency (and so will be trained poorly by exercises involving peak forces at very short muscle lengths), and (2) stretch-mediated hypertrophy (and so will be trained more effectively by exercises involving peak forces at very long muscle lengths). This is not common knowledge. Most fitness professionals assume that only muscles that cross two joints (such as the hamstrings) can experience active insufficiency, and that all muscles can benefit from being trained with full ranges of motion. Neither of these beliefs are true.

• Active insufficiency — When a sarcomere is shortened excessively during a joint action, it ceases to produce force effectively because its actin and myosin myofilaments no longer overlap sufficiently. This is called active insufficiency. Therefore, when a muscle contains sarcomeres that can reach very short lengths, it will respond poorly if it is trained with peak forces at very short muscle lengths (as when using partial ranges of motion or elastic resistance). Conversely, if a muscle contains sarcomeres that cannot reach very short lengths, it will *not* respond poorly if it is trained with peak forces at short muscle lengths, even if it is a two-joint muscle such as the hamstrings!
• Stretch-mediated hypertrophy — When a sarcomere is lengthened to a substantial extent during a joint action, titin contributes meaningfully to force production. This enhances the mechanical tension that the muscle fiber produces (and experiences), this causes greater muscle growth. This is why full ranges of motion often (but not always) cause more hypertrophy than partial ranges of motion. Indeed, when a muscle contains sarcomeres that can reach long lengths, it will respond better if it is trained with peak forces at long muscle lengths (as when using full ranges of motion) than if it is trained with peak forces at short muscle lengths (as when using partial ranges of motion). Yet, if a muscle contains sarcomeres that cannot reach long lengths, it will respond similarly to being trained with peak forces at long or short muscle lengths. In fact, full and partial ranges of motion will cause similar hypertrophy.

#5. Susceptibility to muscle damage

What it is: The susceptibility of a muscle to damage is simply how easily a muscle is damaged by a standard workout. Some muscles experience a great deal of damage in response to a workout (like the biceps brachii), while other muscles experience little damage (like the quadriceps). The susceptibility of a muscle to muscle damage after a workout is affected by: (1) its fiber type proportion, (2) the level of voluntary activation that can be attained for that muscle, and (3) the working sarcomere lengths of its muscle fibers.

• Fiber type proportion — more oxidative (slow twitch) muscle fibers are far less easily damaged than less oxidative (fast twitch) muscle fibers, mainly since they possess a larger amount of mitochondria, which are protective against the muscle damage mechanisms triggered by the calcium ions that cause excitation-contraction coupling to occur. Thus, when a muscle has a higher fast twitch fiber proportion, it will be more easily damaged by a workout compared to a similar muscle that has a lower fast twitch fiber proportion.
• Voluntary activation — muscles that are easy to activate fully will be more easily damaged than muscles that are very difficult to activate. This is quite relevant, as the voluntary activation capacity (which reflects how many motor units can be accessed voluntarily) differs widely between muscles. Since high-threshold motor units contain most of the fast twitch muscle fibers, the voluntary activation capacity affects how many fast twitch fibers can be activated. Thus, when a muscle has a high voluntary activation capacity (such as the hamstrings), it will be more easily damaged by a workout compared to a muscle with a similar fast twitch fiber proportion but with a low voluntary activation capacity (such as the quadriceps).
• Working sarcomere lengths — muscles that contain sarcomeres that can be stretched substantially will be more easily damaged than muscles that contain sarcomeres that are not substantially stretched, since the mechanical tension that the fibers are exposed to is greater.

Why it matters: The amount of muscle damage that a muscle experiences after a workout is the main determinant of the frequency we can use for training it. When a muscle is easily damaged, we will either need to train it with less workout volume than other muscles, or with a lower frequency. In addition, it affects the cost-benefit calculation of using (1) exercises that involve peak forces at long muscle lengths, since these cause more muscle damage than exercises that involve peak forces at short muscle lengths, and (2) single-joint and single-limb exercises, since these cause more muscle damage than multi-joint and two-limb exercises, and (3) types of training that enhance muscle damage, such as short rest periods, advanced techniques (such as drop sets and pre-exhaustion), and large ranges of motion.

Specifically, we can see that when a muscle is easily damaged because it is fast twitch and has a high voluntary activation capacity, but cannot benefit from being trained at long muscle lengths because its sarcomeres do not reach long lengths (like the biceps brachii), then training with a full range of motion (or with an exercise that involves peak forces at long muscle lengths) will actually be a net negative compared to training with a partial range of motion (or with an exercise that involves peak forces at short muscle lengths), since the amount of hypertrophy that is stimulated in each case will be similar, but the muscle damage will be greater when training with a full range of motion.

How can we acquire this information?

Much of this information can be found in the research literature, although not all of it exists for every muscle. I have summarized a lot of it for the following major muscles in articles and infographics as follows:

1. Elbow flexors (article, infographic)
2. Triceps brachii (article, infographic)
3. Pectoralis major (article, infographic)
4. Deltoids (article, infographic)
5. Latissimus dorsi (article, infographic)
6. Trapezius (article, infographic)
7. Quadriceps (article, infographic)
8. Hamstrings (article, infographic)
9. Gluteus maximus (article, infographic)
10. Calves (article, infographic)

If you would like to ask me a question about any of these muscles, the easiest way to make sure that I see it (and so guarantee a response), is to ask it on the most recent (free to use) questions thread on my Patreon account.

What is the takeaway?

We can use the research literature to enhance our training programs if we search for information about the gross anatomy, regional anatomy, and internal moment arm lengths of a muscle, in addition to its working sarcomere lengths, and susceptibility to muscle damage. Each of these factors provides information that is useful for different reasons, as follows:

• Gross anatomy describes the locations of the attachments of the muscle to the skeleton. Learning the basic anatomy of a muscle helps us figure out suitable exercises, and also helps us see how we might alter them to target different muscles within a group.
• Regional anatomy describes the way in which a muscle divides into several internal regions, and this tells us whether we are going to need multiple exercises to train the muscle.
• The internal moment arm lengths of a muscle determine its leverage on the joint, and therefore its contribution to a joint moment, relative to other agonist muscles. This allows us to see where peak force in an exercise joint range of motion needs to be, to target one muscle within a group (or one region of a muscle). We can alter the point where peak force occurs by our exercise selection and by our choice of external resistance type.
• The working sarcomere lengths describe the lengths of the sarcomeres inside a muscle over its joint angle range of motion. It allows us to see if the muscle can experience (1) active insufficiency (and so will be trained poorly by exercises involving peak forces at very short muscle lengths), and (2) stretch-mediated hypertrophy (and so will be trained more effectively by exercises involving peak forces at very long muscle lengths).
• The susceptibility of a muscle to damage is how easily a muscle is damaged by a workout. It is affected by: (1) muscle fiber type proportion, (2) its level of voluntary activation, and (3) the working sarcomere lengths of its muscle fibers. The amount of muscle damage that a muscle experiences after a workout is the main determinant of the frequency (and volume) we can use for training it, and it also influences our choice of exercises (single-joint vs. multi-joint, single-limb vs. multi-limb, and full vs. partial range of motion).