How can we design a strength training program that will maximize the growth of the quadriceps (commonly called “quads”)? What factors do we need to take into consideration, and how do each of these factors affect the different variables within the training program?
What information do we need?
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 (read more).
- 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).
Famously, there are four quadriceps muscles (the rectus femoris, vastus lateralis, vastus intermedius, and vastus medialis), which are all innervated by the femoral nerve. Of these four, the rectus femoris is the only two-joint muscle (it flexes the hip and extends the knee) while the other three are single-joint muscles (they only extend the knee). Anatomically, the vastus lateralis is located on the outside (lateral side) of the leg, the vastus intermedius is located in the middle of the leg, and the vastus medialis is located on the inside (medial side) of the leg. The rectus femoris lies over the top of the vastus intermedius, thereby hiding it.
The rectus femoris originates on the anterior inferior iliac spine of the ilium of the pelvis. The vastus lateralis has a very wide range of origins, including the greater trochanter of the femur and the upper half of the linea aspera. The vastus intermedius originates along a line running along the upper two-thirds of the anterior femur. The vastus medialis originates along the whole length of the linea aspera and the medial condyloid ridge. Although each of the quadriceps have slightly different origins, they all insert into the patellar tendon, which is attached to the tibial tuberosity.
The rectus femoris has an intramuscular tendon, which runs longitudinally down the muscle on its anterior side. At the proximal end of the muscle, the tendon lies more to the medial side of the muscle. As it approaches the distal end, it moves more centrally, while also decreasing in size. Some researchers have proposed that this intramuscular tendon may influence the nature of the strain injuries that can occur in this muscle during sport, but whether this has any clinical importance in practice is not clear.
Although the quadriceps muscles are so-called because there are four of them, some recent research has embarrassingly revealed that there may actually be five. Even so, this debate has not yet been settled, and some authors maintain that the fifth muscle may just be an anatomical variation in some individuals. Further research seems necessary to unravel this mystery.
In practice, the quadriceps muscles are knee extensors. While they have different origins, they have similar insertions, so can likely all be trained by exercises that involve knee extension (either alone or in combination with hip extension). Even so, the rectus femoris is a hip flexor and can also be trained by using hip flexion exercises, while its antagonist role during hip extension might lead us to expect it to play only a minor role during combined hip and knee extension exercises.
#2. Regional anatomy
All of the quadriceps muscles (however many there are) are innervated by the femoral nerve. Yet, there is clear scope for each of the muscles to be activated preferentially as a result of differences in their innervation. Moreover, there is evidence that there are separately-innervated regions inside each quadriceps muscle. Overall, the quadriceps muscles can be subdivided into proximal and distal regions (the vastus lateralis and rectus femoris) or proximal, middle, and distal regions (the vastus medialis) based on their innervation.
Indeed, although a main trunk of the femoral nerve innervates the vastus lateralis, it then splits into two branches. One innervates the proximal region, while the other which innervates the distal region. These regions also receive branches into their proximal and distal subregions. Similarly, although a posterior trunk of the femoral nerve innervates the vastus medialis, it splits into two branches (a medial branch and a lateral branch). The lateral branch does not seem to permeate far through the vastus medialis, but remains in its upper, lateral region. In contrast, while some of the terminal endings of the medial branch can be found in the middle region, the majority are located in the most distal region of the muscle. Like the vastus medialis, the rectus femoris is innervated by a posterior trunk of the femoral nerve, which then splits into two divisions, one which enters the proximal region of the muscle posteriorly, while the other enters the middle region of the muscle from its medial side.
Since the quadriceps appear to have regions that are innervated separately, some researchers have investigated whether this affects function. With regards to the rectus femoris, it has been identified that hip flexion leads to greater activation of the proximal region, while knee extension leads to greater activation of the distal region. Such regional activation can also be observed during the walking gait cycle and during cycling, and it seems to vary distinctively between movements. Thus, there seems to be a functional effect of the separately innervated regions within the rectus femoris . This has key implications for strength coaches seeking to develop the rectus femoris as a hip flexor to enhance sprint running ability, since training its knee extension function is unlikely to bring about optimal results.
While the innervation and muscle activation patterns make it very clear that most of the quadriceps muscles can be further subdivided into autonomous regions proximally-to-distally, there is also evidence that the muscles can be subdivided based on their anatomical features. As might be expected, the anatomical analysis often also reveals the presence of different proximal and distal regions.
Famously, the vastus medialis muscle has historically been subdivided based on its anatomical features into two regions: a proximal portion (the vastus medialis longus), and a distal region (the vastus medialis obliquus). It has been suggested that the vastus medialis longus is a knee extensor, while the vastus medialis oblique is mainly a (medially-directed) stabilizer of the patella during knee extension.
While it is much less well-known, the vastus lateralis has also historically been subdivided into two regions following similar anatomical analysis: a long head (the vastus lateralis longus), and a short or oblique head (the vastus lateralis obliquus). Some researchers have therefore suggested that the oblique head of the vastus lateralis acts as a (laterally-directed) stabilizer of the patella during knee extension movements.
In practice, there is good evidence that several of the quadriceps have regions that can be (and are) activated separately during different movements or exercises. These regions seem to be organized proximally-to-distally, and in the case of the rectus femoris, are separately activated by hip flexion and knee extension. This suggests that a variety of exercises may be optimal for training the quadriceps, in order to stimulate all regions maximally.
#3. Internal moment arm lengths
When studying the rectus femoris alone, researchers have found that the muscle displays a gradually reducing internal moment arm length with increasing knee flexion angle, although there may be a slight curve to the line, with a peak occurring at approximately 45 degrees from full knee extension. The extent of the reduction is quite substantial, with the internal moment arm length being twice as long when the joint is extended than when it is fully flexed. Even so, there is still a meaningful internal moment arm length present in full knee flexion, which allows the muscle to contribute to knee extension in this position, such as in a deep squat.
When studying multiple quadriceps muscles individually, researchers have shown that the internal moment arm lengths follow essentially identical trajectories, peaking in the range between 0–45 degrees from full knee extension, and declining gradually towards full knee flexion. This makes it very difficult to target different quadriceps muscles by altering the point in the exercise range of motion where peak forces are experienced, because all of the muscles have similar leverages at each point, and will therefore be recruited to a similar extent.
However, when the internal moment arm lengths of the quadriceps muscles are input into a model of the squat exercise, the rectus femoris is shown to have minimal leverage throughout the exercise range of motion, while the single-joint quadriceps have strong leverage, as might reasonably be expected. This explains why the rectus femoris is not very active in the squat, or indeed in any multi-joint hip and knee extension exercise, and why training with the squat exercise alone does not cause rectus femoris muscle growth.
While the rectus femoris is known to have a meaningful hip flexion internal moment arm length, its length can vary substantially between individuals. The rectus femoris hip flexion internal moment arm length reduces gradually with increasing hip flexion, and is longest in full hip extension. In contrast, the other main hip flexors, the iliacus and psoas, display either an increase in internal moment arm length with increasing hip flexion (the iliacus), or no clear change in internal moment arm length with hip flexion (the psoas). Even so, the rectus femoris still has a meaningful internal moment arm length in full hip flexion, which means that the muscle will still be an antagonist to the hip extensors in a squat exercise, even when using a deep squat variation.
Thus, it seems likely that we can maximize the contribution of the rectus femoris to hip flexion movements by placing the point of peak contraction at full hip extension (the start of the concentric phase), such as by using weight instead of elastic resistance in a straight-leg or bent-leg hip flexion exercise. In contrast, we can maximize the contribution of the other hip flexors by placing the point of peak contraction at full hip flexion (the end of the concentric phase), such as by using elastic resistance in a similar exercise.
In practice, altering the point at which peak force occurs during a knee extension exercise is unlikely to alter the contribution of each of the single-joint quadriceps. Yet, in combined hip and knee extension exercises (like the squat), the rectus femoris has poor leverage and contributes little to the turning force at the knee. Thus, both combined hip and knee extension exercises (like the squat) and single-joint knee extension exercises (like the knee extension or reverse Nordic curl) are likely necessary for full quadriceps development. When using hip flexion exercises to develop the proximal region of the rectus femoris, the peak force should occur at the start of the concentric phase (in full hip extension).
#4. Working sarcomere lengths
Overall, it seems likely that the single-joint quadriceps do not work on the ascending limb of the length-tension relationship, but only work on the plateau region and descending limb. This means that they are highly unlikely to experience active insufficiency, but will easily experience stretch-mediated hypertrophy. This is probably why most studies that have assessed the effects of strength training with long and short ranges of motion on muscle growth of the quadriceps muscle have found a superior effect of long ranges of motion, (while similar studies in the triceps brachii have not).
Moreover, there are indications that the three single-joint quadriceps may behave in exactly the same way on the descending limb. Indeed, the vastus intermedius seems to operate for a much shorter period on the descending limb than the vastus lateralis and vastus medialis. Additionally, the vastus medialis seems to reach a slightly longer sarcomere length at 90 degrees of knee flexion than the vastus lateralis.
In contrast, the rectus femoris does work on the ascending limb in addition to the plateau region and descending limb of the length-tension relationship, although it does not descend as far down the descending limb as the vastus lateralis and vastus medialis.
In practice, the single-joint quadriceps can experience stretch-mediated hypertrophy but not active insufficiency. Using larger ranges of motion in combined hip and knee extension exercises will likely enhance single-joint quadriceps muscle growth, and may cause slightly more vastus medialis growth, due to longer working sarcomere lengths being reached in this muscle. The rectus femoris does work on the ascending limb, so may be at risk of active insufficiency when trained at very short lengths. Thus, knee extension exercises may benefit from using peak forces at longer muscle lengths (as occurs when using standard machines with weight) to ensure that this does not occur.
#5. Susceptibility to muscle damage
Although lifters anecdotally report that leg day takes a long time to recover from, the literature overall shows that the quadriceps are actually the fastest recovering muscle. Certainly, they recover far faster than small muscles, like the biceps brachii.
While various strange hypotheses have been put forward to explain why the quadriceps recover so quickly in research studies, we can very easily see why it happens once we appreciate that the ability to activate (and cause fatigue) to fast twitch fibers is the main factor that determines the amount of muscle damage that occurs after exercise.
Studies often report a wide variation in fiber type of the same muscle. This is probably because there is a great deal of inter-individual variation, and the sample sizes are not quite large enough to generate a truly average result. Even so, the vastus lateralis has been studied more than almost any other muscle, making it easier to obtain an accurate measurement. Across a number of studies, the vastus lateralis is comprised of approximately 50% type I and 50% type II muscle fibers. To the extent that this makes it more slow twitch than other muscles, it will therefore experience less muscle damage.
Whether there are substantial variations in the fiber types of the individual quadriceps is not clear. Some research has suggested that the rectus femoris might be slightly more fast twitch than the single-joint quadriceps, making it more likely to be damaged after a bout of exercise.
The knee extensors are famous for being harder to activate voluntarily than other muscles. Indeed, this effect is extremely marked, and while most other muscles suffer a voluntary activation deficit of >5%, the quadriceps routinely display deficits of 15–20% below the expected level of force during voluntary contractions. This indicates that a meaningful number of the high-threshold motor units in the quadriceps are not recruited, and/or the firing rates of those motor units are very low. Consequently, the most fast twitch muscle fibers of the quadriceps, which are also the least oxidative fibers, will rarely be activated, and therefore do not experience damage after exercise.
In practice, the extremely low levels of voluntary activation of the quadriceps make it difficult to damage. Thus, it recovers more quickly than most (if not all) other muscles, even though it has a moderate fiber type proportion. This muscle group can therefore be trained quite frequently.
What is the takeaway?
There are (probably) four quadriceps muscles (the rectus femoris, vastus lateralis, vastus intermedius, and vastus medialis), which are all innervated by the femoral nerve. Of these four, the rectus femoris is the only two-joint muscle (it flexes the hip and extends the knee). The others are single-joint muscles (they only extend the knee).
Several of the quadriceps have regions that are activated separately during different movements or exercises. These regions seem to be organized proximally-to-distally, and in the case of the rectus femoris, are separately activated by hip flexion and knee extension. This suggests that a variety of exercises may be optimal for training the quadriceps, in order to stimulate all regions maximally. In particular, separate hip flexion and knee extension exercises may be useful for training the proximal and distal regions of the rectus femoris.
In combined hip and knee extension exercises (like the squat), the rectus femoris has poor leverage and contributes little to the turning force at the knee. Thus, both combined hip and knee extension exercises (like the squat) in addition to single-joint knee extension exercises (like the knee extension or reverse Nordic curl) are likely necessary for full quadriceps development. When using hip flexion exercises to train the proximal region of the rectus femoris, peak force should occur at the start of the concentric phase (in full hip extension). This can be done by using weight as resistance during a standard straight-leg or bent-leg hip flexion exercise.
The single-joint quadriceps can experience stretch-mediated hypertrophy but not active insufficiency. Using larger ranges of motion in combined hip and knee extension exercises will likely enhance single-joint quadriceps muscle growth, and may cause slightly more vastus medialis growth, due to longer working sarcomere lengths being reached in this muscle. The rectus femoris does work on the ascending limb, so may be at risk of active insufficiency when trained at very short lengths. Thus, knee extension exercises may benefit from using peak forces at longer muscle lengths (as occurs when using standard machines with weight) to ensure that this does not occur.
The extremely low levels of voluntary activation of the quadriceps make it difficult to damage. Thus, it recovers more quickly than most (if not all) other muscles, even though it has a moderate fiber type proportion. It can therefore be trained quite frequently.