Why might individual lifters display different responses to the same strength training program?
The principle of individuality has been less well-described in the literature in comparison with the principle of specificity. Even so, we can draw inferences from basic physiology about how the principle of individuality might be applied to writing strength training programs. In this article, I aim to explain the physiological mechanisms that might cause individualized responses to a strength training program, with a focus on bodybuilding and hypertrophy. With that in mind, we can state that training for bodybuilding, individual variation in response to any given strength training program can be observed in at least three key areas:
- the overall amount of hypertrophy that occurs in response to any given workout can vary (the most obvious example of this phenomenon being that some lifters have grown accustomed to performing more training volume in recent months than others, leading them to experience a smaller stimulus from a given workout volume);
- the location of the hypertrophy produced by an exercise can vary (the most obvious example of this being that some lifters tend to use the leg muscles more during a deadlift, leading to more leg muscle hypertrophy, while other lifters tend to use the back muscles to a greater extent, leading to more back muscle hypertrophy); and
- the rate at which a lifter recovers from a strength training set or a single strength training workout can vary. Some individuals recover quickly and can perform additional sets after only a few minutes of recovery as well as similar workouts after only one or two days of recovery. Other lifters recover much more slowly.
Let’s look at each of those areas in turn.
#1. Differences in the amount of hypertrophy caused by a single workout within the program
The amount of hypertrophy that results from a given strength training workout seems to depend on [A] inherent genetic factors, and [B] training status of the lifter. We can take both of these factors into account in order to write the optimal strength training program for maximizing hypertrophy for an advanced lifter.
A. Inherent genetic factors
In terms of the inherent genetic factors, relatively little is known. However, researchers have shown that muscles that are predominantly fast twitch (where the muscle contains more fast twitch fibers than slow twitch fibers) grow more quickly than muscles that are predominantly slow twitch (where the muscle contains more slow twitch fibers than fast twitch fibers). For this reason, it is logical that lifters who possess a higher proportion of fast twitch fibers in their muscles also display a faster rate of hypertrophy in response to a strength training workout than lifters who possess a lower proportion.
This is important, because individuals do display an tendency to possess higher or lower proportions of fast twitch muscle fibers not just in individual muscles, but across all muscles of their body. In other words, individuals can be classified as “generally more fast twitch” or “generally more slow twitch” lifters, and this will have important implications on their rate of muscle growth over the course of a strength training program (lifters who are “generally more fast twitch” will experience a faster rate of growth).
In practice, this means that those individuals who are classified as “generally more slow twitch” lifters may need to continue following training program cycles for longer in order to reach the same point as individuals who are classified as “generally more fast twitch” lifters. Much of the time, it will be fairly obvious if a lifter has a high proportion of either fast or slow twitch muscle fibers. However, it might also be inferred by reference to the gradient of a force-velocity profile (because fast twitch lifters will tend to be more velocity-dominant) or the ratio of fast movement test performance (such as vertical jump height) to maximum strength test performance (such as 1RM back squat). Fast twitch muscle fibers benefit maximum speed to a much greater extent than they benefit maximum strength, which is why this comparison is useful.
B. Training status (and recent training history)
In terms of training status and recent training history, there are at least three factors that differ between individual that seem to influence the magnitude of the hypertrophic response to any given strength training program: (1) the current size of the more responsive muscle fibers of the high-threshold motor units in relation to their maximum possible size; (2) the recent training history of the lifter, particularly in relation to training volume, but also in relation to other training variables such as exercise range of motion and exercise selection; and (3) the voluntary activation capacity that can be achieved when a maximal effort is applied.
#1. CURRENT SIZE OF RESPONSIVE MUSCLE FIBERS
The current size of the muscle fibers of the high-threshold motor units in a muscle (in relation to their maximum possible size) very likely influences the amount of growth that can occur in response to a strength training workout. Indeed, it seems likely that all muscle fibers have a maximum possible size, at least under normal circumstances. Exactly what places a limit on muscle fiber size is not known. Nevertheless, when lifters possess many such muscle fibers that have either reached this limit (and therefore can no longer grow at all) or are approaching this limit (and therefore grow only a small amount), their rate of muscle growth after a strength training workout will be very slow, because fewer muscle fibers are capable of growth. For this reason, the same strength training workout will produce a large amount of muscle growth in beginner lifters but only a very small amount of muscle growth in more advanced lifters. While this may seem obvious and not worthy of comment, it actually has a very important implication.
Since beginners display lower levels of voluntary activation than more advanced lifters, their greater hypertrophy cannot arise from the growth of muscle fibers belonging to the very responsive, higher high-threshold motor units. Rather, it must arise due to growth of muscle fibers belonging to the less responsive, lower high-threshold motor units. From this observation, we can now deduce that as training experience increases, the ability to achieve muscle growth in the muscle fibers belonging to the less responsive, lower high-threshold motor units is lost or at least greatly reduced. This is logical, since these muscle fibers are probably closer to their maximum possible size at the start of a training program anyway. In other words, while beginners might achieve growth in the muscle fibers belonging to all of the high-threshold motor units, more advanced lifters may only achieve growth in the muscle fibers of the highest high-threshold motor units.
This has important programming implications.
Essentially, it means that as overall strength training experience increases, the importance of achieving a very high level of motor unit recruitment similarly increases. We can address this issue in two ways. Firstly, we can use training methods that permit the highest possible levels of motor unit recruitment during a workout (for example, we can train with heavier loads, more stable exercises, and exercises that involve less muscle mass such as single-joint or single-arm variations, which all reduce the impact of central nervous system fatigue). Secondly, we can increase our ability to produce high levels of motor unit recruitment in the long-term through specific training methods. Typically, this means including heavy strength training blocks within a training program or including some high-velocity movements within the workout warm-ups.
#2. RECENT TRAINING HISTORY
The recent training history of a lifter affects their ability to achieve certain adaptations. We can observe this in relation to various training variables, including:  training volume,  range of motion (or more accurately, maximum muscle length), and  exercise selection.
Indeed, recent research has revealed that lifters become accustomed to training with a certain level of training volume. Critically, this acclimatization to a current level of training volume is not related to current level of muscle size. Indeed, both beginners and more advanced lifters can each accustom themselves to the same, high level of training volume, but the advanced lifter would still have a larger muscle size than the beginner. Conversely, two relatively advanced lifters might have the same overall amount of muscle mass, but be accustomed to different levels of training volume. In practice, if those two relatively advanced lifters both started a new training program with the same, moderate level of training volume, the lifter that was previously accustomed to a higher level of training volume would likely experience less hypertrophy than the lifter that had previously become accustomed to a lower level of training volume. This is important, because higher training volumes cause more sustained fatigue after a workout, so if we can achieve the same result for a lower level of volume, then that would be an ideal result.
Similarly, we know that strength training with a larger range of motion often (but not always) causes greater gains in muscle size, because the larger range of motion leads to a longer maximum muscle length being achieved at the start of each concentric phase, which generates more passive mechanical tension. This passive mechanical tension stimulates increases in muscle fiber length through sarcomerogenesis rather than increases in muscle fiber diameter due to increases in the number of myofibrils in parallel. However, the passive mechanical tension is produced by the elongation of the stiff segment of titin, which can only occur if the individual sarcomeres of the muscle fiber work on the descending limb of the length-tension relationship. As new sarcomeres are added, the ability of the sarcomeres to reach onto the descending limb is reduced, thereby reducing the passive mechanical tension. Eventually, once enough additional sarcomeres have been added, the stimulus from using a larger range of motion disappears, and no further increases in muscle fiber length (and therefore no extra hypertrophy) are produced. In practice, this means that training with a large range of motion will have substantial benefits for lifters who have previously not been training with large ranges of motion, but much smaller effects in lifters who have used a large range of motion previously. This is particularly important, because training with a larger range of motion (or a long maximum muscle length) causes more post-workout fatigue, and it may therefore be helpful to use partial ranges of motion to avoid this fatigue if the larger range of motion is not actually beneficial for hypertrophy.
Additionally, the principle of neuromechanical matching tells us that each individual exercise (and exercise variation) requires a slightly different contribution from the various muscles (and regions of each muscle) that work at any given joint. For this reason, when a lifter has been performing the same exercise for a long period of time, they are likely to experience smaller gains from that exercise in the future in comparison with a lifter who has been training the exact same muscle group but with a different exercise or exercise variation (since regional hypertrophy differs between exercises). In practice, this means that tracking exercise use over time is an important feature of any training diary, and varying those exercises from one program cycle to the next is a key feature of any long-term programming plan.
#3. VOLUNTARY ACTIVATION CAPACITY
Some lifters are able to achieve higher levels of voluntary activation than others. Most obviously, those lifters with a more advanced training status can achieve higher levels of motor unit recruitment during maximal efforts than lifters with a less advanced training status. Similarly, those individuals who have previously trained with heavier loads or with maximal efforts in an unfatigued state (such as during velocity-based training) can also achieve higher levels of motor unit recruitment during maximal efforts than lifters without the same experience.
In both cases, the difference between lifters probably occurs because repeated practice of high efforts in the absence of central nervous system fatigue is what increases the ability to achieve high levels of motor unit recruitment, because it maximizes the central motor command signal during a strength training set. The ability to achieve a high level of motor unit recruitment in each set determines how many muscle fibers are trained in that set, which in turn affects the total amount of muscle growth that can be achieved. This seems to be why some recent research has shown that bodybuilders might benefit from performing blocks of heavy strength training, since these are optimal for improving the ability to access high-threshold motor units, and this allows more muscle fibers to be trained in subsequent moderate load training blocks. In practice, those bodybuilders who have performed little heavy strength training previously will therefore benefit most from carrying out a training program cycle involving heavy (1–5RM) loads.
#2. Differences in the location of the hypertrophy caused by a single workout within the program
The exact location of the hypertrophy that results from a given strength training exercise seems to depend on [A] the leverage of the muscles (and regions of muscles) in accordance with the principle of neuromechanical matching, and [B] the sarcomere lengths of the muscles (and regions of muscles). Since these can vary between lifters, we must take both of these factors into account in order to write the optimal strength training program for maximizing hypertrophy for an advanced lifter.
Leverages of muscles
According to the principle of neuromechanical matching, muscles (and regions of muscles) contribute to a movement to the extent that they have leverage. The greater the leverage that a muscle (or a region of a muscle) has relative to the other muscles (or regions of a muscle) that can work at the same joint, the more it contributes. Importantly, the leverage that each muscle (or region of a muscle) has during an exercise varies greatly between individuals, and this means that the same exercise can cause muscle growth to occur in different parts of the body in different lifters.
For example, during deadlifts, some lifters use a hip-dominant lifting strategy that challenges the back muscles to a greater extent, while other lifters make use of a more knee-dominant lifting strategy that challenges the leg muscles to a greater extent. This choice of strategy has been linked to the relative strengths of those muscles, such that the lifter chooses to use a hip-dominant lifting strategy when their back muscles are strong compared to their legs (likely because the back muscles have very good leverage), while they choose to use knee-dominant lifting strategy when their leg muscles are strong compared to their back (again, since the leg muscles have good leverage). In practice, this means that the use of the deadlift primarily challenges those muscles that are strongest (and most well-developed), and provides a smaller stimulus for those muscles that are weaker (and less well-developed).
The same phenomenon occurs in the bench press, which involves the pectoralis major, anterior deltoid, and triceps brachii working together as prime movers. Some lifters use the pectoralis major to a greater extent during the bench press, other lifters tend to make greater use of the anterior deltoid, and still others make greater use of the triceps brachii. Importantly, recent research has shown that the relative contributions of each muscle to force production during the bench press exercise can be changed by a long-term training program that implements single-joint exercises that increase the strength and size of certain muscles relative to others. For example, when the pectoralis major was trained using single-joint exercises, it started to contribute comparatively more to the bench press. This clearly shows that multi-joint exercises primarily challenge those muscles that are strongest (and most well-developed), and provides a smaller stimulus for those muscles that are weaker (and less well-developed). For this reason, long-term training with the same multi-joint exercise probably leads to an increasingly greater stimulus for certain muscles at the expense of others.
In practice, this suggests that more advanced bodybuilders will benefit from identifying their weaker muscle groups during multi-joint exercises and implementing single-joint exercises to strengthen those weak points, in order to make sure that the hypertrophy produced by multi-joint exercises is evenly apportioned over the muscles being trained (in very much the same way as powerlifters and other strength athletes already commonly do). To a lesser extent, it may also be valuable to identify weak points within an exercise range of motion during single-joint exercises and strengthen those weak points by using different types of external resistance, for the same reasons.
Individual lifters likely display different muscle fiber lengths in different muscles within a group (or regions within a muscle). This can lead them to experience greater stretch-mediated hypertrophy for those muscles or regions of a muscle. In practice, this can lead to the same phenomenon as is observed when one muscle (or region of a muscle) naturally has better leverage than the other muscles (or regions of a muscle) within a group, and can therefore be addressed with the same solution, which is to implement a wide variety of multi-joint and single-joint exercises, and to aim to develop strength at a wide range of joint angles.
Additionally, individual lifters might display different levels of stretch tolerance for any given muscle group. When a lifter has a high stretch tolerance, then they will be able to lengthen muscles to a greater extent, and thereby cause any activated muscle fibers in an exercise to experience high levels of passive tension, leading to more hypertrophy of the muscle in general and of the distal region in particular. This suggests that when a lifter has very poor mobility, it may actually help their ability to achieve hypertrophy if they were to improve that through a static stretching program.
#3. Differences in amount of fatigue caused by a single workout within the program
Strength coaches know well that some lifters experience a large amount of fatigue after a single set of training, while others display very little fatigue. This causes some lifters to drop a lot of reps from one set to the next (unless they use long rest periods between sets), while other lifters can maintain the same number of reps from one set to the next with relatively little rest. Similarly, some lifters experience a large amount of fatigue after a workout, while others display very little fatigue. This means that some lifters can fail to make progress from one workout to the next (unless they take more recovery days between workouts), while other lifters can perform workouts much more regularly with few days of recovery between them.
Fatigue after each set
During strength training sets performed to muscular failure, there are many types of fatigue, including (spinal and supraspinal) central nervous system fatigues, calcium ion-related fatigues, and metabolite-related fatigues.
Importantly, those individuals who possess a larger proportion of fast twitch muscle fibers (or who have access to more of their fast twitch muscle fibers because they can achieve a greater level of voluntary activation) tend to display a higher proportion of metabolite-related fatigue, and therefore also likely display a greater amount of supraspinal central nervous system fatigue due to afferent feedback from the metabolite accumulation. It is quite likely that these fatigue mechanisms contribute to a reduced ability to perform subsequent sets with the same number of reps when short rest periods are used, which can reduce the number of stimulating reps achievable in the later sets of a workout.
In practice, this means that those lifters who have a high proportion of fast twitch muscle fibers or who have access to a larger proportion of their fast twitch muscle fibers (which might be estimated by the gradient of the force-velocity profile or the ratio of fast movement test performance such as vertical jump height to maximum strength test performance such as 1RM back squat) are likely to need to take more rest between sets in order to achieve maximal hypertrophy from any given workout. This can easily be monitored by simply monitoring the loss of reps from one set to the next, and implementing more rest if the number of reps that are lost is excessive (the exact number will vary depending on the rep range being used).
Fatigue after a workout
After a strength training workout, fatigue is caused by muscle damage, excitation-contraction coupling failure, and central nervous system fatigue secondary to the inflammatory response to muscle damage. Importantly, the research literature has established that fast twitch fibers are more easily damaged after exercise, and this seems to be because they have fewer mitochondria, which means that they are less able to defend themselves against the influx of calcium ions and the subsequent calpain signaling that leads to muscle protein breakdown.
In practice, this means that those lifters who have a high proportion of fast twitch muscle fibers or who have access to a larger proportion of their fast twitch muscle fibers (which might be estimated by the gradient of the force-velocity profile or the ratio of fast movement test performance such as vertical jump height to maximum strength test performance such as 1RM back squat) are likely to experience greater fatigue after any given workout, and therefore find that they cannot perform exactly the same workout volumes with the same training frequencies as lifters with a greater proportion of slow twitch muscle fibers.
Additionally, since experiencing excessive muscle damage is likely what lead to overreaching and even overtraining, it is unsurprising that recent research has shown that athletes with a higher proportion of fast twitch fibers are at higher risk of overreaching. In practice, this means that greater care should be taken by lifters with a higher proportion of fast twitch muscle fibers to avoid increasing training volume too quickly.
What are the takeaways?
We can observe differences between lifters in terms of their responses to a strength training program in three main areas: (1) the overall amount of hypertrophy that occurs in response to any given workout can vary due to genetic reasons, and also due to recent training history; (2) the location of the hypertrophy produced by an exercise can vary due to anatomical differences; and (3) the rate at which a lifter recovers from a strength training set or a single strength training workout can vary, mainly due to differences in muscle fiber type proportion and/or voluntary activation capacity.
By drawing inferences about how these differences occur (by looking carefully at the underlying physiology), we can determine how to alter training programs in order to benefit specific populations (beginners, intermediates, and advanced) and also how to benefit specific individuals according to their recent training history and specific exercise strengths and weaknesses. Importantly, such alterations are not always immediately obvious by referring to common sense, but rather require an understanding of physiology in order to identify the optimal approach.