Why do individual athletes display different responses to the same training program?
If you like this article, you will also like my first book (see on Amazon).
Why do individual athletes display different improvements in measures of athletic performance (such as sprinting speed, vertical jump height, or throwing distance) after the exact same training program involving a combination of both heavy strength training and fast movement training? While there are likely a great many factors that contribute to the answer to this question, in this article I aim to identify the most important ones.
#1. Individual force-velocity profile
In any single movement (such as a sprint, a jump, or a throw), an individual athlete has a unique force-velocity profile that can be described by reference to a maximum theoretical force, a maximum theoretical velocity, and a gradient. The data from this line can be used to derive a power output curve, which in turn identifies a point on the force-velocity profile (at a certain force and a certain velocity) that corresponds to maximum power output. When the sprint, jump, or throw is done at a force and velocity that is closer to this maximum power output, the performance in that movement will be maximized (for a given maximum theoretical force, and a given maximum theoretical velocity).
Consequently, the force-velocity profile necessarily differs between athletes (as well as between movements). Some athletes will naturally display actual force-velocity profiles that are very close to the optimal one that maximizes their performance, based on their underlying capabilities for high-velocity force production. In contrast, other athletes display actual force-velocity profiles that are not close to the optimal one, and these athletes are said to have a force-velocity imbalance.
When athletes display a force-velocity profile that is very different from the optimal one for the movement that is being tested, the imbalance must exist either due to [A] maximum theoretical force being too high relative to maximum theoretical velocity (in which case the athlete has a velocity deficit) or because [B] maximum theoretical velocity is too high relative to maximum theoretical force (in which case the athlete has a force deficit).
Effects of training on the force-velocity profile
When training with fast movements, an athlete tends to preferentially improve maximum theoretical velocity. After doing heavy strength training, they typically display greater improvements in maximum theoretical force. Thus, after each type of training, the force-velocity profile is altered. If an athlete who has a velocity deficit trains using fast movements, they will remove their velocity deficit, and this will cause a large improvement in performance in the tested movement. However, if they carry out heavy strength training, they will further increase the size of their velocity deficit, and performance in the tested movement will not improve, and could even worsen. Similarly, if an athlete who has a force deficit performs heavy strength training, they will remove their force deficit, and this will enhance performance in the tested movement. However, if they carry out fast movement training, they will further increase the size of their force deficit, and performance in the tested movement will not improve, and could even worsen. Thus, different athletes can experience different results to the same type of training simply because of their actual force-velocity profiles before commencing the training program.
Most athletes perform a combination of heavy strength training and fast movement training. In this context, it is the relative amount of each type of training that matters. A program can be constructed to involve similar amounts of heavy strength training and fast movement training. In such cases, the force-velocity profile will not change. This option is ideal for athletes who already have an optimal force-velocity profile. However, programs can also be constructed to contain a higher proportion of heavy strength training, or a higher proportion of fast movements. These types of program are ideal for athletes who have either force deficits or velocity deficits. In any event, it is clear that when different athletes follow the same training program involving a combination of both heavy strength training and fast movements, their starting force-velocity profiles will influence their results from that program, and they could be very different from one another.
#2. Training status (in terms of neural adaptations)
The ability to recruit motor units is very important for force production at all speeds, and the ability to achieve high motor unit firing rates is critical for force production at high speeds. When athletes already have achieved the ability to recruit most of their high-threshold motor units and to produce high motor unit firing rates, they will display different responses to a strength training program from other athletes who have not already achieved those same adaptations.
Motor unit recruitment
All of the muscle fibers within a muscle are grouped into specific motor units. According to Henneman’s size principle, motor units are recruited in order of size. Low-threshold motor units (which control small numbers of slow twitch fibers) are recruited first in any movement, while high-threshold motor units (which control large numbers of fast twitch fibers) are recruited later.
The level of effort employed during a movement determines the number of motor units that are recruited, although untrained people cannot recruit all of their motor units even during maximal efforts. Only after training with maximal efforts repeatedly over a period of time can the last high-threshold motor units be recruited. Since both heavy strength training and fast movement training involve maximal efforts, they both cause improvements in the ability to recruit high-threshold motor units. Thus, when an athlete has a history of either heavy strength training or performing fast movements (such as jumping or throwing with maximal efforts), they will likely have already attained the ability to recruit many of their high-threshold motor units for the muscles that were used in all of those movements.
Even so, studies that have examined the effects of heavy strength training with maximal efforts in athletes have reported improvements in maximum strength that almost certainly derive from increases in the ability to recruit high-threshold motor units. Why this happens is not clear, but may be due to the more controlled nature of the gym-based exercise. Indeed, researchers have often shown that when efforts are employed under more stable conditions or in the context of single-joint movements instead of multi-joint movements, higher levels of motor unit recruitment can be attained (I will write another article to explain this observation in more detail). Thus, it is entirely possible that a history of performing explosive movements in the context of a sporting environment does not lead to as high levels of motor unit recruitment as a history of gym-based exercises (which could involve heavy strength training or fast movements).
Motor unit firing rate
Unlike the level of motor unit recruitment, which describes how many motor units are switched on during an effort, motor unit firing rate describes how many signals the central nervous system (CNS) sends to the muscle per second. Importantly, force is maximized during slow movements (such as during heavy strength training) at a much lower motor unit firing rate (approximately 30–50Hz) compared to in fast movements (approximately 200Hz). Therefore, motor unit firing rate plays a much larger role in high-velocity force production than during low-velocity force production. Moreover, most research studies have found that heavy strength training, because it does not involve high motor unit firing rates, probably does not increase motor unit firing rates above the levels necessary for maximizing force in slow movements (approximately 30–50Hz). Yet, fast movement training, because it does involve very high motor unit firing rates, does seem to enhance motor unit firing rates to a higher level after training.
For this reason, when an athlete has a long history of heavy strength training, and has already attained the ability to recruit most of their high-threshold motor units, they still likely will not have the ability to achieve a sufficiently high motor unit firing rate to maximize their potential during fast movements. Indeed, unless they have also completed several blocks of fast movement training (preferably under very controlled conditions to enhance the levels of motor unit recruitment), they will still have the potential to achieve gains in high-velocity force production by increasing motor unit firing rates through fast movement training.
#3. Training status (in terms of peripheral adaptations)
While high-velocity force production is very strongly determined by neural factors, including the ability to coordinate the tested movement, the ability to recruit more high- threshold motor units, and the ability to achieve faster motor unit firing rates, there are certain peripheral factors that can also affect high-velocity strength.
Fast twitch fiber proportion
The maximum speed at which a muscle can shorten is a key determinant of the ability to produce force at high speeds. This maximum speed is determined primarily by the fastest fibers in the muscle. As the speed during a movement test increases, more and more of the muscle fibers in the muscle cease to be able to contribute to force production. The slow twitch muscle fibers are the first to stop contributing. Above approximately 0.5–1.0 fiber lengths per second, they do not produce any force, even when they are activated. This is a very important physiological observation that is often forgotten, but is very relevant to understanding how high-velocity force production works.
The moderately-fast (type IIA) muscle fibers are the next to stop contributing. Above approximately 3.0–4.0 fiber lengths per second, they no longer produce force, again, even though they are activated. Above this speed, only the fastest muscle (type IIX) fibers are able to contribute to force production in a movement. For this reason, if an athlete has performed a lot of fatiguing, heavy strength training, and has converted many of their fastest muscle fibers to moderately-fast muscle fibers, then they will be at a disadvantage in comparison with athletes who have done only a minimum of heavy strength training. Nevertheless, moderately-fast (type IIA) muscle fibers are able to convert back into fast (type IIX) muscle fibers if heavy strength training is ceased for a period of time. In this period of time, it is still possible to perform fast movement training.
For this reason, different types of training with produce different effects in athletes, depending on their training history. If an athlete has a history of a high volume of heavy strength training, and has therefore converted most (if not all) of their fast (type IIX) fibers to moderately-fast (type IIA) fibers, a block of training that involves only fast movements and no heavy strength training would be expected to produce a large improvement in high-velocity strength due to fiber type conversions. Conversely, if an athlete had a history of only a low volume of heavy strength training, they would not be expected to experience such a large improvement, since they would already have a meaningful proportion of their fast twitch fibers remaining. Similarly, if a successful athlete has a history of virtually no heavy strength training (or a very low volume of this type of training), and they therefore still have almost all of their fast (type IIX) fibers, a block of heavy strength training might have a detrimental effect. Starting this type of training would be expected to convert most of the fast (type IIX) fibers to moderately- fast (type IIA) fibers, and this would impair the ability to produce force at fast speeds.
#4. Susceptibility to fatigue
Sustained fatigue occurs after workouts that involve high levels of motor unit recruitment (such that fast twitch fibers are activated) for sustained periods of time. This is because the sustained fatigue is caused by a combination of three main factors: [A] excitation-contraction coupling failure, [B] muscle damage, and [C] central nervous system (CNS) fatigue. Both excitation-contraction coupling failure and muscle damage are the result of calcium ions accumulating inside muscle fibers and causing biochemical signaling processes that damage either the triadic junction that enables excitation-contraction coupling to occur, or the myofibrils themselves (which allow crossbridges to form). Since they have many mitochondria, it is difficult to cause calcium ion accumulation inside slow twitch muscle fibers. Therefore, these two mechanisms of sustained fatigue only really occur in the fast twitch muscle fibers of a muscle. CNS fatigue seems to be a secondary factor that only arises when muscle damage is present.
Fast twitch fiber proportion
Since the two primary peripheral mechanisms of sustained fatigue (excitation-contraction coupling failure and muscle damage) only really occur to any meaningful degree in the fast twitch fibers, the proportion of fast twitch fibers that an athlete possesses is very important for determining the extent that they experience fatigue after any given workout. When an athlete has a very large proportion of fast twitch fibers, they will experience more sustained fatigue after a workout. In contrast, if they have a small proportion of fast twitch fibers, they will only experience a small amount of sustained fatigue. For this reason, athletes who have a greater proportion of fast twitch fibers will probably benefit from training less often, and taking longer recovery times between workouts.
Alternatively, if the frequency of training cannot be altered, they may benefit from performing training that causes less excitation-contraction coupling failure and muscle damage. Since it is sustaining a high level of muscle activation that causes calcium ions to accumulate, these fatigue processes can be minimized by a number of techniques during heavy strength training (which is the main type of exercise performed by athletes that causes sustained fatigue), including performing a lower volume of training, stopping sets further from failure, taking longer rests between sets, and implementing cluster set routines that involve rests between reps in each set. Importantly, as far as most athletes are concerned, these techniques can be implemented with little or no negative effects on the adaptations being targeted.
Motor unit recruitment
Since fast twitch fiber proportion is the key factor that determines the amount of sustained fatigue that occurs after a workout, the level of motor unit recruitment that an athlete can attain is also important. When an athlete has access to more of their high-threshold motor units, they therefore also have the ability to activate more of their fast twitch fibers, and this will cause far more sustained fatigue after a workout. In contrast, if they have access to fewer of their high-threshold motor units, they will therefore only be able to activate a smaller amount of their fast twitch fibers, and they will experience far less sustained fatigue. For this reason, athletes who have a greater ability to recruit high-threshold motor units will probably benefit from training less often, and taking longer recovery times between workouts.
Taken together with the fact that athletes with a greater proportion of fast twitch fibers will also experience more sustained fatigue after workouts (and will therefore probably benefit from training less often, and taking longer recovery times between workouts), this suggests that the fastest athletes will likely be those that need the most recovery after workouts (both because of their higher proportions of fast twitch fibers and also because of their greater ability to recruit high-threshold motor units). This is a factor that may need to be taken into consideration when writing training programs for athletes of different levels of ability (and not just for athletes with different training histories).
What is the takeaway?
Individual athletes can respond differently to the same training program involving both heavy strength training and fast movement training, for a number of reasons. If we are able to take the major factors into account when planning a strength training program, we can maximize the beneficial effects of the training programs.
Firstly, athletes likely have different force-velocity profiles, such that some have a force deficit (and therefore respond best to heavy strength training), while others have a velocity deficit (and so respond best to fast movement training). Secondly, athletes have different levels of motor unit recruitment and motor unit firing rates. Athletes who have already achieved high levels of both of these qualities will not display large increases in high-velocity strength after training, while athletes who have not achieved high levels of one or both of these qualities have the potential to make large improvements after specific types of training.
Thirdly, individual athletes will likely have differences in their relative proportions of very fast (type IIX) and moderately-fast (type IIA) fibers. When athletes have high proportions of very fast (type IIX) fibers, starting heavy strength training has the potential to produce a negative effect, by converting these fibers to type IIA fibers. This negative effect would not happen if an athlete has already converted most of their very fast (type IIX) fibers to moderately-fast (type IIA) fibers. Conversely, when athletes have high proportions of moderately-fast (type IIA) fibers, stopping heavy strength training has the potential to produce a positive effect, by converting these fibers back into type IIX fibers. This positive effect would not occur if an athlete still possessed most of their very fast (type IIX) fibers.
Finally, when athletes have a very high proportion of fast twitch fibers and/or they have an ability to recruit a large proportion of their high-threshold motor units (which allows them to activate most of their fast twitch muscle fibers), they will experience more sustained fatigue after each workout. Given that these qualities also give athletes the ability to produce high levels of force at high velocities, such athletes are likely to be the fastest in their sports. These athletes will likely not be able to recover as easily from one workout to the next, and may therefore not respond well to as high a training frequency or as high a training volume as other athletes, unless aspects of the workout are altered to reduce the amount of fatigue that occurs.
If you like this article, you will also like my first book (see on Amazon).