Why are strength gains velocity-specific after heavy strength training?

If you enjoy this article, you will like my first book (see on Amazon).

We can calculate a force-velocity profile for an athlete in an exercise by measuring the bar speed (and the force) produced at a range of loads. Measuring this force-velocity profile at varying times during a training block can be used to monitor the effects of a training program (similar information can be gained by measuring velocity with a range of different loads, although this is called a load-velocity profile).

Very often, the gradient of the force-velocity profile will change after a block of a certain type of training. This tells us that the program block has unique effects on force produced at different speeds (some types of training have larger effects on force at slow speeds, while other types of training have larger effects of force at fast speeds). This is important to know for both coaches and athletes, since most athletes need to perform at fast speeds, while some need to produce force at slow speeds.

We can also consider how the underlying physiological adaptations that cause strength gains (at slow or fast speeds) might change the force-velocity profile. We can then use our understanding of physiology to predict which ways of strength training will be most transferable to improvements in high-velocity strength (speed) as well as in maximum strength.

We can group known adaptations into categories, according to their likely effects on the force-velocity profile.

#1. Adaptations that increase maximum force without changing maximum velocity

Some adaptations that occur after heavy strength training increase maximum force without changing maximum velocity. This causes a shift in the gradient of the force-velocity profile that looks like this:

Increase in maximum force (but no change in maximum velocity)

The adaptations that produce this effect are neural in nature, and include:

  1. an improvement in the coordination of the tested exercise (since such coordination improvements are fairly velocity-specific, so they do not enhance or reduce lift performance at higher velocities); and
  2. a reduction in antagonist coactivation levels (since such alterations are probably also quite velocity-specific). Antagonist coactivation is the extent to which the opposing muscles are active (and producing force) when the agonist muscles are performing the lift (so a higher level of antagonist coactivation reduces lift performance).

Practically speaking, improvements in coordination are most likely to be generated by improving movement skills. This can be done by providing (visual or auditory) feedback, implementing an external focus of attention, and avoiding fatigue during exercise (since fatigue impairs motor learning). To avoid fatigue while lifting heavy loads repeatedly, cluster sets might be useful, as might longer inter-set rest periods (and these are well-known to enhance strength gains).

The practical methods for improving reductions in antagonist coactivation are unclear. Yet, reducing antagonist coactivation very likely occurs through similar mechanisms to coordination. Indeed, stability-specific strength gains are often the result of reductions in antagonist coactivation. Therefore, we can probably accelerate reductions in antagonist coactivation by using the same methods as we use to improve coordination.

#2. Adaptations that increase maximum force but reduce maximum velocity

Many adaptations that occur after heavy strength training increase maximum muscle fiber force while also reducing muscle fiber shortening velocity. This causes a shift in the gradient of the force-velocity profile that looks like this:

Increase in maximum force (but a reduction in maximum velocity)

The adaptations that produce this effect are mainly local (inside the muscle) in nature, and include:

  1. an increase in the amount of lateral force transmission. When the sarcomeres in muscle fibers shorten to produce a muscular contraction, they cause the surrounding endomysium (the layer of collagen that wraps around each fiber) to shorten because they are connected to it by linking structures called costameres. By shortening, this endomysium pulls on the tendons at each end of the muscle, causing the whole muscle to shorten (the sarcomeres do not pull directly on each other, nor do they pull directly on the tendon). When the number of costameres increases after strength training, this increases the number of connections between the sarcomeres of a muscle fiber and its surrounding endomysium. Muscle fiber shortening speed is reduced when a new costamere is added between neighboring sarcomeres for the same reason that short muscle fibers shorten slower than long muscle fibers. When working together in series, two sarcomeres (or two muscle fibers) shorten twice as far in the same duration of time as a single sarcomere (or single muscle fiber); and
  2. an increase in muscle fiber diameter (hypertrophy). While hypertrophy itself does not change the muscle fiber shortening velocity, it does cause a simultaneous increase in the internal moment arm length of the muscle, as well as an increase in tissue inertia, both of which have negative effects on muscle fiber shortening velocity. Increasing the internal moment arm length of the muscle requires muscle fibers to shorten further for the same joint angle range of motion, and this means that they must shorten faster for the same joint angular velocity. When shortening faster, they produce a smaller force, because of the force-velocity relationship.

Practically speaking, the factors that increase lateral force transmission are not known. Therefore, it is unclear how it might be enhanced or avoided. Some researchers think that elements of costameres (such as the dystrophin–glycoprotein complex) may be involved in creating the repeated bout effect. This may suggest that costameres are produced in response to more muscle-damaging exercise, such as eccentric contractions. Alternatively, a large amount of force transmitted laterally inside the muscle might trigger the adaptation. This would imply that high intramuscular pressures (such as are achieved when lifting heavy loads or in eccentric contractions) might be most effective for producing the effect, while using lighter loads might avoid it.

In contrast, the practical factors that increase hypertrophy are well-known. Higher volumes of training and training with a closer proximity to failure (both of which increase the number of stimulating reps that are performed in a workout) are known to enhance gains in muscle fiber size. Therefore, it can fairly easily be avoided (by minimizing volume and fatigue). Again, in this respect, cluster sets might be very valuable.

#3. Adaptations that increase maximum force and also increase maximum velocity

Some adaptations that occur after heavy strength training increase maximum force and also increase maximum velocity. This means that the gradient of the force-velocity profile does not change, and the whole line is shifted vertically upwards, like this:

Increases in maximum force and in maximum velocity

The adaptations that produce this effect are both neural and local (inside the muscle) in nature, and include:

  1. an increase in voluntary activation, or the ability to recruit high-threshold motor units. Untrained individuals do not have the ability to access all of their high-threshold motor units, so they cannot activate the muscle fibers that they control. After heavy strength training, this ability improves, and more motor units become available. Since these motor units control very fast twitch muscle fibers, they can contribute similarly to force production at both slow and fast speeds; and
  2. an increase in myofibrillar density. Recently, it has become clear that this change can occur after strength training, although it may take several years to become apparent (and short-term training blocks may actually cause a reduction instead of an increase). Even so, it essentially produces the same beneficial effects as hypertrophy (an increase in the number of myofibrils in parallel, leading to an increase in the number of actin-myosin crossbridges that can form) but without the increase in internal moment arm length that reduces maximum fiber shortening speed.

Practically speaking, the exact training variables that might enhance gains in voluntary activation are unclear. Yet, strength gains are enhanced (either during a single workout or after long-term training) by many psychological techniques (including giving autonomy, providing a challenging target to hit, or issuing feedback) that by enhancing motivation and therefore effort. Since effort is closely linked to motor unit recruitment, it is likely that techniques that improves effort in a workout lead to increased changes in voluntary activation both instantly and also over time.

In contrast, the practical methods that might increase gains in myofibrillar density are unclear. Indeed, it is totally unknown how this adaptation might be produced (especially since studies have found no effect of load or volume, despite the fact that previous researchers have suggested that heavier loads and lower volumes might enhance myofibrillar hypertrophy, while lighter loads and higher volumes might enhance sarcoplasmic hypertrophy).

#4. Adaptations that do not increase maximum force but decrease maximum velocity

One very well-known adaptation that occurs after heavy strength training does not affect maximum force but still decreases maximum velocity. It is a shift in muscle fiber type from the very fast type IIX fibers to the moderately fast type IIA fibers. This shift means that the gradient of the force-velocity profile alters, like this:

No change in maximum force but a reduction in maximum velocity

Practically speaking, it is known that muscle fiber type shifts occur to a greater extent after higher volumes of training and when experiencing more fatigue during exercise. To minimize fiber type shifts, the optimal approach to training would be to use the least training volume necessary to achieve a progression, and to use techniques that avoid incurring excessive fatigue during training (such as long inter-set rest periods, and intra-set rest periods such as during cluster training).

What is the takeaway?

Heavy strength training often changes the gradient of the force-velocity profile. Usually, it produces an overall increase in maximum force alongside a reduction in maximum velocity. This is the net effect of various adaptations, each of which produce different changes to the force-velocity profile.

Some adaptations increase maximum force without altering maximum velocity (coordination and antagonist coactivation). Some adaptations increase maximum force while reducing maximum velocity (lateral force transmission and hypertrophy). Some adaptations increase maximum force and also increase maximum velocity (voluntary activation and myofibrillar density). And some adaptations just reduce maximum velocity without changing maximum force (fiber type shifts).

In practice, maximum velocity is reduced most when muscle fibers experience high training volumes or a lot of fatigue. While training volume cannot easily be reduced in well-trained individuals, unnecessary volume can be avoided. Equally, fatigue can more be reduced, by using intra-set rest periods and longer inter-set rests. Also, strength gains can be achieved without reducing maximum velocity if certain adaptations are targeted specifically. This can be done by using techniques that enhance coordination (feedback, avoiding fatigue, and an external focus of attention) and voluntary activation (by enhancing effort in each workout through psychological techniques).

If you enjoyed this article, you will like my first book (see on Amazon).

Figuring out how strength training works. See more of what I do: https://www.patreon.com/join/SandCResearch