When strength training was first introduced as a preparation method for sports, some sports coaches were skeptical, believing that lifting heavy weights would slow athletes down.
Nowadays, after a great deal of anecdotal evidence (and many scientific studies) it is largely accepted that strength training does typically increase athletic performance, even in high speed events like track sprinting.
And yet, there are cases where heavy strength training has not been effective for improving athletic performance in some high-level power athletes, including track sprinters, and it has actually slowed them down.
So that leads to three questions:
(1) Why does heavy strength training produce *smaller* gains in high-velocity strength?
(2) When *can* heavy strength training improve high-velocity strength?
(3) When might heavy strength training even *reduce* high-velocity strength?
To answer these questions, we first need to understand what determines strength at different speeds.
What determines strength at different speeds?
After a period of strength training, improvements in the ability to produce force at a low velocity (against a heavy load) are mainly caused by:
- increases in muscle size
- increases in activation levels of the prime mover muscles
- increases in lateral force transmission within the muscle
- increases in tendon stiffness
- increases in load-specific coordination
When compared to high-velocity (light load) strength training, low-velocity (heavy load) strength training causes greater adaptations in all of these ways (this ignores training to muscular failure, which produces similar effects on muscle size at all loads).
Conversely, gains in strength at a high velocity (against a light load) are mainly caused by:
- higher single fiber velocity
- greater type IIX fiber proportion
- greater activation levels of the prime mover (agonist) muscles in the early phase of a contraction
- lower activation levels of the opposing (antagonist) muscles
- velocity-specific coordination
These are very different sets of adaptations!
Why does heavy strength training produce *smaller* gains in high-velocity strength?
Heavy strength training does *not* produce the same adaptations as high-velocity, light load strength training. So it will never be quite as effective for improving high-velocity strength.
Inside the muscle and tendon, the changes are very different.
Heavy strength training produces increases in muscle size, increases in lateral force transmission within the muscle, and increases in tendon stiffness. In contrast, high-velocity strength training with light loads does not produce these same adaptations, but instead maintains type IIX fiber proportion and increases single fiber contraction velocity.
Even the improvements in muscle activation and coordination differ, although there is some degree of cross-over.
Muscle activation after heavy load strength training improves over the whole exercise range of motion, while muscle activation after high-velocity strength training increases primarily in the early phase of a contraction, because of increases in rate coding in the first 50–100 milliseconds.
Similarly, while coordination of a movement is similar between heavy load and high-velocity variations of the same exercise, it is *not* identical, and coordination has a habit of being extraordinarily specific to the exact movement pattern performed.
These differences in the adaptations produced by each type of strength training explain why heavy load strength training most often produces smaller improvements in high-speed athletic performance compared to high-velocity strength training.
When *can* heavy strength training improve high-velocity strength?
Although heavy strength training does not improve high-velocity strength through the mechanisms that high-velocity strength does, it can still have a positive effect.
By increasing muscle size, heavy strength training can improve the ability of a muscle to produce force under any conditions.
Additionally, while the increases in muscle activation of the prime movers after heavy load strength training are not identical to the increases after high-velocity strength training (where the gains are mainly in the early phase), there is some transfer, and the same is probably true for coordination.
This why strength gains in beginners (and anyone else who can gain a great deal of muscle size very quickly) are much less specific than you might expect, and it also explains why so many standard heavy strength training programs are still recommended by strength coaches for playing sports, even though they are almost certainly not the optimal way to improve athletic performance in high-velocity movements.
When might heavy strength training even *reduce* high-velocity strength?
Heavy strength training produces some side-effects.
These side effects do not impede the ability to produce maximum force, but a few are unhelpful for producing force at high velocities.
One side-effect of heavy load strength training (especially with higher volumes) is that it produces a shift in muscle fiber type from type IIX to type IIA. Since type IIX fibers are faster than type IIA fibers, this causes a reduction in muscle fiber contraction velocity.
Another side-effect of heavy load strength training is that it tends to increase the amount of muscle activation in the opposing (antagonist) muscles. This in turn increases their resistance to the movement, and this is probably a beneficial adaptation, in order to improve joint stability. Yet, this is the exact opposite of what happens after high-velocity strength training, where the activation decreases, and this allows us produce higher speed contractions, and thereby increase high-velocity strength.
Finally, since heavy load strength training improves lateral force transmission (likely by increasing the number of costameres between the muscle fiber and the surrounding collagen), it is possible that this could reduce force at higher speeds, by reducing maximum contractile velocity.
What is the takeaway?
Heavy strength training produces smaller gains in high-velocity strength because each type of strength training produces different adaptations. Heavy strength training produces adaptations that are useful for increasing maximum strength, while high-velocity strength training causes changes that are mainly beneficial for increasing high-velocity strength.
Yet, heavy strength training can still improve high-velocity strength if it is able to increase muscle size, which is most often in beginners or in other athletes who are for some reason able to gain a large amount of muscle mass.
Even so, under some circumstances, heavy strength training might even *reduce* high-velocity strength, because it produces adaptations that have a negative effect on high-velocity force production, including a shift to slower fiber types, an increase in opposing muscle activation, and an increase in lateral force transmission.