How does heavy strength training transfer to fast sporting movements?

Heavy strength training is commonly used to prepare athletes for sport, but exactly how it increases force production during a fast sporting movement is not as obvious as you might assume.

When reading articles about strength training, you will come across the idea that heavy strength training increases “strength,” and that this increased “strength” allows athletes to produce more force in a sprint or a jump.

This oversimplification causes a lot of confusion.


What is strength?

Strength is the ability to produce force, and our ability to produce force is dependent upon our current level of fatigue. A heavy strength training program may fail to record increases in strength, if fatigue is still present at the point when the measurement is taken.

For example, immediately after a workout involving lifting heavy weights, our ability to display maximum concentric strength (1RM) is reduced. The adaptations caused by the workout take time to appear, while the fatigue produced causes a temporary reduction in our ability to produce force.

Our ability to produce force is also dependent upon the conditions in which we measure it. Force production differs according to the length of the muscle, the speed of the contraction, and whether the muscle is shortening or lengthening. Moreover, training under a certain set of conditions will not always improve our ability to produce force in another situation.

For example, heavy strength training using an exercise variation with a partial range of motion (ROM) tends to increase our ability to produce force mainly in the partial ROM exercise variation, and not in a full ROM variation. The specific adaptations that occur after training allow us to produce more force in the one exercise variation, but not in the other.

Ultimately, strength is something that we express in a given situation, and while it can be modified after a strength training program, the way in which it changes is most accurately described by referring to the underlying adaptations.


What is heavy strength training actually doing?

Heavy strength training is primarily judged based on its ability to improve maximum concentric strength (1RM) in a given exercise.

Once post-workout fatigue has passed, our ability to perform a 1RM is enhanced by heavy strength training because of four types of underlying adaptation, as follows:

  1. Technique
  2. Intermuscular coordination
  3. Voluntary activation
  4. Changes in muscle-tendon unit properties

All of these ways represent adaptations through which 1RM can be improved, but only two (voluntary activation, and changes in muscle-tendon unit properties) might contribute to our force-producing capacity in high-velocity movements, including key sporting actions like sprinting and jumping.

Technique only relates to our ability to perform a lift efficiently, and does not reflect the capacity of our muscles to exert force. Performing a deadlift with poor technique (such as allowing the bar to drift too far forwards) will limit our 1RM, and correcting this may allow us to lift a heavier weight. Yet, our muscles are not producing more force, we are just lifting a heavier weight using a more efficient movement pattern.

While intermuscular coordination can enhance our ability to produce force, it is highly specific to the exact movement pattern being performed. So even though it represents a true increase in our ability to exert force in a movement, switching from the gym exercise to another movement (like a sporting movement) means that we lose the benefit.

So the real questions are: how exactly do increases in voluntary activation and changes in muscle-tendon properties after heavy strength training improve force production (strength) in fast movements?


How does increased voluntary activation improve force production (strength) in fast movements?

What is voluntary activation?

Voluntary activation refers to the extent to which the muscle fibers of a muscle (which are governed by motor units) are actively producing force to their full capacity during a maximal contraction. It is determined by main two factors:

  1. Degree of motor unit recruitment
  2. Rate coding (motor unit firing frequency)

All other things being equal, higher degrees of motor unit recruitment and higher rate coding both lead to greater voluntary activation, and therefore greater force production.

N.B. Older review papers and textbooks often refer to motor unit synchronization as a factor that determines voluntary activation, and therefore force production. To the extent that action potential discharges of motor units are synchronized, this can theoretically affect force production. However, new research has shown that the influence of synchronization is fixed, because muscle force only depends on the degree of common drive. Thus, we don’t need to consider motor unit synchronization when thinking about the effects of strength training.


How does voluntary activation increase after training?

Heavy strength training involves sustained muscle contractions. When lifting very heavy weights, muscles are contracting nearly isometrically, and during isometric contractions, maximum force can actually take a couple of seconds to be reached.

Changes in voluntary activation after training with sustained muscle contractions likely reflects improved motor unit recruitment only. Isometric training using sustained (3-second) maximal contractions increases voluntary activation, but does not increase rate coding.

Conversely, increases in voluntary activation after explosive strength training (whether isometric or dynamic) likely reflect large increases in rate coding in the early time windows.

Thus, the mechanisms through which heavy strength training and explosive strength training increase voluntary activation are probably different. While heavy strength training achieves it through increased motor unit recruitment, explosive strength training accomplishes it by increased rate coding.


Why does voluntary activation increase in different ways?

Heavy strength training and explosive strength training likely increase voluntary activation in different ways *not* because of differences in the contraction velocity, but because of differences in the type of voluntary motor command, which can be preprogrammed, or controlled continuously.

Explosive contractions (whether isometric or dynamic) involve pre-programmed motor commands. The muscular contraction is very brief and is immediately followed by the relaxation of the same muscles, such that any further dynamic movement occurs by momentum. The command is said to be preprogrammed because it cannot be canceled once it has begun, and the movement must run its course through to the end.

Sustained contractions, in which force is ramped upwards, do not involve this element of preprogramming, and are constantly subject to control by the central nervous system. They can be stopped at any time.

The nature of the voluntary command is linked to the level of rate coding. Preprogrammed, ballistic contractions involve very high levels of rate coding at the start of the contraction, but sustained contractions do not. Thus, pre-programmed, ballistic contractions can easily lead to improved rate coding, while sustained contractions do not.


What does this mean?

Fast movements involve high levels of rate coding at the start of the contraction for one reason: it is the most effective neural strategy for producing high levels of force at fast speeds.

Similarly, explosive strength training causes large increases in the levels of rate coding at the start of the contraction for one reason: it is the most effective modification that can be made to the neural strategy for increasing force at high speeds.

So while increasing motor unit recruitment through heavy strength training will probably help, it is very unlikely to be as effective as explosive strength training.


How do changes in muscle-tendon properties affect force production (strength) in fast movements?

What changes happen after heavy strength training?

When we lift heavy weights, we produce several changes in the muscle-tendon unit. The main ones are:

  1. increased muscle size
  2. increased lateral force transmission within the muscle
  3. increased tendon stiffness
  4. fiber type shifts

#1. Muscle size

Increases in muscle size are almost universally agreed to be a good thing, but clearly there must be a limit to this when considering the ability to produce force at high velocities, otherwise track and field athletes would look like professional bodybuilders.

Indeed, it is a curious phenomenon that increases in muscle size never quite improve force production at high velocities by quite as much as they do at low velocities. Exactly why this occurs is not known for certain, but may relate to a change that occurs because of the increased mass, and a change that occurs subsequent to the increased size.

Increases in muscle mass cause an increase in the tissue inertia, which reduces the maximum contraction velocity of the muscle. Increases in muscle size increase internal moment arm lengths, which increases the extent to which the muscle must shorten for the same change in joint angle. This in turn reduces force at high joint angular velocities where maximum contraction velocity is a limiting factor (because a faster muscle contraction velocity is needed for the same joint angular velocity).


#2. Lateral force transmission

Most of the force produced by muscle fibers is transmitted laterally to the collagen layer surrounding them, and then onwards to the tendon and to neighboring muscle fibers.

This force is transmitted through lateral links, called costameres. The number of costameres increases after strength training, which increases the amount of lateral force transmission that occurs.

Since the increase in the number of lateral linkages creates an *effective* increase in the number of sarcomeres working in parallel, alongside an *effective* decrease in working muscle fiber lengths, this increases the maximum amount of force that muscle fascicles can produce, while reducing their maximum contraction velocity.

Lateral force transmission is a very important mechanism by which maximum strength is increased after heavy strength training, and is likely why research has shown that training can increase maximum strength per unit cross-sectional area without changes in power output per unit muscle volume.


#3. Tendon stiffness

Tendons elongate when muscles produce force, which causes muscles to shorten faster than the change in muscle-tendon unit length.

When muscles shorten faster, they produce less force. Therefore, the extent to which a tendon elongates at the start of a contraction is a key factor that can influence force production.

When tendons are stiff, this allows the muscle to shorten at a similar speed to the muscle-tendon unit, and they therefore produce maximal force. When tendons are compliant, muscles shorten at a faster speed to the muscle-tendon unit, and produce less force.

Since heavy strength training increases tendon stiffness, this is one way in which force production can be enhanced in many other types of muscular contraction. Yet, the stretch-shortening cycle often depends on a somewhat compliant tendon, and increasing tendon stiffness may not be always optimal for some high-velocity movements.


#4. Fiber type shifts

Heavy strength training causes a very quick shift in very fast (type IIX) fibers to moderately-fast (type IIA) fibers. Other than that (and the reduction in the number of hybrids), fiber type shifts after strength training are fairly unclear, at least in humans.

In contrast, high-velocity strength training does not tend to alter type IIX fiber proportion by anywhere near as much.

Type IIX fibers have very fast contraction velocities, and by reducing their proportion, heavy strength training potentially decreases the maximum contraction velocity of the muscle, which likely has a negative effect on high-velocity force production.


What does this mean?

Heavy strength training causes increases in muscle size, lateral force transmission, tendon stiffness, and a shift in fiber type proportion from type IIX to type IIA.

Increases in muscle size and lateral force transmission have a smaller effect on high-velocity force production than you might expect, because of their negative side effects, and the shift in type IIX to type IIA fiber proportion is inherently negative for high-velocity strength. Whether tendon stiffness is beneficial or not likely depends on the way in which the stretch-shortening cycle is used in a fast movement.


What else is going on?

Heavy strength training with free weights usually involves an initial reduction in antagonist (opposing) muscle activation in the exercise being performed, which represents an improvement in intermuscular coordination.

This appears to happen because synergist muscles learn to perform the supporting role that the antagonist muscles were initially performing, and this in turn allows greater forces to be displayed at the joint, because the synergists do not impede agonist (prime mover) force in the same way as the antagonist.

Even so, long-term heavy strength training can also lead to increases in antagonist (opposing) muscle activation, which may or may not be limited to the exercise being performed.

This appears to happen in order to increase joint stability, and may be a benficial adaptation in order to protect the body from the increased forces that can be exerted. Yet, one adverse effect of this strategy is that the effect is carried over into fast movements.

In contrast, the exact opposite effect happens after high-velocity strength training, where antagonist activation decreases, and this allows us produce higher speed contractions, and thereby increase high-velocity strength.


What is the takeaway?

Heavy strength training *can* cause increases in the ability to produce force in high-velocity movements, because of increases in voluntary activation and changes in muscle-tendon properties. Yet, it is almost certainly not as effective as high-velocity (ballistic/power) training.

Increases in voluntary activation after heavy strength training likely arise through increased motor unit recruitment, rather than increased rate coding, and changes in motor unit synchronization probably do not happen at all. While increased motor unit recruitment can improve force production in high-velocity movements, it is unlikely to be as effective the increases in early phase rate coding produced by explosive strength training.

Heavy strength training also causes increases in muscle size, lateral force transmission, tendon stiffness, and a shift in fiber type proportion from type IIX to type IIA. Increases in muscle size and lateral force transmission have a smaller effect on high-velocity force production than you might expect, because of their negative side effects, and the shift in type IIX to type IIA fiber proportion is inherently negative for high-velocity strength. Whether tendon stiffness is beneficial or not likely depends on the way in which the stretch-shortening cycle is used in a fast movement.