What is the relationship between muscle growth and strength gains?

When we do strength training, we get stronger in the exercises that we use in our training program, and we also experience growth in the muscles we have been working.

But we often observe strength gains *without* simultaneously achieving muscle growth. And we sometimes attain muscle growth without increasing strength.

So just how much does muscle growth contribute to gains in strength after training?

To answer this question, we need to understand (1) the various factors that determine strength gains, and (2) how the contribution of each of these factors differs according to the strength test we use.


What factors determine strength gains?

Many factors can influence our ability to produce muscle force.

Here are the most important factors affecting strength, with factors in green being inside the muscle, factors in blue being inside the tendon, and factors shown in purple being inside the central nervous system (CNS).


Factors inside the muscle and tendon

  • Muscle cross-sectional area is the most common way of measuring muscle size. A larger whole muscle cross-sectional area mainly reflects a larger muscle fiber diameter. Larger fiber diameters mean that individual fibers (and hence the whole muscle) can exert more force.
Increasing muscle cross-sectional area likely causes strength gains in all strength tests.
  • Fascicle length is the most common way of measuring muscle fiber length (fascicles are bundles of fibers). An increase in fascicle length reflects an increase in fascicle volume. So in one respect, an increase in fascicle length is another way in which muscle size increases. However, an increase in fascicle length also reflects an increase in the number of contractile units (sarcomeres) in series, and this shifts the joint angle at which peak force is produced to longer muscle lengths. It may also impact on contraction velocity, because all sarcomeres in a fiber contract at the same time.
Increasing fascicle length likely contributes more to strength gains when strength tests involve peak contractions at longer muscle lengths.
  • Pennation angle is the angle at which individual muscle fibers are arranged in a muscle, relative to its longitudinal axis. After strength training, pennation angle increases. This allows more fibers to fit into the same amount of space, but also reduces the proportion of force transmitted by each single fiber to the tendon. Also, pennation angle changes *during* a muscle contraction, because the fibers rotate. This allows more force to be exerted in fast contractions, because the whole muscle can shorten more for the same amount of single fiber shortening. This is key, because more pennated fibers seem to rotate more than less pennated fibers, so increasing pennation angle may have benefits for maximum muscle shortening velocity.
Increasing pennation angle may contribute more to strength gains as test speed increases, because more pennated muscles rotate more than less pennated muscles.
  • Muscle moment arm length is determined by individual anatomy but also by muscle size, making it another way in which muscle growth can increase strength. Muscle force is transmitted to the outside world by net joint moments (turning forces) and the size of these joint moments is equal to the muscle force multiplied by the muscle moment arm length. Yet, because of the force velocity-relationship, the beneficial effect of increasing muscle moment arm length decreases as the speed of the strength test increases.
If moment arm length increases after strength training, a faster muscle shortening velocity will be needed to achieve a given joint angular velocity during lifting, and this will reduce muscle force at this speed.
  • Fiber type is a way of classifying muscle fibers, with type IIX fibers contracting the most quickly, and type I fibers contracting slowly. In contrast to the differences in contraction speed between slow and fast fibers, there are *much smaller* differences in maximum isometric force production, but type II fibers are slightly stronger than type I fibers. Yet, fibers only shift type after strength training in a way that is detrimental for force production, by shifting from type IIX to the slightly slower type IIA, and this effect is therefore most marked at fast contraction speeds.
Fiber type shifts after strength training likely reduce strength more when the speed used in a strength test is higher, because of their detrimental effect on contraction speed.
  • Ability to transmit force laterally is a key determinant of strength. Although we tend to think of muscle fibers transmitting force longitudinally from one end to another, they actually transmit most of their force laterally, to the surrounding collagen layer. After strength training, we increase the number of links (called costameres) between the fibers and this collagen layer, and this increases our ability to transmit force, and therefore our strength. Yet, the practical effect of increasing the number of these links is to decrease the functional length but increase the number of fibers in parallel inside a muscle, which reduces maximum fiber contraction velocity. So increasing lateral force transmission is less helpful for high-velocity strength gains.
Increases in lateral force transmission ability do contribute to strength gains, but the effect is probably smaller at high speeds.
  • Myofilament packing density refers to the relative volume of contractile material packed into an individual muscle fiber. It has been proposed as a mechanism by which strength might increase without changes in other factors. If this were to happen, we would always observe increases in the maximum isometric force of single muscle fibers when extracted by biopsy before and after strength training, and this only rarely occurs. Currently, it is doubtful whether this effect can actually happen, but if so, it would improve strength under all conditions.
An increase in myofilament packing density would increase strength under all conditions, if it happened.
  • Tendon stiffness refers to the extent to which the tendon elongates when a muscle attempts to shorten, to produce force in the lifting phase of an exercise. If the tendon is very stiff, the muscle only shortens to the extent that the joint angle changes. If the tendon is not stiff, then the muscle can shorten faster than the joint angle changes (because the tendon elongates). A muscle produces less force when it shortens more quickly, so a stiffer tendon generally leads to greater force production. Increasing tendon stiffness likely therefore contributes to strength gains under all conditions.
An increase in tendon stiffness may increase strength gains under all conditions.

Factors inside the central nervous system

  • Voluntary activation refers to the extent to which an electrical stimulus superimposed upon a maximal voluntary contraction can increase force production. If it cannot produce any incremental effect, the muscle is already fully activated under voluntary conditions. Voluntary activation is achieved initially by increasing levels of motor unit recruitment, and then by increasing the frequency of the signal from the CNS (called rate coding). Full voluntary activation likely reflects a state of tetanus, in which all motor units are recruited and rate coding is fast enough such that the muscle fibers do not relax between signals from the CNS. Yet, rate coding is >5 times faster in high-velocity contractions, so increases in rate coding likely exert a greater effect on strength at faster speeds.
Increases in voluntary activation (by either motor unit recruitment or rate coding) can increase muscle force after strength training, but changes in rate coding are likely more influential when strength tests involve faster speeds.
  • Antagonist coactivation is the same thing as voluntary activation of the agonist (prime mover) muscles, except it refers to the activation of antagonist (opposing) muscles in a given agonist contraction (like the triceps in a biceps curl). When antagonist activation is increased, this reduces our ability to exert force, even if our agonist muscle activation (or muscle size) increases. In general, antagonist activation increases after heavy strength training, but it can reduce after high-velocity strength training, especially when testing strength at fast speeds.
Increases in antagonist activation reduce muscle force after strength training; this tends to happen more after heavy strength training, and less after high-velocity strength training.
  • Synergist (stabilizer) activation is the same thing as activation of antagonist (opposing) muscles in a given agonist contraction, but refers to muscles that provide stability (like the shoulder girdle in biceps curls). According to gym lore, older lifters say “you cannot shoot a cannon from a canoe” when explaining this fact to novices. In other words, when synergist activation is increased, this improves our strength by improving the stability of the system in which we are working, and this allows greater joint moments for the same amount of agonist muscle force.
Increases in synergist activation increases strength by improving stability, which likely causes strength gains in tests where such stabilization is needed.
  • Coordination is still largely a black box, and we know very little about it. Yet, there is clearly a large role for coordination in achieving strength gains, especially in multi-joint movements, although such coordination is almost certainly both load-specific (differs between a 1RM and a 5RM) and velocity-specific (differs between a jump squat and an unloaded vertical jump).
Increases in coordination certainly contribute to strength gains, but improvements are likely greatest when the strength test is identical to the type of training performed.

So what does this all mean?

Clearly, it means that *many* factors can affect strength gains, and they differ in relative importance depending on the type of strength training we do, and the type of strength we want to develop.

So a good way to approach the question is to consider the effects of all of these factors on two very important types of strength for athletes: (1) maximum strength, and (2) high-velocity strength.


#1. What is the relationship between changes in *maximum* strength and muscle growth?

When we are improving maximum strength, we use conventional, heavy strength training, which produces the following effects:

  1. Increases in muscle size
  2. Increases in lateral force transmission
  3. Increases in voluntary activation
  4. Increases in tendon stiffness
  5. Increases in load-specific coordination

If we reduce the weight on the bar so that we can do more reps (as in a bodybuilding program), we will likely achieve smaller improvements in lateral force transmission, voluntary activation, tendon stiffness, and load-specific coordination.

Powerlifting programs that use very heavy loads will likely achieve large gains in maximum strength through all of these factors (and therefore report only a moderate relationship between strength gains and muscle growth).

In contrast, bodybuilding programs (and volume-based powerlifting programs) will probably only achieve gains in maximum strength through increases in muscle size (and will therefore report strong relationships between strength gains and muscle growth).


#2. What is the relationship between *high-velocity* strength gains and muscle growth?

When we are improving strength for sports performance, we tend to use high-velocity strength training exercises like plyometrics and loaded jump squats to improve force production in fast athletic movements like sprinting, changing direction, and vertical jumping.

Strength gains in high-velocity strength tests are achieved through various mechanisms that occur only after high-velocity strength training or plyometrics, including:

  • Increases in rate coding, particularly at the start of a contraction
  • Reductions in antagonist (opposing) muscle activation
  • Increases in velocity-specific coordination
  • Increases in single muscle fiber contractile velocity
  • No shift in muscle fiber type, despite small increases in muscle size

Consequently, there are large improvements in force production in high-velocity strength tests, despite minimal muscle growth. So the relationship between strength gains and muscle growth is almost always going to be fairly weak.


N.B. Force and power

Some people struggle with the idea of high-velocity strength tests (like force in jump squats), because they associate such tests with measuring “power,” which is force multiplied by velocity.

Yet, this is just a matter of convention, because force is always a measure of strength irrespective of the velocity we choose to measure it at, whether it is zero velocity (isometric), a very slow velocity (as in a 1RM), or a high velocity (as in a vertical jump). Other than historical convention, there is no reason why power should only be measured at high velocities, and force at slow velocities.

Moreover, power is actually a poor measure for sports performance, because it is almost always measured at an arbitrary velocity that maximizes the value of power, but in sport force is often produced at higher velocities. So force at a maximum velocity, or a given sports-specific velocity would be a better measurement.


What is the takeaway?

Although we are all keen to identify the *exact* relationship between strength gains and muscle growth, we will likely never find one, because so many different adaptations can influence strength gains, and their relative importance changes depending on the type of training that was done, and the type of strength being measured.

The closest relationships between strength gains and muscle growth *may* occur after training that provides a large stimulus only for muscle growth, and not for other adaptations, like bodybuilding programs using high volumes of moderate loads, and submaximal lifting speeds.