Strength is Specific: The key to optimal strength training for sports

by Chris Beardsley

If you can display “strength,” that means you can produce force. If you increase your “strength” by getting stronger, then that means you can produce more force than when we last tested you, in the exact same exercise.

Others relate to the way in which our brain and central nervous system coordinate force production at different joint angles, speeds, loads, and conditions of stability.

The length-tension relationship is the observation that muscle fibers have an optimal length for producing force.

Consequently, strength training programs are designed to improve our ability to produce force at most muscle lengths, contraction speeds, and contraction types (shortening or lengthening). However, they do *not* improve them all to the exact same extent.

Strength is something you can only *display* on a given occasion, and it depends on the environment, your state of fatigue, and many other factors.

Secondly, when we lift a light weight to muscular failure, the CNS usually recruits all available motor units towards the very end of the set.

Thirdly, when we lift a light weight (between 30–40% of maximum force) with a fast bar speed, the CNS recruits most of our motor units. Throwing a medicine ball, doing jump squats, or plyometric push ups, all very likely achieve (nearly) maximal motor unit recruitment.

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.

Voluntary activation is achieved initially by increasing levels of motor unit recruitment, then by increasing the frequency of the signal from the CNS (called rate coding).

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.

Lifting to failure allows light weights to recruit more high-threshold motor units. This seems to be important (perhaps even necessary) to cause muscle growth when training with light weights, and muscle growth is probably the only way that this type of training causes strength gains.

In reality, much of that gain in strength is an improvement in balance. As a result, it does not transfer as well as you might expect to strength gains under more stable conditions.

In other words, to achieve meaningful strength gains for sporting movements in more advanced athletes, we need to learn how to apply the principle of specificity.

improvements in the ability to produce force at a low velocity (against a heavy load) are mainly caused by: (1) increases in muscle size, (2) increases in activation levels of the prime mover muscles, (3) increases in lateral force transmission within the muscle, (4) increases in tendon stiffness, and (5) increases in load-specific coordination.

Conversely, gains in strength at a high velocity (against a light load) are mainly caused by: (1) an increased single fiber velocity, (2) greater type IIX fiber proportion, (3) greater activation levels of the prime mover (agonist) muscles in the early phase of a contraction, (4) lower activation levels of the opposing (antagonist) muscles, and (5) velocity-specific coordination.

Muscle activation after heavy load strength training improves over the whole exercise range of motion. In comparison, 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.

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.

This greater mechanical loading on the whole muscle-tendon unit could cause any of the following adaptations that we know increase maximum lifting strength: (1) increased tendon stiffness, (2) increases in the amount of lateral force transmission inside the muscle, (3) increases in the activation of the prime mover muscles, and (4) increases in load-specific coordination.

(Contrary to popular belief, fiber type has a large effect on contraction velocity, but only a minor effect on force production.)

Every muscle has a length at which it produces maximum force. We can call this the optimum muscle length for force production.

Since more overlap means more force can be exerted, the optimum muscle length for force production is where sarcomere overlap is greatest and all the sarcomeres in a chain are neither too short nor too long.

Muscles can alter their ability to produce force at different lengths by increasing in size at certain specific regions and not others. This is called “regional hypertrophy” and appears to occur in tandem with joint angle-specific strength gains, although exactly why is still unclear.

mainly-neural origin of the strength gains explains why the strength gains after partial range of motion training are so localized to one particular joint angle. It also explains why partials tend to produce less muscle growth.

If you want to improve strength at high velocities, they do have an advantage over free weights. This may be useful for team sports athletes, who need to sprint, throw, jump, and change direction quickly.

Even so, several years ago, researchers identified that superior sprinting performances were determined more by the ability to produce greater force in a horizontal direction, rather than in a vertical direction.

It is often now programmed to improve the ability to produce force in a horizontal direction when sprinting, even though there is a stronger case for using it to improve back squat and deadlift performances in powerlifting.

Therefore, it is the hamstrings that need to be exposed to very heavy loads (ideally while lengthening) to improve sprinting ability. In contrast, the gluteus maximus needs to be able to produce force very quickly, which means using light loads and a maximal bar speed, to develop explosive strength in that muscle.

However, it seems very likely that hip thrusts with a lighter load and a maximal bar speed would be able to enhance sprinting ability, by increasing the ability of the gluteus maximus to exert force at short lengths and a high-velocity.

Within the framework of ways that strength is specific, there are eight ways in which strength can produce targeted effects, as follows: (1) contraction mode (eccentric or concentric), (2) velocity, (3) joint angle of peak contraction or range of motion, (4) the number of reps, or the point on the strength-endurance continuum, (5) the degree of stability, (6) external load type (weight or elastic resistance), (7) force vector, and (8) muscle group.

Overall, this means that we should train hip extension for sprinting using high-velocity exercises that load the muscles at short muscle lengths (in the contracted position), like jump squats, hex bar jumps, and heavy kettlebell swings.

After reaching full extension, the sprinter has to recover this leg as fast as possible, to the point where the thigh is just 20 degrees from a horizontal line parallel with the ground.

Strength gains are very specific to (1) the contraction type (lengthening or shortening), (2) the velocity, and (3) the joint angle used in training.

(Note: elongating the lowering phase by using a slower tempo is *not* the same as using a heavier weight. The former allows nearly the same force as in the lifting phase, while the latter involves a far greater force, which is what causes the beneficial adaptations that improve eccentric strength, and therefore decelerating ability).

Often, individuals with a long history of heavy strength training display profiles that are not ideal for vertical jumping, because their force is too high and their velocity is too low. These individuals need to focus on high-velocity strength training.

Eccentric training increases active muscle-tendon unit stiffness remarkably effectively, because it increases the force that can be produced by the muscle while it is lengthening (which is its eccentric strength) to a degree that other types of training cannot, while also increasing tendon stiffness.

Active stiffness is essentially the same thing as eccentric strength, which is most effectively increased by eccentric training.

Especially where strength needs to be displayed primarily in fast movements, during eccentric contractions, and so on.

Since the squat develops low-velocity, concentric strength for coordinated hip and knee extension from long muscle lengths, with the primary muscles being the adductor magnus and quadriceps, the biomechanical rationale for using the squat as the primary exercise for improving sprinting performance is surprisingly weak, given its popularity.

Such advocates of this modern form of functional training have perhaps forgotten (or maybe never learned) that strength is also specific to (1) whether the muscle is shortening or lengthening, (2) whether the muscle is contracting slowly or quickly, (3) the muscle length when maximum force is developed, (4) the number of repetitions performed, (5) the type of resistance used, (6) the direction that force is exerted, and (7) the muscle group that is worked (which can be affected by many of the above factors, as well as just the exercise itself).

Strength and conditioning does actually have a list of its own “basics” or fundamentals, which are normally referred to as “principles”. There are four key principles, which are (1) progressive overload, (2) specificity, (3) individuality, and (4) variation.

In addition to changes in the central nervous system, there are also changes after training with light loads and maximal bar speeds that happen inside the muscle. Such changes include a greater retention of type IIX muscle fibers, and an increase in single fiber contraction velocity. These adaptations are all very important, because high-velocity strength is very transferable to almost all sporting movements, including sprinting and jumping.

In fact, partial range of motion strength improves after training primarily through increases in neural drive that are specific to the joint angles that correspond to short muscle lengths, and not because of any substantial increase in muscle size.

Although it is not well-known, partial squats do seem to be better than full squats for improving jumping and sprinting performance in athletes.

Ultimately, this means that when training athletes, we do need to make a hard choice about whether to prioritize specific strength gains *or* muscle size in our programming. But I think it is fairly clear which option we need to choose.

This is why deliberately slowing down the lifting (concentric) phase of an exercise has no beneficial effects for bodybuilders.

In contrast, when we lift light or only moderately-heavy loads, we do not achieve maximal motor unit recruitment from the first repetition of a set.

However, by stopping sets before fatigue causes a reduction in bar speed, the proportion of type IIX fibers that are converted to type IIA fibers can be reduced. This may contribute to proportionally greater gains in high-velocity strength.

It also has *adverse* effects when trying to improve high-velocity strength, because it accelerates the conversion of fast to moderately-fast muscle fibers, which reduces contraction velocity.

voluntary activation occur when the central nervous system (either the brain or the spinal cord) increases the size of the signal to the muscle, allowing a larger number of motor units to be recruited and the muscle to be more fully activated.

However, muscle-tendon units might also increase their ability to produce force through other mechanisms, including: (1) an increase in tendon stiffness (stiffer tendons allow muscles to shorten at slower speeds, and therefore exert more force), (2) an increase in the density of myofibrillar parts of the muscle cells, also known as myofibrillar packing density (the sarcoplasm does not contribute to force production), and (3) increases in the number of lateral attachments between the muscle fiber and its surrounding collagen layer (this increases the amount of force that is transmitted laterally, but reduces the effective fiber length).