Nordic curls enhance sprinting ability by increasing hamstrings active stiffness

Not that long ago, I predicted that researchers would eventually find that the Nordic curl improves sprinting performance, based on my analysis of (1) the specific strength qualities that it develops, and (2) the biomechanics of sprint running.

Just recently, a study was published, confirming my prediction, and showing that the Nordic curl does indeed improve sprinting ability, and in team sports athletes no less.

Even so, the results of this long-term trial are being disregarded by some strength and conditioning coaches, because of the opposing opinions of some popular experts.

Some of these experts find it hard to understand how the Nordic curl can achieve improvements in sprinting performance despite being a single-joint exercise that looks nothing like the sprinting movement. Others find it hard to believe that an eccentric-only strength training exercise for the hamstrings could enhance sprinting ability, when the hamstrings muscles may be behaving isometrically in certain parts of the sprinting movement.

Neither of these concerns is actually a problem, however, if you look at the adaptations that eccentric training produces, and how these adaptations may influence muscle-tendon function during sprint running.


What determines sprinting performance?

Although sprint running looks quite complex from the outside, there are actually just three key parts of the movement that contribute substantially to performance.

#1. Energy absorption in swing

At the end of the swing phase of the sprinting gait cycle, sprinters need to decelerate the thigh and the shank as they swing forwards into hip flexion and knee extension. The sprinter decelerates these segments by producing hip extension and knee flexion joint moments (turning forces).

Yet, it is not the moments alone that decelerate the segments. The segments have rotational kinetic energy about the hip and knee. This kinetic energy must be completely absorbed before the segments can be rotated in the opposite direction.

Energy absorption is equal to negative work done, which in a rotating system is the joint moment multiplied by the angular range of motion over which the moment is applied. And energy absorbed at the knee in the swing phase is nearly perfectly associated with running speed, and increases substantially with increasing speed.

In other words, how fast a sprinter runs is very closely linked to how much negative work is done by the hamstrings muscle-tendon unit to produce a knee flexion joint moment over an angular range of motion.

Clearly, the sprinter can increase the amount of energy absorbed by either increasing the size of the joint moment, or increasing the range of motion over which the moment is applied. The limiting factors to the amount of negative work that can be done are therefore (1) the size of the joint moment, and (2) the range of motion over which the moment can be applied.

  1. The size of the joint moment is determined by how much force the hamstrings muscle-tendon unit can produce while performing high-velocity lengthening in a cyclical, stretch-shortening cycle movement, and
  2. The range of motion over which the moment can be applied is determined by the point at which the sprinter begins decelerating the segments, which is an iterative function depending on how much positive work was done on the segments in the first half of the swing phase.

Essentially, the sprinter regulates the speed of the sprinting gait cycle so that the kinetic energy produced in the first half of the swing phase is absorbed in the second half. By cutting the first half of the swing phase shorter, the kinetic energy produced is reduced, while the available joint angular range of motion for absorbing kinetic energy in the second half is increased.

This is why both hip flexor positive work done in the first half of swing and knee flexor negative work done in the second half of swing are both similarly capable of explaining running speed.


#2. High-velocity hip flexion

The ability to perform high-velocity hip flexion at the start of the swing phase is the other half of the iterative equation to the ability to absorb kinetic energy in the terminal swing phase. The faster the sprinter can accelerate the thigh and shank about the hip and knee, the sooner they can start decelerating them again.


#3. High-velocity hip extension

In the terminal swing and stance phases, the hip extensors must shorten at high velocities, to drive the leg backwards, and push the athlete forwards.

Since the hip is not very flexed at the point when the foot reaches the ground, and finishes the movement in full hip extension, the hip extensors have to achieve this fast movement at very short muscle lengths. The gluteus maximus has the longest muscle moment arm for hip extension in this range of motion, so the hamstrings probably play a smaller role than the glutes while the feet are in contact with the ground.


What adaptations happen after Nordic curl training?

The Nordic curl is an eccentric-only exercise, which means that it places supramaximal loading on the working muscles while they are lengthening. This produces unique adaptations.

While greater increases in fascicle length are often identified as the key unique adaptation after eccentric training, eccentric training also causes preferentially larger increases in eccentric strength than in other types of strength, such as isometric or concentric strength.

Such preferentially larger increases in eccentric strength (called “eccentric-specific” strength gains) probably occur through several mechanisms.

Some commentators attribute eccentric-specific strength gains entirely to neural adaptations. This cannot be true, because we observe the same effect when measuring strength using involuntary (electrically-stimulated) contractions, where the amount of neural drive to the muscle is taken out of the equation.

Eccentric training likely changes the amount and structure of muscle collagen, increases the number of lateral links (costameres) between muscle fibers and the surrounding layer of muscle collagen, and increases the amount of titin. This causes an increase in eccentric strength even without a correponding increase in muscle size or maximum isometric strength.

Even so, eccentric training does also produce large gains in voluntary activation of a muscle, potentially through both spinal and supraspinal mechanisms. This adaptation contributes substantially to the ability to produce force in all types of contraction, but the gains in strength are most noticeable in eccentric contractions, because the starting deficit is greater for that contraction mode.


How do these adaptations enhance sprinting ability?

Sprinting performance is (partly) determined by the ability to decelerate the rotation of the thigh and shank in the second half of the swing phase. When a sprinter decelerates these segments more quickly, they run faster.

Deceleration occurs when the sprinter does negative hip extension and negative knee flexion work, and absorbs the kinetic energy of the segments. This energy must be either stored as elastic strain energy in the muscle-tendon unit, or dissipated as heat.

When an object changes length in a linear system, as the hamstrings muscle-tendon unit does, the amount of elastic strain energy absorbed is equal to the area under the force-distance curve. If there is a linear relationship between force (F) and distance (x), then elastic strain energy absorbed = ½Fx. Since the stiffness of the object (k) is equal to F/d, we can restate this as E = ½kx².

Essentially, the ability of the object to absorb elastic strain energy is proportional to its stiffness.

Greater stiffness of the hamstrings muscle-tendon unit therefore leads to superior absorption of elastic strain energy in the swing phase of sprinting, which will permit faster running speeds.

If the hamstrings muscle is inactive, the stiffness of the muscle-tendon unit is determined by the passive stiffness of the muscle and the stiffness of the tendon. Eccentric training tends not to increase muscle-tendon unit stiffness when the hamstring muscle is inactive, because it decreases passive muscle stiffness while increasing tendon stiffness. This probably happens because of the increase in muscle fascicle length, which makes the muscle easier to elongate to a given length.

But in sprinting, the hamstrings muscle is often very active!

When the hamstrings muscle is active, the stiffness of the muscle-tendon unit is determined by the active stiffness of the muscle and the stiffness of the tendon. 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.

The Nordic curl improves sprinting ability by increasing eccentric strength by the mechanisms outlined above, which increases its active muscle stiffness, which enhances the ability of the sprinter to store energy in the swing phase of the running gait cycle.


Problem #1. Nordic curls are single-joint exercises

Some experts have suggested that since Nordic curls are single-joint exercises, they cannot improve sprint running performance.

This assumes that strength training works by improving or altering coordination patterns. Yet, as the above analysis shows, strength training does not work like this, and produces adaptations inside the muscle and inside the central nervous system that have nothing to do with coordination. These effects alter the eccentric strength (and therefore the active stiffness of the muscle) regardless of the movement pattern.

  • Inside the muscle — eccentric strength can be increased by increases in muscle size, changes in the amount and structure of muscle collagen, increases the number of lateral links (costameres) between muscle fibers and the surrounding layer of muscle collagen, and increases in the amount of titin. These factors will increase eccentric strength (and therefore active stiffness of the muscle) regardless of the movement pattern.
  • In the central nervous system — eccentric strength can be increased by increases in voluntary activation, both through spinal and supraspinal mechanisms. This increased voluntary activation can be used regardless of whether the muscle is used to perform hip extension or knee flexion, and is therefore again independent of the movement pattern.

This is not to say that other exercises cannot improve performance in sprint running by improving intermuscular coordination, just that the Nordic curl is perfectly capable of increasing sprinting ability even though it does not look particularly “functional.”


Problem #2. Sprinting may not involve any large amount of hamstrings muscle lengthening

Work in animal models suggests that muscles tend not to change length as much as you might expect during cyclical stretch-shortening cycle movements like sprinting, because the tendon lengthens and shortens very quickly, in tandem with the changes in length of the whole muscle-tendon unit.

This strategy allows a large portion of the kinetic energy expended in one cycle to be absorbed as elastic energy, and then returned again as kinetic energy in the next cycle, making the movement more efficient than it would otherwise be.

This movement strategy is more efficient because tendons are very good at returning energy that they absorb. The more that the muscle remains the same length during a cyclical stretch-shortening cycle contraction (such that the tendon changes length more), the more efficient the movement becomes.

Some researchers have therefore suggested that elastic energy storage can be increased during sprinting if we can train the hamstrings muscles to lengthen less. This makes a lot of sense. The same researchers have suggested that this training effect can be best achieved by doing strength training exercises at a static, constant joint angle (called “isometrics”), rather than by using dynamic, eccentric (lengthening) contractions as in the Nordic hamstring curl. I do not think this is the case.

In this model of cyclical stretch-shortening cycle contractions, the goal of training is to develop an athlete from a point where they are typically performing active hamstrings muscle lengthening during sprint running, to a point where they are capable of maintaining the hamstrings muscles at a largely constant length. To achieve this, we need to make the muscle stiffer, and therefore harder to elongate. When the muscle is extremely stiff, even very high forces will only cause small amounts of muscle lengthening. Active stiffness is essentially the same thing as eccentric strength, which is most effectively increased by eccentric training.

Once the athlete is capable of maintaining the hamstrings muscles in a largely isometric state at the appropriate point in the same running movement, then doing strength training exercises at a static, constant joint angle may be more effective. However, they are unlikely to be effective before this stage in training.

Also, on a practical level, strength training exercises that use a static, constant joint angle (isometrics) are quite problematic. They produce gains in strength that are very specific to the joint angles used in training, making the room for error quite small. What is more, these joint angle-specific strength gains are determined by multiple factors.

Muscle length and joint angle are two such factors, and the muscle length at a given joint angle in a strength training exercise may not be the same as in sprinting. So simply identifying the joint angles used in sprinting may not identify the right muscle length to be trained with an exercise. In fact, it may not even be possible to recreate in a strength training exercise the exact same muscle length and joint angle, at the same time, that is observed during sprinting.


What is the takeaway?

Sprinting performance is (at least partly) determined by the ability to decelerate the rotation of the thigh and shank in the second half of the swing phase. A greater ability to decelerate these segments is closely linked to faster running performances. The Nordic curl enhances the ability to absorb energy in the terminal swing phase of sprinting, by increasing the active stiffness of the hamstrings muscle-tendon unit.

Even though the Nordic curl is a single-joint exercise, this does not affect its transfer to sprinting performance, since it helps improve sprinting by increasing the eccentric strength of the hamstrings muscle in ways that are not dependent on movement patterns. Similarly, even though the Nordic curl involves hamstrings muscle lengthening, this does not mean that it cannot transfer optimally to force production during the swing phase of sprint running, where a constant muscle length may be optimal, because high levels of active stiffness are the key to preventing muscle lengthening, not maximal isometric force.