How do strength gains transfer to vertical jumping?
Identifying which strength training exercises are best for improving vertical jump height by asking experts is a very challenging task, as many of them disagree quite substantially with one another.
Fortunately, by looking at the factors that determine vertical jumping performance, we can identify the muscles that are used, and the types of strength that they need to display. Then we can work backwards from there to identify the best exercises.
What determines vertical jump height?
Greater vertical impulse
In simple terms, jump height is determined by the relative vertical impulse that can be produced by the lower body muscles in the upward phase of the jump, while the feet are still in contact with the ground.
Vertical impulse is calculated as the force produced multiplied by the duration of time over which is is exerted. And relative vertical impulse is simply the vertical impulse divided by bodyweight.
This is why using a slightly deeper countermovement often increases jump height, because the larger range of motion allows the muscles to exert force for a longer duration of time before take-off. Jump height *can* increase even though the force produced is almost always smaller. (Force is smaller when the countermovement is deeper partly because shortening through a longer range of motion leads to a faster contraction velocity, on account of the force-velocity relationship, and partly because the leverage of bodyweight on the lower body joints is larger with a deeper countermovement).
Longer duration of time
Since vertical impulse is determined by both the force produced and the time over which that force is exerted, anything that alters the amount of time available to produce force will affect vertical jump height. And there are at least three factors that play a role.
Firstly, the rate at which force is developed an important factor. When force rises faster at the start of the jump, this leads to a greater vertical impulse (and therefore a higher jump height), even when peak force is the same.
Yet, rate of force development is likely less important for vertical jumping than for faster athletic movements, such as sprinting. This is because the time that is available for force production is *ten times* longer in the vertical jump than in sprinting. Sprinters often take their foot off the ground before their lower body muscles have achieved maximum force (which takes approximately 150ms), but this early period of rising force production plays only a very small role during vertical jumping.
Secondly, in addition to the rate of force development, the size of the force itself produces a negative feedback effect on vertical impulse, because higher forces lead to faster accelerations, which in turn reduce the time spent producing force before take-off. This is *partly* why drop jumps tend to involve higher forces, shorter ground contact times, and yet similar jump heights to countermovement jumps.
Thirdly, interactions between the muscle and tendon likely have an impact on the countermovement depth that an athlete uses, and this in turn affects the vertical impulse, because a greater countermovement leads to a longer time spent producing force.
Vertical jump training and assisted vertical jump training (essentially with a negative load) can each increase vertical jump height through increases in countermovement depth, even while actually reducing peak force produced in the jump. This seems to happen because the tendon becomes more compliant after these types of training, which means they elongate more during the countermovement phase of the jump.
When tendons elongate to a greater extent during a jumping movement that is preceded by a countermovement, the muscle lengthens less. This produces two effects. Firstly, the greater elongation of the tendon means that more elastic energy is stored during the countermovement, which is then released in the subsequent jumping phase. Secondly, the smaller elongation of the muscle means that countermovement depth can be greater for the same shortening velocity in the subsequent jumping phase, because the muscle never lengthened that much to begin with. Since shortening velocity determines force, this allows the same muscle force to be produced, despite the larger joint range of motion.
In addition to the duration of time, any factor that affects the amount of force that can be produced will affect vertical impulse, and therefore alter vertical jump height.
In comparatively fast movements like jumping, the main factor that affects force production is the force-velocity relationship.
The force-velocity relationship during muscle shortening occurs because the number of simultaneously attached crossbridges between the myofilaments inside the working muscle fibers determine the amount of force that a fiber can produce. The number of attached crossbridges at any one time is dependent upon the fiber shortening velocity, because the detachment rate of the crossbridges at the end of their working stroke is higher at faster shortening speeds.
This means that the amount of force that can be produced at a given velocity depends very strongly on the force-velocity profile of the lower body muscles working together in the squat/jump pattern.
The force-velocity profile can be described by three elements: (1) maximum strength, (2) maximum velocity, and (3) the slope of the force-velocity gradient, because this is what determines whether the balance between force and velocity is optimal at the desired speed for force production. Each of these factors is an independent predictor of vertical jump height.
When an individual has a force-velocity gradient angled such that force is too high and velocity is too low, they benefit most from high-velocity strength training exercises with light loads. Conversely, when an individual has a force-velocity gradient angled such that force is too low and velocity is too high, they benefit most from low-velocity strength training exercises with heavy loads. 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, so they need to focus on high-velocity strength training.
Larger muscle forces
When performing a vertical jump, the athlete exerts force at the low back, hip, knee, and ankle joints. The spine flexes as the athlete squats downwards, and then is extended by the spinal erectors over the course of the jump. The hip extensors (gluteus maximus, hamstrings, and adductor magnus) work to move the trunk and the thigh apart, which pushes the torso up and backwards. Meanwhile, the knee extensors (quadriceps) contract to extend the knee, and the calf muscles contract to move the shin backwards, towards the vertical.
Many models have been constructed to identify the most important muscles in the vertical jump, with some conflicting results. Some have suggested that movement is governed by the gluteus maximus and quadriceps, while others have proposed that the hamstrings, quadriceps, and calf muscles are key. Importantly, no model has yet explored the role of the adductor magnus, which is the primary hip extensor in the barbell squat. This is relevant, as many studies have found that the squat is an ideal exercise for improving jump height, and maximum back squat strength is closely associated with vertical jump performance among athletes.
Even so, the back squat does differ in important ways from the vertical jump. Primarily, it involves a much greater trunk extension turning force, because of the barbell weight on the upper back, and this likely contributes to the more hip-dominant nature of the squat over the vertical jump. Secondly, it is often performed to a deeper depth, which can alter the relative contribution of each of the hip extensors to the movement, because of their different leverages at each joint angle. And thirdly, it only involves accelerating up to midway through the movement, while the vertical jump involves accelerating right up until take-off. This also affects the relative contribution of the hip extensors, as force production will be required in the jump even when the hip is nearly fully extended, while this is unnecessary in the squat.
Finally, to make things even more complicated, it is likely that the roles of the lower body muscles may differ according to if: (1) the jump is maximal or sub-maximal, (2) long-term training has occurred, and (3) the individual has a “hip-dominant” or a “knee-dominant” technique. Indeed, the vertical jump is more dependent upon the hip extensors in maximal jumps, compared to in sub-maximal ones. And after long-term jumping training, the increase in the amount of work done in the jump by the hip extensors is related to the increase in height, while the increase in the amount of work done by the knee extensors is not.
The vertical jump involves coordinated spine, hip, knee, and ankle extension to produce force in a vertical direction very quickly, while the muscles are shortening through to a very short muscle length. Since the time available for producing force is long compared to other athletic movements, this reduces the importance of rate of force development. Yet, the force-velocity relationship is the primary determinant of the amount of force that can be exerted at a given movement speed. Therefore, maximum force, velocity, and the force-velocity gradient all affect vertical jump height.
Exactly which muscles are most important for improving the vertical jump is still relatively unclear, and may differ between individuals. Clearly, the spinal erectors, hip extensors, quadriceps, and calf muscles are all involved in the jumping movement, and the hip extensors and quadriceps are likely the prime movers, but which of the hip extensors is the primary muscle is very unclear. Importantly, since force production is required right up until take-off, the lower body muscles must produce force from moderate through to short muscle lengths, which differs from the barbell back squat exercise.
Implications for training
The back squat and jump squat are the two most commonly-used strength training exercises for increasing vertical jump height. The back squat is clearly more effective for improving maximum force, while the jump squat can be used to shift the force-velocity gradient towards a more “velocity-oriented” profile when required. In addition, the jump squat has the secondary benefit of training force production right through until the muscles are contracting at short lengths, because of its longer acceleration phase. Even so, it is unclear whether squat variations are optimal for improving vertical jump height, because the center of mass is in a different place from in the vertical jump.
Slightly better results could be achieved using exercises that involve placing the load closer to the center of mass of the body, rather than on the upper back. Such exercises include the trap (hex) bar deadlift and trap (hex) bar deadlift jump, vertical jump while holding dumbbells or halteres, and vertical jump with a weighted vest.
Using only a lifting (concentric) phase for strength training exercises could also be more effective for improving vertical jump height than traditional, stretch-shortening cycle exercises under load, for two reasons. Firstly, using only a lifting phase involves faster rate of force development through higher rate coding, and this may increase high-velocity strength more over the long-term. Secondly, doing stretch-shortening cycle exercises under load *might* cause the tendons to increase stiffness to a greater extent. This would make the muscle lengthen more in the countermovement phase of a jump, and thereby reduce muscle force for a given countermovement depth.
What is the takeaway?
For improving vertical jumping ability, the back squat and jump squat have been used for many years with great success. Depending on the exact force-velocity profile of the athlete, either back squats or jump squats should be effective for improving vertical jump height. Even so, exercises that shift the load towards the center of mass of the body, such as hex bar deadlifts and weighted vest jumps could be superior.
Also, using the lifting (concentric) phase of these exercises only, rather than both lowering and lifting phases, *might* further improve results. This is partly because lifting phases involve faster rate coding, and partly because this strategy might potentially help avoid optimizing stretch-shortening cycle function for lifting heavy weights, rather than for jumping.