Since strength training was first studied as a method for preparing strength and power athletes for competition, it has been recognized that peak force takes some time to reach. And yet some athletic movements are completed in just 100 – 150ms. Therefore, the rate at which force is developed is likely at least as important for athletes as maximum strength.
Training with maximal force isometric contractions is quite effective at increasing maximum strength, but it does not increase force by much in the early phase of a contraction. Maximal force isometric training is similar to conventional, heavy strength training, as both involve sustained contractions of 3 seconds or more.
Training with explosive isometric contractions is far more effective at increasing force in the early phase of a contraction. Explosive isometric contractions involve sudden, rapid force production followed immediately by relaxation. They are therefore very similar to ballistic strength training.
Strength gains are velocity-specific. This means that high-velocity, light load strength training produces greater gains in force at high velocities than low-velocity, heavy load strength training.
However, force production at high velocities likely increases through two totally different mechanisms:
- Increases in rate of force development (RFD)
- Increases in maximum contractile velocity (MCV)
Changes in RFD likely arise from the intention to produce force maximally, which is why “intent” is known to be a key factor determining gains in both strength and RFD. RFD can be therefore be increased by contractions at any speed (including isometric contractions). Increasing MCV is probably more velocity-specific.
This important study compared isometric contractions with explosive intent and sustained intent, in order to assess their different effects on strength gains in various time windows.
The two training approaches produced very different effects on:
- maximum strength (during a sustained contraction of several seconds)
- force production in early time windows (0 – 150ms).
Importantly, vertical jumping, sprinting, and changes of direction often involve durations of time of 100 – 200ms, suggesting that explosive strength is more important than maximal strength for many athletic movements.
This shows that strength coaches , trainers and rehab specialists should be very clear about what type of strength is required for their sport (maximum or explosive), and they should then choose the right exercises and cues in order to develop these qualities.
Why do these different adaptations happen? Well, for several reasons, we think that the neural adaptations that occur after explosive isometric strength training are responsible for increasing force production in the early phase of the contraction.
Recently, we have seen that explosive strength is a very different quality to maximum strength, even in elite athletes. But why is this?
This study investigated the determinants of explosive strength at 50ms and 100ms. The main factor influencing explosive strength at 50ms was neural drive, while the main factors influencing explosive strength beyond that point were peripheral to the muscle itself.
This was observed in two ways:
- the ratio of voluntary-to-involuntary force only increased at 50ms, and
- EMG amplitude increased by much more at 50ms, than at 100ms
This shows that central factors are very important for the earliest phase of force production, and require training specifically if they are to be developed. Since this phase is key for many athletic movements, this is very important for strength coaching.
Importantly for strength coaches, trainers and rehab specialists working with athletes, the ability to produce isometric force in the early time windows is related to sprinting ability. Sprinting is an athletic movement that is well-known to involve very short ground contact times of just 100 – 150ms. It is therefore not surprising that the ability to produce force more quickly during the first half of these ground contact phases is a key determinant of sprinting ability, even over quite short distances.
We saw in the previous study that strength training with sustained contractions was better for improving maximal strength, while strength training using explosive contractions was better for increasing explosive strength. This was true even when the velocity was the same in both training programs (because isometric contractions were used). This is important because vertical jumping, sprinting, and changes of direction can involve short durations of time of 100 – 200ms, suggesting that explosive strength is more important than maximal strength for many athletic movements.
However, even within these athletic movements, some involve much shorter periods of time than others. In particular, sprinting is known to involve very short ground contract times (<100ms) in elite athletes.
On the other hand, when a countermovement is performed, the vertical jump can involve quite long ground contact times (>500ms). This study investigated the predictive value of explosive strength in different time windows on both sprinting and vertical jumping. Exactly as you might anticipate, sprinting ability was predicted by early phase force, while vertical jumping was predicted by force in the later phases.
This shows that when training or rehabbing athletes, we should be very clear about what type of strength is required for their sport (maximum or explosive), and they should then choose the right exercises and cues in order to develop these qualities.
Although these studies all involve isometric contractions, the wider literature (particularly in relation to the adaptations produced in rate coding by ballistic training) makes it clear that the findings can be extrapolated to dynamic training. The bottom line is that conventional heavy strength training (even when pushing the bar “with maximal intent”) will likely not improve explosive force production in the early phases of a contraction. Ballistic strength training (e.g. jump squats) is probably essential for achieving increases in force production during the early phases that transfers most effectively to athletic performance.