By Chris Beardsley
Chris Beardsley graduated from Durham University with a Masters Degree in 2001. He since contributed to the fields of sports science and sports medicine by working alongside researchers from Team GB boxing, the School of Sport and Recreation at Auckland University of Technology, the Faculty of Sport at the University of Ljubljana, the Department of Sport at Staffordshire University, and the College of Health Solutions at Arizona State University. He is also a Director at Strength and Conditioning Research Limited
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. The rate at which force is developed can most easily be expressed as force produced at certain time points (50ms, 100ms, etc.) after the onset of a contraction.
Training with maximal force isometric contractions is quite effective at increasing maximum strength, but it does not increase force by much in these early time windows. 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 these early time windows. Explosive isometric contractions involve sudden, rapid force production followed immediately by relaxation. They are very similar to ballistic strength training.
Why do these different adaptations happen?
Neural adaptations after explosive isometric strength training are likely responsible for increasing force production in the early phase of the contraction. We can see this because:
1️⃣ The ratio of voluntary-to-involuntary force at the first time point (50ms) increases substantially after explosive isometric training; and
2️⃣ Muscle activation (as measured by EMG) also increases over the first time window (0 – 50ms) after explosive isometric training.
Importantly for strength coaches 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.
On the other hand, vertical jumping ability is more closely related to force in the later phases of a contraction. Again, this makes sense, as countermovement jumps typically take 500ms for the eccentric phase and 300ms for the concentric phase.
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.
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 likely essential for achieving increases in force production during the early phases that transfers most effectively to athletic performance.