So now I'm going to return to this rate of forced development issue that we talked about a little bit earlier. You can estimate an athlete's rate of force development by using one of these strategies that I've got listed here. You can measure the index of explosive strength. You can measure the reactivity coefficient. And we're going to come back and talk about each of these. You can measure the S-gradient, where S stands for start. And you can measure the A-gradient, where A is equal to acceleration. So, let's go back now and take a look at each of these measures, and see what they tell us about the athlete's ability to apply their strength. The first that we'll take a look at is the index of explosive strength. The explosive strength index is equal to the peak force generated during the movement. And you divide that by the time it takes to reach that peak force. Now, this is a fairly simply to calculate. You just need two measurements. You need their peak force while performing the skill, and the time it takes to reach that peak force. Now, I think you can appreciate the value of this new technology for helping you to attain these types of measures. You can also measure their reactivity coefficient. And this equation for reactivity coefficient is the athlete's peak force divided by the time taken to reach that peak force. And then you multiply that by the athlete's weight, or the weight of the implement. Now, reactivity coefficient is typically highly correlated with jumping performances. It's a measure of how quickly the athlete's legs can stop down movement of the body in response to the body weight, and leave the ground, so that the jump can be performed. It's a really important measure for giving you insight into how stiff the athlete's leg is. And you can measure the S-gradient, where S is for start, and this measure provides insight into how quickly the athlete can begin the muscular effort. How quickly can they recruit all those muscles and the muscle fiber? So, here is the S-gradient. It's the time during which half of the force is applied the equation for the S-gradient is one-half of the athletes maximal possible force, their Fm, that's their performance force. And this is divided by the time taken to reach 50% of their performance force, the Fm. Okay and you can also measure the A-gradient where A ,it stands for acceleration. Now, to get the, and you can see the A-gradient here, it's that later part of the curve. To get the A-gradient, you take 50% of the athlete's maximum force generated, and you divide that by the total time it takes the athlete to perform the movement. And you subtract that time from the time it took them to reach 50% of their maximum force, or the Fm, and that gives you that little portion of the curve right there that's showing in red. Now, the A-gradient is quite valuable, because it gives you some idea of the acceleration that's occurring over the latter stages of the movement. So the S-gradient is how quickly they can get the movement going. And then the A-gradient is how quickly they can keep the movement going. And they're quite often not correlated. The maximum force, and the rate of force development, in particular, the S-gradient, like I said, are not correlated, so the maximum strength and the rate of force development, there's hardly any relationship there. Strong people don't necessarily possess a high rate of force development. Okay, so here are some key points, just to go back and quickly summarize them again, that we can derive from this particular equations. Oops, we missed this chart, didn't we? So, this is just showing the lack of correlation between their maximum force, their performance force, their Fm, and their maximal force that they can produce. They're not correlated. Okay, so let's move on to the training implications then. If the objective of training is to increase maximal force production, that is the maximum strength, their Fmm. Explosive type exercises are not useful, you just have to train the maximum strength, not explosiveness. In turn, heavy resistance exercise that take longer to perform than then movement time of skill are not useful for enhancing the rate of force development. So, heavy strength training does not train speed of movement. Improving an athlete's explosive strength, or rate of force development, requires a two-pronged approach. The first is to increase the athlete's current maximal strength or their Fmm. So, you do that first. And this strategy is particularly important when the athlete's explosive strength deficit is much less that 50% of their maximum strength. That is their maximum strength or Fmm as rather low. So, if I got a low Fmm, you want to work on building that. The second approach you can use is to increase the average rate of force development by using explosive type of exercises. Heavy resistance exercises are not suitable in this case, especially for elite athletes who already have a high level of strength. They can't use 50% of that maximum strength to perform the skill of this sport. So, if they've got a high level of strength, and they are way below 50%, you know that you need to work on their rate of force development. All right, so here's a summary of ways to measure rate of force development, once again. And I want you to see if you can explain each of these methods, because they seem a bit confusing, but they soon will make sense to you. You can measure the index of explosive strength, and this gives you an idea of how the athletes explosive power, or their rate of force development is improving. So, you measure that over time, and you should see an improvement there. You can measure reactivity coefficient. This provides insight into how quickly the brain activates muscles to change movement direction, and is particularly important for jumping and sprinting. So, you would like to have this information. You can measure the athletes S-gradient while performing a specific movement as fast as possible. And this provides you with insight into how quickly the athlete is able to activate the muscles, and you can measure their acceleration, or the A-gradient. And this gives you an idea is to how well the muscles synchronize to keep the body parts accelerating. And this is particularly important in any kind of throwing activity, or any movement that involves a release of an implement.
