Human movement relies on contraction of muscle-tendon complexes. Different movements require different types of contractions, which rely on different combinations of strength, explosiveness, springiness, and flexibility. The last three factors of that list all affect how well strength is utilized for high-speed movements. Explosiveness determines how fast muscle tension can be generated; springiness determines how quickly and effectively energy can be stored and utilized; flexibility affects how fast body segments can move. The commonality between the factors of strength utilization is speed. To put it simply, all athletic movements depend on some combination of strength and speed. From that comes the concept of the strength-speed spectrum.

The faster a movement is, the less it relies on strength and the more it relies on the abilities that determine speed. Looking at movements in which the whole body is accelerated, full-speed sprinting is at the far speed end of the spectrum. Moving from that end toward the strength end, there is long jumping, then bounding, then 1-leg running vertical jumps, then 2-leg jumps from an approach, then standing vertical jumps. Things continue to slow down as the load increases. There are weighted jump squats, then hang power snatch, then power snatch, then full squat snatch. Snatch variations are getting into the realm where strength is a larger factor than speed. Moving on, we have the hang power clean, power clean, and full squat clean, followed by heavy strength training, with maximum lifts being completely reliant on strength. Speed is not a factor at this end of the spectrum. However, it’s important to understand that at the speed end of the spectrum, strength is still an important factor, because the load being moved is your body. Even if you can squat three times your weight, your body easily weighs enough that accelerating and projecting it through space is largely influenced by strength. All movement is driven by muscle tension, so no movement can possibly not rely on strength at all, but an example of a movement in which strength is a small factor is throwing a baseball. Yes, you move your whole body to throw, but the end result is just the whipping motion of the arm. The arm and ball make up the load that has to be accelerated. Since that load is so light compared to what your body is capable of moving, and because there is a large elastic contribution, the movement is very fast, and the importance of strength is greatly reduced. Thus there are pitchers who throw over 90 mph and are not particularly strong at all.

The strength-speed concept is useful for understanding athletes’ abilities and designing training. Let’s say everyone has a 1-10 rating for strength and for speed. (When I refer to a number on that scale, it is arbitrary. I don’t actually have a measuring system.) All great athletes have a high rating (8+) for speed. Athletic movements are fast, so you cannot be bad at the speed end of the spectrum and expect to excel at them. The primary difference between a really good sprinter or jumper, let’s say a high school state champ, and a world-class sprinter or jumper is the world-class athlete is also going to have a high rating for strength. A typical Olympic athlete is naturally gifted and well-trained in the speed abilities, someone you would call a speed athlete, but is also very strong from years of strength training or from genetic blessing. Looking at more typical people, what I see is a lot of athletes who aren’t very advanced in either area. Let’s say that average athletes naturally end up with a 3-5 rating for strength and speed by the time they’re fully grown. With some good training those ratings can both go up. The problem I see is that people tend to only improve on one end of the spectrum. Athletes who are naturally a little stronger seem to get more into lifting; those who are naturally fast and bouncy tend to like lighter, faster training more or they’ll just play their sport a lot . The culture of the sport also plays a big role. For example, football players tend to love the weight room, while basketball players tend to like plyometric circuits. The result is a lot of athletes who are good on just one end of the spectrum, and it’s usually the end on which they were naturally better to start. For example, look at imaginary high school linebacker, Johnny Strong. Johnny began lifting during his freshman year on the football team. He excelled at it, so he stuck with it for his entire career. As a senior, he is 180 cm tall (5’11”) and weighs a lean 93 kilos (205 lbs). He squats 180 kilos (397 lbs), the best mark at his school. However, as the movements get faster other athletes catch up to Johnny. He power cleans 108 kilos (238 lbs), as do a couple other football players. He can only power snatch 68 kilos (150 lbs), which a few of the basketball and track guys can do despite not being nearly as strong. Johnny has a solid standing vertical of 72 cm (28 in), but he wonders how the wide receivers on his team jump over 80 cm. Lastly, he runs a very average 5.1-second 40-yard dash. This lack of speed prevents him from being a standout player on the field. Johnny is an example of a strength athlete. He excels on the slow end of the spectrum, but gets less and less impressive with increasingly faster movements. Johnny is in dire need of speed training. On the other end of the spectrum is Tommy Springs. He weighs 93 kilos at a height of 188 cm (6’2″). He’s a football receiver and plays a wing position for the basketball team. He can dunk well with an approach and runs a 4.5-second 40-yard dash. He only squats 125 kilos (275 lbs). Tommy is a speed athlete and could reach elite levels of athleticism if he improves his strength. I realize that these are two pretty clear-cut examples, but honestly most people’s abilities are not hard to evaluate. For an idea of where your abilities rank in different areas, read Long-Term Training Goals.

Some good complimentary information to understand along with the strength-speed spectrum is the force-velocity relationship of muscle contractions. There are three common classifications of muscle contractions: concentric contractions when the muscle is shortening, isometric when the muscle is not changing length, and eccentric when the muscle is lengthening. Concentric contractions are considered to have positive velocity, isometric to have a velocity of zero, and eccentric contractions to have negative velocity. In that frame of reference, contraction velocity is inversely related to the amount of muscle tension that can be generated, meaning that as velocity increases, maximum possible tension decreases. It’s dangerous to associate high velocity with low force, so let me clarify this. This does not mean that moving a load slowly requires more force. It means that, with maximum effort, moving a light load fast involves less muscle tension than moving a heavier load more slowly.

force velocity curveHere is a picture of contraction velocity vs maximum muscle tension (force). As you can see, the highest possible tension occurs during eccentric contractions. This explains why lowering a weight and stopping it is much easier than lifting it and why it’s much easier to drop from a given height and land than it is to jump up to that height. High-speed (low velocity, since it’s negative) eccentric contractions in particular can generate extremely high muscle tension. Sprinting, jumping, and other plyometric movements begin with high-speed eccentric contractions during the loading phase, which is one reason why ground reaction force is highest during these movements and not during strength training. It’s also the reason that you may be extremely sore after a plyometric workout if your body is not acclimated to that type of training. The picture also shows that tension drops significantly during concentric contractions, particularly fast ones. Athletic movements all rely heavily on these contractions. Eccentric contractions only serve to stop; the following concentric contraction is what provides the go. Given the low tension of high-speed concentric contractions, it’s extremely important to enhance that muscle action as much as possible with proficient force absorption and storage of elastic energy during the eccentric phase of athletic movements. This is why an athlete’s “springiness” has remarkable effect on jumping and sprinting.

The graph above represents maximum muscle tension at different contraction velocities. However, maximum tension is not generated instantaneously. It takes a small amount of time. Muscular force development follows a curved path up to the maximum. That path is often called the force curve. An athlete’s force curve reflects athletic ability. Look at the graph below. The curves represent force as it scales up from zero to maximum in three different trained athletes. Don’t think of this as muscle tension during an athletic movement. In that case, the length of the muscle and the contraction velocity are both changing, so the curves would look way different. This is muscle tension during an isometric contraction with a limb locked into a machine. So those dynamic muscular factors are eliminated, and the curves purely represent explosiveness as a product of the nervous system and muscle fiber twitch speed. (This is not actual data. It’s a model made to illustrate the concept.)

force curvesNotice that the sprinter curve goes up the fastest in the beginning, followed by the jumper and then the weightlifter. The sprinter and the jumper end up at the same max tension. But because of the different paths to get there, the sprinter has the highest tension around 0.1 seconds, roughly the time of a sprinting foot contact, and the jumper has the highest tension around 0.2 seconds, roughly the time of a takeoff plant. The weightlifter reaches higher max tension but takes longer to get there. It is only in a longer-duration movement that the weightlifter is able to produce higher force than the other two. All three are highly explosive athletes within the time frame of the movement that they excel in. But none of them can dominate the entire strength-speed spectrum, because their force curves are shaped by the demands of their sport. No one gets to be the fastest, jump the highest, and be the strongest.

So let’s talk training. The goal for an athlete is to raise a small section in the first quarter second of the force curve. The idea with strength training is to accomplish that by raising the entire curve. For someone in the early stages of athletic development or someone who is just beginning training, that may be possible for a short time. Lift, get stronger, get more athletic. Easy process. However, in the long run, strength training by itself pulls the force curve in the direction of the weightlifter curve in the picture. That is not good for high-speed movements. So if you are a running and jumping athlete, you cannot fall in love with the weight room. Explosive training always needs to be included, and you have to be willing to stop lifting in order to get your force curve shooting up faster in that first quarter second. The other point I want to make with the force curve is that all explosive training is not equal. People like to put training into either the strength or the explosive category. Anything full effort done with just the body is grouped into the same category. Truth is the strength-speed spectrum really is a complete spectrum. It’s not just fast or slow. Jumping and sprinting are not equal speed, and their effect on the force curve is not the same. The people that need to be careful with this are sprinters, long jumpers, and triple jumpers. There is temptation to take a break from your sport and spend the off-season doing 2-leg jumping exercises, medicine ball throws, and olympic lifts. It’s all explosive. It’s all the same, right? Not quite. While those are all great exercises, they do not feature the same contraction velocity and short time to develop force that sprinting does. As a sprinting athlete, you do not want to spend too much time training at slower speeds without training at the speed of your sport. If your force curve slows down, it is not an easy thing to fix.

Looking back at our example athletes above, Johnny Strong has something like the weightlifter force curve. He has developed a slow curve with too much emphasis on strength training. To speed up his curve, he should do the fastest possible training that he is capable of executing well. He should sprint, or, if he is terrible at it, start with something a little slower and work towards sprinting. He should not even worry about lifting until he improves the first quarter second of his force curve. However, being Johnny Strong and loving the weight room so much, he will probably say, “Oh, I need to get more explosive? I’ll start doing snatches at the beginning of my lifts.” That will not cut it. Snatch is not fast enough to target the changes he needs to make. That’s why Johnny Strong will remain Johnny Strong. Tommy Springs, on the other hand, already has something like the jumper force curve. If he wants to jump higher, no change to the shape of his force curve is necessary. He just needs to get stronger to raise the whole thing. It probably does not matter too much what explosive training he does, as long as he does something. What he needs to focus on is getting stronger.

So start thinking about your abilities along the strength-speed spectrum. See if you can get an idea of what your force curve looks like and what it should look like for your sport. Then get on the track, the field, the court, or in the weight room, and start making those changes.