THE STRENGTH-SPEED SPECTRUM
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 will rely on strength and the more it will depend on the abilities that determine speed. Looking at sports movements and training exercises in which the body is transported around the playing field or projected into the air, full-speed sprinting is at the far speed end of the spectrum. Moving from that end toward the strength end, there is 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 snatch, then full snatch. Snatch variations are getting into the realm where strength is a larger factor than speed. Moving on, we have the hang clean and then the full 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 my 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 4.9-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 my Long-Term Training Goals article.
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 tension, so let me clarify this. A different way to state it is lifting a heavier load as fast as possible will have a lower velocity and require more muscle tension than lifting a lighter load. That should make more sense.
Here is a picture of the tension-velocity curve of muscles to illustrate the relationship. (click it for a clearer view) A person's actual curve is not smooth like the picture, but it does follow the same general pattern. 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 force absorption phase, which is why force output 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. Looking at the picture again, there is a curve for a speed athlete and a strength athlete. Both are identical to the general curve, but they are shifted on the graph so that the two athletes excel in different areas. The speed athlete generates higher muscle tension in both high-speed eccentric and concentric contractions and thus runs faster and jumps higher, because those are fast movements. The strength athlete has higher tension in slower contractions close to zero velocity. This is the range of the curve where strength training takes place, so the strength athlete has higher max lifts.
The strength-speed spectrum is a very simple way to identify strengths and weaknesses in athletes. The muscle tension vs contraction velocity curve provides a physiological background to allow even further understanding of abilities. For example, an athlete may excel at high-speed movements like the hang snatch and paused jumps, but be less impressive at vertical jumps from an approach. All are high-speed movements, but the running jumps are largely influenced by a high-speed counter-movement. This athlete has good explosiveness, which produces a good amount of concentric muscle tension. What is lacking is high eccentric muscle tension during the vertical jump counter-movement. Plyometric training would be valuable for this athlete, particularly depth drops and other variations of force absorption. Improving in that weak area will greatly enhance the contribution of the strengths the athlete already has. Bottom line, the way to incredible athleticism is to be well-trained in all areas of the strength-speed spectrum.