There is a lot of disagreement on the role that strength training plays in athletic development. Some say squatting is the only thing you need to do to increase your vertical jump. Others say heavy lifting is slow-twitch activity. Some point out that elite sprinters are extremely strong. Others claim that the skinny guy at their gym can't even squat his own body weight but jumps really high (this is not true). Some report that they made amazing athletic gains through strength training. Others say they have gotten very strong but cannot run faster or jump higher. So what's the deal? Does lifting work or not?
Truth is heavy lifting and explosive athletic movements are very different activities, so they have some different adaptations. So why is heavy lifting useful? Why is any activity ever effective for improving a different activity? To answer that question we have to look at the physiological adaptations of the activities in question.
All movement is driven by muscle tension, which comes from the contraction of muscle fibers. Recruitment of muscle fibers always starts with the slower, weaker ones and progresses to the faster and stronger. The more tension required for the movement and the longer the contraction lasts, the more fibers will get activated. In explosive athletic movements only a percentage of fibers can be activated, because the muscle contraction only lasts for a fraction of a second. In a heavy lift the same fibers are activated plus more on top of that, because there is more time to generate tension. The main point is the same muscle fibers that are used to sprint and jump are used to lift a heavy weight. There is not a separate pool of fibers for fast and slow movements. Strength training provokes higher neuromuscular strength in those fibers than what is developed by explosive movements. Increased strength means they produce more force during all movements. More force equals faster running and higher jumping. This is the means by which strength training can increase athleticism.
While strength does contribute to athleticism, there are other adaptations to strength training which can have a negative impact. Consider the effect on muscle fiber twitch-speed. Research has shown that muscle fibers default toward being fast-twitch when they are not used. Any activity promotes some level of efficiency in muscle fibers, which requires shifting toward the slow end of the spectrum. Low effort, long duration contractions slow down muscle fibers the most. Max effort, short duration contractions slow them down the least. Explosive athletic movements use quick, max effort contractions. Strength training uses max effort, but the contractions last for a relatively long time. Thus it promotes some efficiency. Heavy lifting is still fast-twitch dominant activity, but it pulls fibers a little further down toward the slow end of the spectrum than sprinting and jumping.
Now consider the different characteristics of the neural drive during high-speed movements and heavy, slow movements. Sprinting, jumping, throwing, etc use quick bursts of neural drive to stimulate fast activation of muscle fibers and a high rate of muscular force development. Strength training uses maximum neural drive to stimulate maximum tension, but the rate that the neural drive builds up is slower. It can be made faster by accelerating the weight as fast as possible, but in the end heavy weight demands more control. Rate of force development (RFD) during heavy lifting cannot be as high as during high-speed movements. So strength training and explosive training develop two different abilities of the nervous system, generating maximum drive and generating drive quickly. It is hard to make a firm statement on how those abilities interact with each other, because we have very little understanding of the brain, which is where neural drive comes from. But generally speaking different abilities conflict with each other. No one gets to have the best of both worlds. Do not allow lifting to make up a majority of your activity unless you want to be a hybrid between a lifter and an athlete. There may be times when strength is the focus. But looking at a year as a whole, your sport and high-speed training need to dominate. You do not get great at a skill or a movement by doing something completely different.
The other issue that comes into play with heavy lifting is nervous system fatigue. Hard strength training demands more from the nervous system than any other activity. After months of consistent lifting, fatigue can accumulate. Neural fatigue is not a tired feeling after a workout or soreness the next day. It's not something that you can really feel. We do not have great understanding of this fatigue, but we do know that it exists, we know it can lower performance across the board, and we know it can last for a long time.
Strength training has the positive adaptation of increased neuromuscular strength, but it also has some negative adaptations that can decrease explosiveness. These adaptations are reflected in the way that strength training shapes the isometric force curve. (See video below.) In order to use strength training effectively in athletic development, proper strategies must be used to develop a high level of strength without interfering with fast force production.
There is a certain approach that running and jumping athletes should take to strength training. It is not the same approach as a bodybuilder, a powerlifter, or a weightlifter. There are a few recommendations in the video below. Rather than being redundant and listing them here, I'll share some other thoughts.
There are a ton of problems I see with strength training for athletes. Many athletes in some sports never do enough strength training to see the real benefits it can provide. (I'm looking at you, basketball players.) Other athletes may put in work but never eat enough food to allow the body to adapt. Another big problem is just complete lack of education on executing exercises in an effective manner. Then there is the obsession with trying to be sport-specific, an effort which often produces worthless exercises.
On the other hand, naturally strong athletes and those in sports that have a strength training culture often fall in love with it too much. Football is probably the biggest culprit here. Here's an example. Through Four Years' Training, College Football Players Gain Strength and Size. Research was conducted on over 150 football players over the course of seven years at Oklahoma State University. They found that the athletes generally did not improve speed or vertical jump over the course of their career. The article attributes it to genetic limitations with muscle fiber types. To be blunt, that's a bunch of BS. The lack of improvement was a result of ineffective training methods, particularly (I'm guessing based on the article) too much time spent in the weight room and too much focus on max strength. We do need to acknowledge that weight gain and strength may be greater priorities for college football players, but do we really think this team’s coaches were planning on not improving their players’ speed? OSU plays Big 12 football. The sport is big deal at the school. But even among the "experts", the professionals, the people with the top positions, etc in the field of sports training, few people truly know how to train advanced athletes to get even better. I saw the same problem at my own university. The football players' off season training was 90% lifting. And somehow that was supposed to produce increased speed on the field? If lifting is what you do, a lifter is what you become.
I could rant on and on about mistakes made in training, but I won't. I just want to make it clear that developing elite speed and jumping ability is not an easy process. In fact, successful athletic development for advanced athletes is pretty rare. A lot of people are "gettin after it" and "puttin in work" but few actually increase their athleticism year to year. So we have to have an open mind and be willing to challenge the current norm, even if it is used by "top coaches" at the university of wherever. No one has all the answers, and I would certainly never claim that I do. But I have found an approach to strength training that works well for athletic development. It is covered in the video below, and there is more info the article, Long-Term Athletic Development.