I’d like to talk a little about explosive potential and how it relates to your innate muscle fiber and neurological characteristics. There are different combinations of fiber and neurological characteristics and they go a long way in determining your potential. You have to look at things at both a muscular level and a neurological level. For the most part the muscular characteristics are an indicator of your fast twitch dominance - how much muscle mass you carry naturally, how strong you are, and how easily you gain muscle. People with more fast twitch oriented muscular systems easily gain strength at an accelerated rate.
The neurological characteristics are basically an indicator of the "excitability" of your nervous system and your ability to ramp up rate coding. The best indicator of that in my opinion is natural quickness - how fast are your hands and feet in unloaded movements? There's not really a test you can do that says, "Oh you're quick or you're slow", so don't read into this TOO much. Just ponder it a bit and think of how you relate to other people in the natural quickness department.
Ideally one would have both a fast twitch physique at the fiber level and a great nervous system along with a good skeletal structure. Athletes with these characteristics make easy gains, but they also tend to be explosive even without any training. Unfortunately, they don't tend to train consistent and hard. But here are the various combinations of muscular/nervous system and the traits of each:
Fast Twitch Muscles and Slow Twitch Nervous System
One subset of athletes I run across have a fast twitch oriented muscular system and a slow twitch oriented nervous system. These people will be strong and muscular but slow. Lots of powerlifter types fit into this group. They're naturally big, thick, strong, and gain muscle & strength extremely easy, but are not typically all that quick. He may hate me for saying this, but the first guy that comes to mind is a guy like Mark Rippetoe - big, strong, and thick. In the NBA a guy like Nick Collison comes to mind. These individuals likely will have a very easy time gaining the required strength they need, but will need to spend more time working on various speed-strength and plyometric drills to improve rate of force development. Oftentimes a big challenge for this group is getting their body-fat down. They tend to be thick and often struggle staying lean.
Fast Twitch Muscles and Fast Twitch Nervous System
One can also have a fast twitch oriented muscular system and a fast twitch oriented nervous system. These are the true freaks of the athletic world. They respond to weight training EXTREMELY quickly and easily transfer any gains they make into increased power production. The majority of the NFL fits into this group as do "highlight reel" dunkers like Golden Child, T-dub, and many others. In the NBA Lebron James would fit into this group. These people don't need to do anything special to make gains and will often respond to ANYTHING. They can pretty much look at a squat rack and put 5 inches on their vert. They typically don't need or respond all that well to plyometric training because they already move very well & naturally have everything they'd get from plyos (increased RFD, movement proficiency etc.) - they simply don't need much of it.
Slow Twitch Muscles and Fast Twitch Nervous System
One can also have a slow twitch oriented muscular system and a fast twitch oriented nervous system. I'd put myself in this group. Kenyan distance runners would fall here. I'd also put a guy like Chris "Birdman" Andersen in this group. The characteristic of this group is a naturally frail build coupled with good natural quickness - skinny and weak, but potentially explosive. They may not actually be all that slow twitch at the fiber level, they just don’t have much natural muscle fiber to work with. The challenge with these people is getting them strong. Even though they may be naturally good at endurance activities, they have good nervous systems, so tend to respond really well to increased strength and size. However, they tend not to have the greatest endocrine profiles (testosterone and stress hormone profiles), and may have recovery issues that tend to make consistent progress difficult. I'd put a high jumper like Steffan Holm in this group as well. Interestingly enough, in my observations a LOT of top high jumpers fall into this group and it's a good example of how fiber type is over-rated. A good nervous system & structure wins out. These people can respond very well to VJ programs, but they need to be consistent and patient over a good period of time - particularly with their strength training.
Slow Twitch Muscles and Slow Twitch Nervous System
Lastly, one can have a slow twitch oriented muscular system with a slow twitch oriented nervous system. Despite shortcomings at both ends (fiber and neural), these types can make good VJ gains because one thing they usually have going for them is a relatively easy time staying lean. These folks respond well to a general all around vert program including all components of proper training: Strength, explosive, and reactive work. You could put most distance athletes in this group. It's also possible to have a lot of slow twitch fiber characteristics but still be relatively "big". In my observation a lot of NFL quarterbacks and NBA centers would also fit into this group - guys like Brad Miller, Peyton Manning, Philip Rivers, Matt Schaub - long but still pretty good sized. People with this profile tend to naturally be "motor morons" and without frequent exposure of movement work they don't move well. Therefore this group needs more sports specific movement work than other types - be it plyometrics, sprinting, or what have you.
So, How do you figure your type?
It should be noted the majority of athletes won't be an all out "pure" type - you'll most likely be a mix. Additionally, these characteristics combine with your natural skeletal structure to determine how efficiently your neural and muscular characteristics leverage force into power. There is definitely a benefit to having SOME amount of length (longer bones), particularly in the right spots such as the lower leg**. However too much tends to hamper explosive neural characteristics.
** If you're interested in digging more into that look into the crural index and Wilt Chamberlain http://hoops-nation.com/community/topic/32938-wilt-chamberlain-athletic-freak/?p=445362
On the neural end if you tend to be "stronger than fast" you're most likely more slow twitch at the neural end, and vice versa. Take a look at how your strength compares reativey speaking to real world performances in sprints, jumps, agility tests etc. Here are some comparison charts: www.higher-faster-sports.com/standards.html
People with good nervous systems also tend to potentiate very well and get a nice boost from stimulation and contrast methods. People with poor nervous systems don't always respond well to this type of training.
At the fiber end just ask yourself a few questions:
1. How much muscle do you carry naturally?
2. How strong are you naturally?
3. How quickly do you make strength and size gains relative to others?
Fast twitch dominant people easily make incredible stength gains with minimal effort and don't lose their strength or overtrain easily.
If you'd like to dig a little deeper, one thing you can do is pick a handful of basic strength training exercises (squat, bench press, etc.), put 85% of your max on the bar, and do as any reps as possible. If you get more than 8 reps you're more likely to be more slow twitch oriented at the fiber level. If you get less than 5 you're more likely to be more fast twitch **.
** Keep in mind on this type of assessment the more training experience you have under your belt the more you will tend to test as being fast twitch dominant.
To summarize, regardless of your genes you CAN make good gains, but everyone has a different innate potential. All you can do is work with what you have. Take a look at my Vertical Jump Bible 2.0 for other special topics such as this!