Cross training. Hitting the gym is great, but volleyball is not a game of slow muscle lifting. Here is my advice.
Running
Lower body strength is a must. You need a solid base. Two big excercises:
- Quick burst sprints up a moderate incline.
- Lateral shift: run your track straddling a line and instead of running straight, each step should be laterally to the other side of the line. This is not a forward speed drill, this is a lateral speed drill.
Whip Drills- Got a net? Stand 2 feet away from it, toss a volleyball 5 feet above your head and smack the air out of it. Do 5 or more sets of 20 consecutive reps a day.
- Same exercise, but jump up to hit the ball instead of at a standing position.
Why
Because volleyball is played in a 900 sq ft space. You need forward and lateral burst speed, not stamina (within reason). You need fast arm whipping arm speed. It's one thing to pound the living daylights out of a ball, but if you have a slow strong swing, you'll get blocked twice as much as a fast weak swing. So practice a fast strong swing and you'll do well.
Predicting
Hitting and running after balls is only a fraction of the game. Reflexes and prediction are what make good volleyball players. During practice and games (always), concentrate on body language of the opposing team. You need to predict where the hitter is going to hit, and when s/he hits, you need the reflexes to react.
Reflexes
A really good habit to get into is before every practice and game, grab your volleyball and the nearest wall, and pepper with IT. Pepper partners are highly overrated (unless you're outdoors

) When I say pepper with it, I mean get close to the wall, say 2 feet for speed passing. Oh, and pass with one arm as well. One arm, two arms, stick a foot out there. Goal is to not let that ball hit the ground under any circumstances. DIVE Dangit! sacrifice!

Hit the ball overhand and react to it. The more you react to fast objects, the more your body becomes accustomed to "reacting" without thinkig because of repetition.
Encouraging Words
Do all of these regularly, (discipline means doing it even when you don't feel like doing it). It's not enough to do it for a week and say "I'm a better volleyball player now." It's a career long discipline. The defense you cannot practice off the court. Play doubles as much as you can, and concentrate on the hitter as well as the setter. Get used to reading what the most likely set and hit is going to be based on body language. Use yoru peripheral vision to watch the ball.
Last last words
You don't need to gain weight... I read that and I pondered... it may be good to have more muscle mass en-toto, however think of the stress on your knees and the fact that hitting with more muscle doesn't necessarily equate to hitting harder, and even so, experience will just stand in front of your power hit and take it like a man. Think of a whip. The only reason for strong muscles is to increase acceleration of the arm. Having a little more beef behind the whip so to speak will help power, but it's not power that grounds a ball. It's quickness... keeping it away from the defense.
For the sake of understanding the problem of gaining weight... go pick up a 10 lb weight... do it right now as a matter of fact. Now think to yourself... How is this 10 pounds of weight going to affect my vertical versus the strength/speed of my hit? How will if affect every tendon attached to my knees? ... trust a veteran on this.
Chris
