•Fact•

"People who worked out more than 100 days throughout the year had a 30 percent reduction in sick days."
Training, Feb 2003

Short rest & higher reps (as in circuit training) actually changes the chemistry of the worked muscle. Short rest / high reps results in increased lactic acid build up which has a positive effect on both metabolic rate and hormone release.

There are 2 types of muscle fibers: Fast Twitch & Slow Twitch.

Fast twitch muscles are used primarily in anaerobic explosive exercise. Sprinting / Anaerobic

Slow twitch muscles are used primarily in aerobic endurance exercise. Long distance / Aerobic.

Slow twitch muscles have a higher concentration of mitochondria, or the cellular source of energy.
Slow twitch muscles are the basis of endurance.
Because there are 2 types of muscle fibers with different functions it is necessary to perform a variety of exercise modalities to stimulate both types of fibers in as many muscle groups as possible.

Exercise Info

Upright Rows:

Though the verdict is out on this exercise, you may be increasing your risk of shoulder injury by performing it, especially if you are using heavier weights or have had a past shoulder or rotator cuff injury. The upright movement is an unnatural motion for the shoulder complex.

Combine these exercises to substitute for an upright row:
Dumbell front raises.
Dumbell lateral raises.

Crunches:

When you are doing any kind of crunches, always keep your chin up and never lock your fingers behind your head to help pull yourself up. Not only are you cheating yourself from getting the most out of the movement by doing so, but you are putting yourself at a greater risk of injuring your neck or spine.

Squats:

Ensure you're getting an adequate range of motion while squatting. Always performing partial reps not only decreases the total workload, but can also increase your risk of knee problems if done too often. Additionally, watch that your heels are on the floor and that your back is relatively flat throughout the movement.

Incline Presses:

Watch for two things: First, if you have to drastically arch your back or lift your hips off the bench while pressing, you're probably using too much weight. Second, make sure the incline is at no more than a 45-degree incline, or the movement becomes more of a shoulder press than a chest press.

Step Machine:

If you are using a step machine or treadmill, you should not be relying on the handrails in front of you or at your sides to support yourself. If you find you have to, there is a good chance you are going too fast or are working on too much of an incline.

Pushdowns:

If you are performing any kind of pushdown movement, take note of how much you are leaning into the weight. Any more than a slight lean forward is a sign you are using too much weight and are starting to use your bodyweight and some momentum to move the weight.

Barbell Rows:

If you regularly use barbell row exercises to add some size and definition to your back, make sure you aren't wasting your time exercising in an inefficient way. Make sure your body is bent forward enough to effectively work the muscles of the back so your shoulders and biceps aren't doing the majority of the work.