Ever feel like the hours you spend on the treadmill don’t yield the results you’re looking for? Your muscles look flatter, you still have fat in troublesome areas, and you feel a general sense of fatigue going into your training session. This is not uncommon, yet people still flock to cardio equipment for long-duration workouts. (Learn how to condition for your sport.)
Steady-state aerobic training, where you maintain a slow to moderate pace for extended durations, can seriously interfere with power and strength development—for several reasons, including:

• It increases the release of cortisol, a hormone that actually breaks down muscle
• It fails to develop fast-twitch muscle fibers, which are primarily active during strength and power movements

There’s also evidence that extreme endurance training, such as that performed by marathoners and triathletes, may cause transient structural cardiovascular changes. Over time, this can lead to of scarring of certain areas of the heart and abnormal heart rhythms. (Source.)
At this point, you may be wondering what type of workout will provide the assumed benefits of cardio workouts without the drawbacks. The answer is high-intensity interval training (HIIT).
HIIT workouts involve alternating intervals of work and rest. The idea is that you are able to train at a higher intensity, increasing the physiological changes in your body. And this can be achieved in a much shorter amount of time.
A popular example of HIIT is Tabata training, which calls for eight 20-second max-effort sprints, each followed by  10 seconds of rest. Compared to traditional aerobic training, Tabata training produces vastly superior results and a more meaningful impact on sports performance. (Find out the detailed results here.)
The great thing about HIIT workouts like Tabata is you can perform them with a variety of exercise options to keep workouts fresh, fun and challenging. For example, you can choose to perform Burpees, Mountain Climbers, Squat Jumps or virtually any exercise that elevates your heart rate.
One final point: compare the physiques of Olympic sprinters with those of long-distance runners. Sprinters wouldn’t look out of place on an NFL field or NBA court, whereas long-distance runners look like they might blow away in the wind. I rest my case.
Sources
Passelergue, P., et al. “Saliva cortisol, testosterone and T/C ratio variations during a wrestling competition and during the post-competitive recovery period.” Int J Sports Med. 1999 Feb;20(2):109-13.
O’Keefe, J., et al. “Potential Adverse Cardiovascular Effects from Excessive Endurance Exercise.” Mayo Clinic Proceedings. June 2012. 87 (6), 587-595.