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The adaptations from and benefits of interval training

Interval training involves high-intensity work efforts followed by recovery periods that are repeated multiple times.

Typically described as work-to-rest ratios, they are performed at an intensity above which you could maintain for an extended period of time. Unlike continuous forms of aerobic exercise which lasts from twenty, thirty, sixty, or more minutes these interval segments last anywhere from sixty seconds to five minutes.

The total time of a workout may last an hour or more but the total on-time (that spend doing intense efforts may last anywhere from 5 to 25 minutes). The rest of the workout time entails the warm-up, recovery time between interval efforts, and the cool-down time.

You can do intervals on a bike, treadmill, track, in a pool, using any form of aerobic activity. Typically they are performed doing the particular sport the individual trains in.

While these intervals are still geared to improve cardiovascular fitness, endurance, and conditioning this form of training focuses more of the stress on the muscular system and - although it might not feel like it at the time (with your elevated HR and breathing) - less on the cardiovascular and respiratory systems.

Think of it like having the cardiovascular system as the oxygen delivery system (and this is trained by your moderate intensity cardio workouts). The muscular system is the oxygen utilization system (trained with high-intensity intervals). If you only do moderate intensity continuous cardio workouts you improve your cardiovascular (CV) condition and ability to deliver oxygen but don't improve your ability to use that oxygen.

Doing intervals improves your muscles aerobic capacity and allows you to use the oxygen more efficiently to produce energy. You could have a strong heart and CV system but if your muscles can't use all the oxygen you can delivery to them you are limited. On the other hand if your muscles improve their aerobic capacity by doing intervals but your CV system was limited you would also be limited. This is what happens when a sea-level athlete goes to altitude and although their CV system is strong the amount of oxygen available to deliver to the muscle is reduced and their performance suffers.

Intervals are just one more means of stressing your overall system and allowing for more complete and full adaptations to take place. While they aren't necessary for everyone, they are essential for overall condition and peak performance.

In general, long, aerobic intervals improve the oxidative capacity of the fast twitch type IIa muscle fibers and improves lactic acid clearance capabilities. Short, anaerobic interval focus more on conversion of type IIb muscle fibers to type IIa (making them more aerobic) and improves neuromuscular efficiency.

As an aside, both types of intervals will also add some tone to the muscles being used - hence the difference in appearance between the legs of a sprinter (trains doing mostly high-intensity sprints) and those of someone who trains doing primarily long steady duration runs (the same pace for the same distance each workout for weeks and months on end).

When interval workouts are combined with continuous longer duration cardiovascular workouts you achieve peak fitness and more significant gains in fitness than with moderate intensity workouts alone.
Written by Dr. Sternlicht for on 3.04.06