Awesome exercise must:
1) Work out the majority of our muscles
There are few exercises that work out the majority of your muscles. Martial Arts, forms of dance, swimming and mountain climbing are some of the more well rounded forms of exercise.
2) Have aerobic and anaerobic components.
You'll want to have both speed, and endurance. While aerobic exercise is excellent for your cardiovascular system, and promotes the muscle's abilities to maintain exertion for long periods of time, anaerobic exercise will develop the muscles' abilities to function for short bursts of high intensity. For a martial artist, the anaerobic provides the ability to hit in short, accurate bursts, and the aerobic provides the ability to recover in between combinations.3) Challenge us.
Without a goal on the horizon, we tend to jog a little slower. Without a black belt on the horizon, we tend to train a little less. Find goals within your exercise, and monitor them.4) Be fun, and be done regularly.
I would not recommend using weight as a goal (as muscle gain can provide frustrating "weight-based" benchmarks, while pants-size goals, or simply aesthetic goals are considerably more beneficial)
What do you honestly do in your life, regularly, that isn't in some way fun? How hard is it to get you to do those things? You want your exercise to be stress-relief, to be fun, to be challenging, and enjoyable! This is why I love martial arts so much. You barely realize you're exercising, and the results speak for themselves.
Recommended Reading: How to choose a martial arts school that's right for you
Sensei Ono, Shinka Martial Arts
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