If you want to improve your level of fitness, you can try interval training, the technique used by athletes in sports requiring speed and endurance such as cycling, skiing, running or swimming. They exercise very intensely, rest, and then alternate intense bursts of exercise and rest until their muscles start to feel heavy or tired.
The American College of Sports Medicine (ACSM) surveyed more than 4,000 fitness professionals and found that High Intensity Interval Training (HIIT) was the most popular trend in fitness for 2018 (ACSM Health & Fitness Journal, Dec 2017). The most efficient, time-saving and health-benefitting way to exercise is to use short intervals (Scandinavian Journal of Medicine & Science in Sports, Jan 2014).
Interval training is far more effective than continuous exercise to increase your maximal ability to take in and use oxygen. The limiting factor to how fast you can move over a distance is the time it takes to bring oxygen into your muscles, and the faster you can bring oxygen into your muscles, the faster you can move. When you can't meet your needs for oxygen, muscles start to burn and hurt, you gasp for breath, and you have to slow down.
Short and Long Intervals
There are two types of intervals: short and long. A short interval takes no more than 30 seconds and does not build up significant amounts of lactic acid in the bloodstream, so you can do lots of repeat short intervals in a single workout. Long intervals are defined as taking two minutes or more and are very tiring, so you can do only a limited number in each workout.
You can apply the concept of interval training to your program at any level of fitness. When you want to start doing intervals, you should do only short intervals lasting 30 seconds or less, rest for as long as it takes for your muscles to stop burning and you to stop breathing hard, and then repeat another interval. Alternate exercising and resting until you feel tired or your muscles feel heavy. Cool down by exercising at a slow pace and then stop for the day. The stronger you get in your sport, the more intense your intervals can become. You should not time the rest between intervals; take as long as you need to get your breath back. The key is to maintain a fast interval pace. If you start your next interval before you recover from your previous one, you will run a slower interval and will decrease gains from your program.
Delayed Onset Muscle Soreness
To make muscles stronger, you need to exercise intensely enough to damage the muscles. You can tell that you are damaging muscles when you exercise hard enough to feel soreness in your muscles eight to 24 hours later, which is called Delayed Onset Muscle Soreness or DOMS. On the next day, you should work out at reduced intensity. Most athletes set up their training programs to damage their muscles by taking a hard workout on one day, and then taking easier workouts until the soreness is gone. The current recommendation for most people is to attempt interval training twice a week, not on consecutive days.
How Muscles Become Stronger
Muscles are made up of thousands of individual fibers just as a rope is made of many strands. Each fiber is a series of blocks called sarcomeres lined up end to end. Each sarcomere is attached to the one next to it at the "Z line." Muscle fibers do not contract equally along their lengths. They contract only at each "Z line" and the damage that you feel from intense exercise occurs only at the "Z lines." See Making Muscles Stronger.
Repeating bouts of exercising your muscles intensely enough to damage them, followed by easy workouts until the muscles heal, makes muscles stronger so they can withstand higher loads and be more resistant to injury. When a muscle is damaged, your immune system sends the damaged tissue the same kinds of cells and chemicals that are used to kill germs when you have an infection. This causes inflammation, characterized by soreness (pain), increased blood flow to the injured fibers (redness), and increased flow of fluid into the damaged area (swelling). The immune cells release tissue growth factors to heal the damaged muscle fibers, and you allow the muscle soreness to disappear before exercising intensely again. Muscle fibers become larger and increase in number by splitting to form new fibers. If you do not wait until the soreness goes away before exercising intensely again, the fibers can be torn, the muscles weaken and you can become injured.
What To Do When You Have DOMS
You can take off when you have DOMS if you want to, but you will become stronger by taking easy workouts while your muscles are sore. Athletes do not usually plan to take off workouts during recovery, even though resting when the muscles feel sore will allow muscles to heal faster than exercising at a low intensity. If you exercise at low intensity during recovery, your muscles will become more fibrous and resistant to injury when you stress them in the next intense bout of exercise.
Interval Training Protects Your Heart
High intensity interval training maximally improves every conceivable measure of heart function and heart strength. It also helps to prevent both the pre-diabetic metabolic syndrome and the heart damage it causes (Exercise and Sports Sciences Reviews, July 2009). Interval training raises the good HDL cholesterol far more than less intense exercise does (Journal of Strength and Conditioning Research, March 2009). Vigorous exercise protects obese people from heart attacks and prolongs their lives, even if they don't lose weight (MSSE, October 2006). Interval training is now recommended even for people who have had heart attacks, because it improves a heart attack victim's maximal ability to take in and use oxygen far more than continuous exercise does (J Cardiopulm Rehabil Prev, Feb 13, 2014). Vigorous exercise can cause a heart attack in susceptible people, so it is a good idea to get approval from your doctor before you increase the intensity of your exercise program.
I am 83 years old and have been exercising all my life. I do interval training on my bike up to four days a week and long slower rides for recovery on the other three days. I warm up for a mile or two, then do standing intervals of 50 pedal strokes (half-rotations) that make me short of breath and cause some muscle burning. Each interval takes about 24 seconds. I then go slowly until I recover my breath completely and my muscles feel fresh, and then start my next interval. I can usually do 21 of these intervals comfortably in a workout. Near the end of my workout, my muscles start to feel stiff and heavy, signaling that it is time for me to stop for the day. You too can achieve a high level of fitness with interval training.
Caution:Intense exercise can cause heart attacks in people with blocked arteries leading to their hearts, so before you start interval training, check with your doctor.