Short, hard workouts: what interval training can and cannot replace

Why intensity saves time

The body adapts to the demand placed on it. Effort close to your maximum stresses the heart, the blood vessels and the muscles' capacity to use oxygen more than a comfortable pace does, and the adaptation is correspondingly larger per minute. A meta-analysis of trials in patients with lifestyle-related cardiometabolic disease found that interval training raised peak oxygen uptake nearly twice as much as continuous moderate training.

The effect on VO2 max is the best documented. See VO2 max for why that number matters.

There is a second, more prosaic advantage. A twenty-minute session can be placed in a diary where a sixty-minute one cannot. For many people in senior roles, the choice is not between intervals and a long run. It is between intervals and nothing.

Three formats that work

1. 4 × 4 minutes. Four minutes at an effort that brings you to about 90% of your maximum heart rate, three minutes of easy movement, four times. With a ten-minute warm-up, about forty minutes. The most studied format, and the most demanding to pace.
2. 10 × 1 minute. One minute hard, one minute easy, ten times. Twenty minutes of work and recovery, plus the warm-up. Easier to tolerate and well suited to a stationary bike.
3. Sprint intervals. Six to eight efforts of twenty to thirty seconds, close to all-out, with two to four minutes between them. Very short, very hard, and best reserved for people who are already fit.

The calculator above gives the real length of a session once the warm-up and recoveries are counted. Sessions advertised as seven or ten minutes rarely include them.

How hard is hard

The most common error is to go too hard on the first repetition and fade. The right intensity is the one you can reproduce on the last repetition.

• For long intervals, you should be able to say a few words but not a sentence. Your heart rate climbs through each repetition and reaches its peak near the end.
• For one-minute intervals, you finish each one glad it is over and ready to start the next after the recovery.
• For sprints, power drops visibly by the last effort. When it has dropped by a fifth, the session is finished.

A heart-rate monitor lags behind effort in short intervals. Use it to check the long ones; go by feel for the short ones.

Choosing the equipment

Intervals are hard on the body, so choose the mode that loads the heart without punishing the joints.

• Bike, rowing machine, ski ergometer: low impact, easy to control, suitable at any weight.
• Running: effective and harder on tendons. Uphill intervals reduce the impact while raising the effort.
• Bodyweight circuits: convenient in a hotel room, though form deteriorates when exercises such as burpees are done to exhaustion. See the hotel room workout.

What intervals do not replace

• Strength. Circuits of light weights done fast raise the heart rate and do little for strength. Lift separately. See strength training for executives.
• Low-intensity volume. Steady moderate work can be done often and recovered from quickly. See zone 2 training.
• Recovery. Hard sessions are a stress added to the others. In a week of short nights and long flights, one interval session is plenty.

How often

One session a week maintains fitness and improves it in someone untrained. Two sessions, separated by at least 48 hours, is a sound upper limit alongside a demanding job and two strength sessions. People who do four or five interval sessions a week usually perform each of them at an intensity that is no longer high.

A simple week for someone with four training slots:

• Monday: strength.
• Wednesday: 10 × 1 minute.
• Friday: strength.
• Sunday: 50 minutes at moderate intensity.

Who should be careful

Vigorous exercise transiently raises cardiac risk in people with undiagnosed heart disease, even though regular exercise lowers it over time. Start with moderate activity for several weeks, and see a physician before beginning intervals, if any of the following applies: you are over 45 and have been inactive; you have high blood pressure, diabetes or a family history of early heart disease; you have had chest pain, fainting or unusual breathlessness on effort.

Supervised interval training is used in cardiac rehabilitation, which shows that it can be safe in the right conditions. The condition is supervision.

A coach paces the first sessions for you: how hard, how long, when to stop. That is how the coaches of the Club begin with every new member. How to get started.