You’ve had a cold or the flu and your symptoms have begun to subside. Your nose has stopped dripping, your cough is clearing and your head and muscles no longer ache.

You’re ready to get off the couch. But is it too early to go for a run? Here’s what to consider when getting back to exercising after illness.

Exercise can boost your immune system – but not always

Exercise reduces the chance of getting respiratory infections by increasing your immune function and the ability to fight off viruses.

However, an acute bout of endurance exercise may temporarily increase your susceptibility to upper respiratory infections, such as colds and the flu, via the short-term suppression of your immune system. This is known as the “open window” theory.

A study from 2010 examined changes in trained cyclists’ immune systems up to eight hours after two-hour high-intensity cycling. It found important immune functions were suppressed, resulting in an increased rate of upper respiratory infections after the intense endurance exercise.

So, we have to be more careful after performing harder exercises than normal.

Can you exercise when you’re sick?

This depends on the severity of your symptoms and the intensity of exercise.

Mild to moderate exercise (reducing the intensity and length of workout) may be OK if your symptoms are a runny nose, nasal congestion, sneezing and minor sore throat, without a fever.

Exercise may help you feel better by opening your nasal passages and temporarily relieving nasal congestion.

However, if you try to exercise at your normal intensity when you are sick, you risk injury or more serious illness. So it’s important to listen to your body.

If your symptoms include chest congestion, a cough, upset stomach, fever, fatigue or widespread muscle aches, avoid exercising. Exercising when you have these symptoms may worsen the symptoms and prolong the recovery time.

If you’ve had the flu or another respiratory illness that caused a high fever, make sure your temperature is back to normal before getting back to exercise. Exercising raises your body temperature, so if you already have a fever, your temperature will become high quicker, which makes you sicker.

If you have COVID or other contagious illnesses, stay at home, rest and isolate yourself from others.

When you’re sick and feel weak, don’t force yourself to exercise. Focus instead on getting plenty of rest. This may actually shorten the time it takes to recover and resume your normal workout routine.

I’ve been sick for a few weeks. What has happened to my strength and fitness?

You may think taking two weeks off from training is disastrous, and worry you’ll lose the gains you’ve made in your previous workouts. But it could be just what the body needs.

It’s true that almost all training benefits are reversible to some degree. This means the physical fitness that you have built up over time can be lost without regular exercise.

To study the effects of de-training on our body functions, researchers have undertaken “bed rest” studies, where healthy volunteers spend up to 70 days in bed. They found that V̇O₂max (the maximum amount of oxygen a person can use during maximal exercise, which is a measure of aerobic fitness) declines 0.3–0.4% a day. And the higher pre-bed-rest V̇O₂max levels, the larger the declines.

In terms of skeletal muscles, upper thigh muscles become smaller by 2% after five days of bed rest, 5% at 14 days, and 12% at 35 days of bed rest.

Muscle strength declines more than muscle mass: knee extensor muscle strength gets weaker by 8% at five days, 12% at 14 days and more than 20% after around 35 days of bed rest.

This is why it feels harder to do the same exercises after resting for even five days.

But in bed rest studies, physical activities are strictly limited, and even standing up from a bed is prohibited during the whole length of a study. When we’re sick in bed, we have some physical activities such as sitting on a bed, standing up and walking to the toilet. These activities could reduce the rate of decreases in our physical functions compared with study participants.

How to ease back into exercise

Start with a lower-intensity workout initially, such as going for a walk instead of a run. Your first workout back should be light so you don’t get out of breath. Go low (intensity) and go slow.

Gradually increase the volume and intensity to the previous level. It may take the same number of days or weeks you rested to get back to where you were. If you were absent from an exercise routine for two weeks, for example, it may require two weeks for your fitness to return to the same level.

If you feel exhausted after exercising, take an extra day off before working out again. A day or two off from exercising shouldn’t affect your performance very much.

Ken Nosaka, Professor of Exercise and Sports Science, Edith Cowan University

This article is republished from The Conversation under a Creative Commons license. Read the original article.

If you spend a lot of time exploring fitness content online, you might have come across the concept of heart rate zones. Heart rate zone training has become more popular in recent years partly because of the boom in wearable technology which, among other functions, allows people to easily track their heart rates.

Heart rate zones reflect different levels of intensity during aerobic exercise. They’re most often based on a percentage of your maximum heart rate, which is the highest number of beats your heart can achieve per minute.

But what are the different heart rate zones, and how can you use these zones to optimise your workout?

The three-zone model

While there are several models used to describe heart rate zones, the most common model in the scientific literature is the three-zone model, where the zones may be categorised as follows:

• zone 1: 55%–82% of maximum heart rate
• zone 2: 82%–87% of maximum heart rate
• zone 3: 87%–97% of maximum heart rate.

If you’re not sure what your maximum heart rate is, it can be calculated using this equation: 208 – (0.7 × age in years). For example, I’m 32 years old. 208 – (0.7 x 32) = 185.6, so my predicted maximum heart rate is around 186 beats per minute.

There are also other models used to describe heart rate zones, such as the five-zone model (as its name implies, this one has five distinct zones). These models largely describe the same thing and can mostly be used interchangeably.

What do the different zones involve?

The three zones are based around a person’s lactate threshold, which describes the point at which exercise intensity moves from being predominantly aerobic, to predominantly anaerobic.

Aerobic exercise uses oxygen to help our muscles keep going, ensuring we can continue for a long time without fatiguing. Anaerobic exercise, however, uses stored energy to fuel exercise. Anaerobic exercise also accrues metabolic byproducts (such as lactate) that increase fatigue, meaning we can only produce energy anaerobically for a short time.

On average your lactate threshold tends to sit around 85% of your maximum heart rate, although this varies from person to person, and can be higher in athletes.

In the three-zone model, each zone loosely describes one of three types of training.

Zone 1 represents high-volume, low-intensity exercise, usually performed for long periods and at an easy pace, well below lactate threshold. Examples include jogging or cycling at a gentle pace.

Zone 2 is threshold training, also known as tempo training, a moderate intensity training method performed for moderate durations, at (or around) lactate threshold. This could be running, rowing or cycling at a speed where it’s difficult to speak full sentences.

Zone 3 mostly describes methods of high-intensity interval training, which are performed for shorter durations and at intensities above lactate threshold. For example, any circuit style workout that has you exercising hard for 30 seconds then resting for 30 seconds would be zone 3.

Striking a balance

To maximise endurance performance, you need to strike a balance between doing enough training to elicit positive changes, while avoiding over-training, injury and burnout.

While zone 3 is thought to produce the largest improvements in maximal oxygen uptake – one of the best predictors of endurance performance and overall health – it’s also the most tiring. This means you can only perform so much of it before it becomes too much.

Training in different heart rate zones improves slightly different physiological qualities, and so by spending time in each zone, you ensure a variety of benefits for performance and health.

So how much time should you spend in each zone?

Most elite endurance athletes, including runners, rowers, and even cross-country skiers, tend to spend most of their training (around 80%) in zone 1, with the rest split between zones 2 and 3.

Because elite endurance athletes train a lot, most of it needs to be in zone 1, otherwise they risk injury and burnout. For example, some runners accumulate more than 250 kilometres per week, which would be impossible to recover from if it was all performed in zone 2 or 3.

Of course, most people are not professional athletes. The World Health Organization recommends adults aim for 150–300 minutes of moderate intensity exercise per week, or 75–150 minutes of vigorous exercise per week.

If you look at this in the context of heart rate zones, you could consider zone 1 training as moderate intensity, and zones 2 and 3 as vigorous. Then, you can use heart rate zones to make sure you’re exercising to meet these guidelines.

What if I don’t have a heart rate monitor?

If you don’t have access to a heart rate tracker, that doesn’t mean you can’t use heart rate zones to guide your training.

The three heart rate zones discussed in this article can also be prescribed based on feel using a simple 10-point scale, where 0 indicates no effort, and 10 indicates the maximum amount of effort you can produce.

With this system, zone 1 aligns with a 4 or less out of 10, zone 2 with 4.5 to 6.5 out of 10, and zone 3 as a 7 or higher out of 10.

Heart rate zones are not a perfect measure of exercise intensity, but can be a useful tool. And if you don’t want to worry about heart rate zones at all, that’s also fine. The most important thing is to simply get moving.

Hunter Bennett, Lecturer in Exercise Science, University of South Australia

This article is republished from The Conversation under a Creative Commons license. Read the original article.