Respiratory System - Breathing Facts

  • Expiration is normally passive (only during heavy exercise workloads does it become forced)
  • Inspiratory muscles play a vital role in the efficiency of breathing at rest and also during exercise
  • The strength of the inspiratory muscles reduce with age (men 25+, women 35+), although fitter individuals may not notice until approximately 45-years of age
  • Weaker inspiratory muscles are usually highlighted with breathlessness, especially in the elderly
  • At rest we use as little as 8-12 litres of air per minute
  • During exercise this can raise to as much as 150 litres per minute and for elite male athletes even up to 240 litres per minute

The diaphragm and chest wall muscles act together like a bellows to pump air in and out of the chest. To breathe in these muscles contract to expand the chest cavity, causing a pressure drop into which the air flows.

To breathe out, you simply relax these 'inspiratory' muscles and the chest springs back forcing the air out of your lungs. During exercise the exhalation is assisted by contraction of the abdominal muscles. Thus, the inspiratory muscles undertake most of the work of breathing. In contrast to our frequent observations of inspiratory muscle fatigue, our research has never identified exercise-induced expiratory muscle fatigue. For this reason we've found it unnecessary to train anything other than the inspiratory muscles. At rest you breathe around 12 litres of air per minute, but during heavy exercise this can rise to over 150 litres per minute, and in elite athletes, this can be as high as 220 litres.

Benefits of inspiratory only vs. inspiratory & expiratory muscle training 

It’s logical to think that if training your inspiratory muscles is good, then training your expiratory muscles too must be even better. Unfortunately, research has shown this not to be the case. In a carefully conducted study on rowers, the independent and combined effects of inspiratory and expiratory muscle training were compared ( Griffiths & McConnell, 2007 ). These authors found no effect of expiratory muscle training upon performance whatsoever. In addition, when inspiratory and expiratory muscle training were combined (inhale/exhale training), the addition of the expiratory training impaired the effects of inspiratory training. In addition, a study of inhale/exhale training in swimmers found no change in swim performance after this type of training ( Wells et al., 2005 ). These studies indicate that, far from being better than inspiratory training, inhale/exhale training is LESS effective than inspiratory training. So why jeopardise the proven benefits of inspiratory training by adding something that is ineffective, and more expensive?

Fig. 1 Work of Breathing

Work of breathing

The figure shows the changes in breathing muscle contraction force and the resulting changes in lung volume at rest (red loop) and exercise (orange loop). The area below the dashed line represents the amount on inspiratory muscle work and the area above, the amount of expiratory muscle work.

At rest, all breathing muscle work is inspiratory.

During exercise it is also clear that inspiratory muscle work is much higher than expiratory muscle work (as represented by the area on the green loop compared with the area of the blue loop).

Weakness of the inspiratory muscles can result from a number of causes, including disease, but a potent influence upon their condition is the amount of exercise they receive. The phrase 'use it or lose it' applies equally well to the inspiratory muscles as it does to your leg muscles. If you get out of breath on the stairs, then you'll take the lift, with the consequence that your inspiratory muscles get less exercise.

As they become weaker, the level of physical activity that brings on the breathlessness gets lower, so you avoid the stairs even more…it's a vicious cycle of breathlessness, lack of exercise and inspiratory muscle weakness. In addition, the use of oral steroid medication (not inhaled steroids) to control lung inflammation in conditions such as asthma and emphysema has been shown to cause weakness of the inspiratory muscles. This weakness can impair lung function and can be counteracted by inspiratory muscle training. N.B. inhaled steroids do not cause inspiratory muscle weakness.

Simple Exercise Principle: USE IT or LOSE IT!

By training the inspiratory muscles the following will be achieved: 

  • Increase in resistance to fatigue
  • Improved efficiency therefore less oxygen is required by the lungs and can be used by working muscles

Result = INCREASED performance

Breathlessness is a common feature of lung and heart disease, but as we know all too well, it’s also a feature of normal exercise. Recent research has shown that the strength of the inspiratory muscles has a direct influence on how hard we can breathe and how breathless we feel whilst doing it.

If the muscles are weakened or fatigued (inspiratory muscles can fatigue by as much as 20%) then we can't breathe as hard and breathing requires greater effort; we experience the effort as breathlessness.

A useful analogy is to think about how much heavier a barbell feels on the 12th repetition than it did on the first. In the same way, if the inspiratory muscles are weakened or fatigued, breathing feels harder.

When we climb hills or stairs, we are suddenly exposed to high intensity exercise that, for most of us, is above our lactate threshold. At these intensities our breathing moves out of its 'comfort zone' and increases steeply. This sudden increase in inspiratory muscle work is perceived as breathlessness.

At low and moderate intensities, breathing is very modest, but as the intensity becomes more strenuous breathing increases steeply becoming almost exponential. During the majority of your everyday activities, your breathing operates well within its 'comfort zone'. Only when you venture above the lactate threshold (hill and stair climbing territory) is breathing stimulated sufficiently for the breathing muscles to be challenged. Exercise above the lactate threshold is usually short and sharp.

In other words, your breathing is not exposed to a suitable training stimulus for a sufficient duration or with sufficient frequency for the breathing muscles to experience a full training adaptation. Even if you could sustain the high intensity exercise, it’s doubtful whether this type of unloaded breathing would provide an adequate training overload to elicit maximal training benefits; it’s akin to a bicep curl without the dumbbell.

This is not to say that aerobic activity doesn't provide any training benefit to your inspiratory muscles; it does, it’s just not sufficient to elicit the full potential of this vital group of muscles. The result is that under normal conditions, the breathing muscles never really get trained to cope with 'heavy breathing' and for this reason it will always present an uncomfortable challenge.

Even when the smaller physical size of women is taken into account, their lungs are still smaller than men's. Women also have narrower airways (breathing tubes), which means it’s harder to move air in and out of the lungs. At rest we breathe around 8-10 litres of air per minute, but during strenuous exercise a woman can raise this to around 120 litres per minute.

Compare this to an elite male athlete who can breathe as much as 240 litres per minute! Because women are unable to 'heavy breathe' as well as men in response to strenuous exercise, research has shown that many women may experience a drop in the amount of oxygen in their blood and a corresponding increase in their breathlessness.

Recent research evidence suggests that during heavy exercise, blood flow (and hence oxygen delivery) to the exercising legs is inversely related to respiratory work.

In other words, if inhalation is made harder by loading breathing with an added resistance, blood flow to the working legs goes down.

In contrast, if inhalation is assisted using a ventilator, blood flow to the legs goes up. What is more, the extra blood delivered to the legs can be put to good use by increasing the maximum power output. What this tells us is that the inspiratory muscles are capable of stealing blood from the locomotor muscles, and in so doing, they can impair performance.