Lack of use
Lack of muscle fitness (deconditioning) is one of the most common causes of muscle weakness. It may occur as a result of an inactive (sedentary) lifestyle. If muscles are not used then the fibres within the muscles are partially replaced with fat. Muscle wasting will eventually occur: muscles become less chunky and more floppy. Each fibre is just as strong but there aren’t so many of them and they don’t contract so effectively. When you feel them they may be floppy and a little reduced in bulk. This leads to easy tiring when you try to do things that would have been easier when the muscles were fit. The condition is reversible with sensible, regular exercise regimes. It gets worse and more marked with increasing age.
Muscle power is greatest and recovery times are shortest in our 20s and 30s. This is why most great athletes are in this age range. However, building of muscles through regular exercise can be done at any age. Many successful long-distance runners are aged over 40. Muscle tolerance for prolonged activity such as marathon running remains high for longer than the powerful, short-burst activities like sprinting.
It’s always good to stay fit, whatever your age. Recovery from muscle and tendon injury, however, also becomes slower with increasing age. At whatever age you decide to improve your fitness, a sensible training regime is essential. You need advice from trainers or physiotherapists, to prevent injury to muscles which, at least at first, may not perform as well as you hope.
Ageing
As we age, our muscles tend to lose strength and bulk and they become weaker. Whilst most people accept this as the natural consequence of age - particularly great age - it is frustrating to be unable to do the things you could manage when younger. However, exercise is still beneficial and it is still possible to increase muscle power and strength with a careful and safe exercise routine. Injury recovery times are much longer with great age, balance is often impaired and thinner bones are easily broken. So, supervision is sensible, especially at first.
Infections
Infections and illnesses are amongst the most common causes of temporary muscle fatigue. This is usually through muscle inflammation. Even though recovery is usual, if inflammation is severe (such as a bad bout of influenza), the weakness can last quite a while. This can sometimes trigger chronic fatigue syndrome (CFS). Any illness with generalised temperature and muscle inflammation can be the trigger. However, some illnesses are particularly prone to cause it. They include flu (influenza), glandular fever (Epstein-Barr virus), HIV, Lyme disease and hepatitis C. Other less common causes, in the UK at least, are tuberculosis (TB), malaria, syphilis, polio and dengue fever.
Pregnancy
During and just after pregnancy, high levels of steroids in the blood, together with a tendency to be lacking in iron (anaemia), can cause a feeling of muscle tiredness. This is normal in pregnancy and whilst some exercise is still sensible, when pregnant you need to be particularly careful when exercising. Remember you are carrying a weight on the front. This can lead to lower back pain if you don’t adapt what you do to take account of your altered posture.