The cause of stiffness usually isn’t your muscles at all – it’s your fascia. These are the slimy layers within and between muscles which allow the muscle fibres to flex and glide around each other. After a period of inactivity, the fascia start to seize up and stiffen, making it harder for your muscles to move around. This is why sitting at a desk all day makes you feel stiff and uncomfortable. Chronic dehydration can also make the problem worse, because dried-out fascia become sticky.
Fascia can contract if they aren’t stretched and worked enough, but there’s a fine balance to strike. If you put too much pressure on the fascia, by working the muscles too hard or moving in a repetitive way, then they can get twisted and inflamed. This is one of the reasons you feel so cranky and sore the day after a hard workout (known as delayed onset muscle soreness, or DOMS because fascia link the whole body together, tightness in one set of muscles can sometimes be caused by the body working to compensate for weakness in another area – for example, weak neck muscles sometimes lead to tight hamstrings, as your body is trying to support the weight of your torso in any way it can. (Source)
Age-related muscle tightness is due to a variety of causes. To put it in layman’s terms, your whole body has less ‘bounce’ – less cartilage in the joints, shorter muscle fibres, damage to the collagen which keeps you flexible. (Source) While some increase in stiffness is inevitable as you get older, it’s lifestyle rather than fate which determines how badly you’ll be affected. You can even train your fascia to become more flexible.
Flexibility is measured by your range of motion (ROM), which is basically how many degrees you can rotate your joint. Interestingly, stiff joints feel like their ROM is limited, but this isn’t always the case – often, you can stretch just as far as usual, you simply have to grit your teeth a bit to get there. So although it’s tempting to give up certain exercises because they make you stiff, it’s worth gently persevering to see if the discomfort goes away with time.
There’s research indicating that certain hormones affect the fascia – for example, human growth hormone (HGH), oestrogen, and stress hormones like cortisol (Source). It’s been suggested that this is why post-menopausal women tend to become much stiffer, why sudden growth spurts in children usually lead to decreased flexibility, and why anxiety can cause muscle pain. But the research is still in its early days, and it’s not easy to change your hormone levels anyway. So although the hormonal influence is interesting, it makes more sense to focus on the physical causes of your muscle tightness.
Note that there’s a difference between normal muscle tightness, and muscle spasticity as a side-effect of other medical conditions. If your stiffness isn’t eased by simple treatments like stretching or mild painkillers, then it should be checked out by an expert as it could be a side-effect of something else – which could be as mild as dehydration, or as serious as lupus.