In my last two blogs we established that joints are the place where bones meet and ligaments are what hold them in place. There is a lot of equipment we use to move, as this blog series highlights, but muscles drive the show. Literally.
Muscles are the tissue that supply the energy to move.
They are the engines that convert chemical reactions into mechanical activity, the same as your car engine does (except muscles use sugar instead of gasoline to combine with oxygen). Other tissues like ligaments and fascia can store mechanical energy, but muscles are the only tissue that can generate mechanical energy.
The car engine comparison goes further. A car engine only does one thing – it makes the drive shaft spin, in the same direction, all the time. Everything else in the car is run by harnessing the motion of the spinning shaft. Muscles also do only one thing mechanically- they get shorter. If one end of a muscle, say your biceps, is anchored to your upper arm and the other end into your lower arm then the two ends of the arm will draw closer together when the muscle contracts. Because you have bones, the ends of the muscle will get closer together by folding the arm at the elbow. But once the elbow is bent the biceps cannot push its’ attachment points apart to straighten the elbow, contraction is a one way trip.
So how do muscles get longer again so they can contract once more?
To understand this, you need to know a little about how a muscle works. Muscles contain two kinds of fibers, called actin and myosin, that overlap with one another. To contract the muscle actively pulls one fiber along the length of the other. In reality you pull hundreds or thousands of individual fibers at a time, but this is a one way pull. The only other thing a muscle can do is release the connection between the fibers. Then it is possible to slide the fibers back where they started but something else has to provide the energy to slide the fibers in that direction. And that something is usually a different muscle or muscles.
Back to our example of the biceps bending the elbow. The way the elbow straightens is by the bicep releasing the grip between its fibers while the tricep (on the other side of the elbow) pulls the bones to make the elbow straight. As the bones move, the fibers of the bicep gets slid back to its starting position.
The other main source of energy for movement is gravity.
You can sit at the table and bend your elbow up. When you relax the muscles – i.e. release the fibers from each other – gravity will cause the bones to change position and straighten the elbow which lengthens the biceps out again. Of course, often we move to oppose gravity so our relationship with gravity can get complicated.
The cool thing about muscles is that they are very adaptable. If you use them more, they get stronger. The upper limit of how strong a muscle can be changes as we age, but the rate you gain strength as you exercise is fairly constant. So, if you start exercising at 60 years old you can gain strength at about the rate of a 25 year old! Your total strength will just max out at a lower level than your 25 year old self. Don’t panic, people rarely reach their maximum strength so most of us always have room to improve.
Next time, we’ll talk about how a muscle is actually connected to a bone. Il’l give you a hint… tendons.
– By John Macy, PT