This is a simple explanation of how the body should work. When one muscle contracts, its opposite number relaxes. In order to contract, a signal is sent via the nerves to ‘innervate’ the muscle – ie make it work. The basic structure of a muscle is rather like a very thin skinned orange in all but shape. A muscle is surrounded by fascia, which could be likened to the orange skin. Within the orange are the segments – which can be likened to the bundles of muscle fibres, together making 
up the whole muscle, with each bundle surrounded by another type of fascia. Finally each muscle fibre within its bundle is also surrounded by fascia – so within each orange segment are tiny pieces of orange within a casing. And, finally, just as when we peel an orange, we can see the individual segments held together, so a muscle has a striated feel – if we feel carefully, we can feel the bundles of muscles. Perhaps the muscle group that shows this the best is the delts – the ‘epaulettes’ on the outside of the shoulders. The orange analogy breaks down in when it comes to shape, since ideally all the fibres run in the same direction.
When a muscle contracts, not every bundle and fibre contract every time. Otherwise when lifting a pie to our mouths, we would smash it into our face with a huge amount of force. We have all felt how our brain judges how much to contract a muscle when we get it wrong – for example when picking up a box from the floor; if the weight of the box is unexpectedly light, the box flies up – and if it is unexpectedly heavy, we grunt and nearly (hopefully, anyway) blow our backs as we suddenly strain to lift it. Since this happens rarely, it shows just how remarkably adept the brain is at this judgement.
Another thing that the brain takes into account is the overall state of our body; if our posture is less than optimal -a common example is the upper back rounded from working at a desk or driving with the head poked forwards, the brain will weaken our muscles to prevent injury, except in times of extremis. What is true is that we are all amazingly strong and there is truth in the urban legends of mothers being able to lift a car off a trapped child. When life is at stake, the brain lets us recruit our full strength. However, the aftermath is pretty terrible for the rescuer. Muscles will have been overloaded and torn, quite possibly joints will have been pulled out of alignment, it is also possible that bones got broken. And so the better aligned we are, the stronger we will be.
At the risk of stating the blindingly obvious, for a muscle to work, it has to cross the joint. So the hamstrings run down the back of the leg and attach below the knee joint on either side. To complicate things a little, many muscles cross 2 joints – again the hamstrings: at the top end, they cross from the top of the leg bone and onto the bottom of the pelvis. The next layer of interest is that where the muscle crosses the joint, it is often no longer the muscle, but the tendon. Tendons attach the muscles to the bones and are made of dense connective tissue with a limited blood supply. So if we feel directly behind the knee, on the edges, we can feel the tendons of the hamstrings as thin, strong wires. Perhaps the best known tendon is the achilles tendon at the back of the heel which runs right underneath the sole of the foot. If this area is uncomfortable, it means the calf muscles running into it are very tight and need attention.
So the muscles and their tendons move the joints. Each joint needs additional support for stability and strength, otherwise the knee, for example, would rotate too much on its axis and damage the surrounding muscles. This strength comes from the ligaments. Ligaments are also composed of dense tissue with a very limited blood supply. They are rather like carbon fibre – which has been used to replace torn knee ligaments. Unlike a muscle or its tendon, which contract or relax as we move, the ligaments remain taut; it is a structural unit that will prevent unwanted movement at the joint, as opposed to a unit causing movement.
Here we see the ligaments of the knee viewed from behind.
In future blogs, I will explain some of the things that go wrong, some stemming from injuries old and new and some from poor posture and chronic overuse. So as a final piece of information: acute means just happened: a broken ankle is an acute injury. Chronic means it has been going on for a long time: carpal tunnel syndrome is a prime example. Only too often acute leads to chronic: a broken ankle leads to hobbling about and pain which, if care is not taken, can lead to a permanent change in how we walk – and in due course this change in gait will lead to chronic pain elsewhere in the body. Old injuries do come back to haunt us – no longer will one muscle relax properly as its opposite number tries to contract.