Muscle fibres are about the thickness of a human hair and can range from 1mm to 4cm long.
Individual fibres don't work on their own (they'd break if they did); instead, they're bundled together, and act together.
Each bundle of muscle fibres is innervated by a nerve - that is, the nerve transmits electrical messages from the brain ordering the muscles to contract to cause movement. The nerve controls a motor unit which is attached to each bundle of muscle fibres.
So when you want to contract a muscle, in order to move part of your body, electrical impulses are sent from the brain via the nerves to the motor units which then cause bundles of muscle fibres to contract.
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But how do we use the right amount of muscle to lift a small weight versus a big weight?
Motor units operate like a binary switch - they're either on or off.
So each motor unit, when it's innervated causes the bundle of muscle fibres to contract. The fibres in the bundle can either be contracted or not contracted - there's no half way measure.
Whilst the muscle as a whole is capable of gradations of muscle contraction, each bundle is either fully contracted or 'off'.
This is called the all or nothing principle of muscle contraction.
So if you want to do a bicep curl with only a small weight, only a small number of motor units will be innervated and only a small number of bundles of muscle fibres will contract to pull the arm up.
If a heavy weight is lifted, more motor units are innervated causing a large number of bundles of muscle fibres to contract at the same time, allowing the weight to be lifted.
Now imagine doing a bicep curl with a heavy weight, and stopping half way. A large number of bundles of muscle fibres will have been innervated and those fibres will have contracted. A large number, but not all. There are still some bundles in reserve.
Eventually, the bundles that first contracted will become tired and unable to maintain the contraction, so the motor unit will switch off - to allow you to keep holding the weight, another muscle bundle (currently not innervated) will need to be switched on to take the place of the one that's switched off.
Eventually, if the weight is so great, there won't be any bundles with sufficient energy to switch on and the whole muscle will begin to fatigue.
As ever, if you need help with muscle problems, give me a call