now let's talk a little bit about how muscles generate force we've seen that muscles are composed of muscle fibers which in turn are composed of myofibrils which are basically end-to-end sarcomeres and the sarcomeres are the functional unit of muscle these are the things that contract and these are the things that shorten muscle and that shortening of the muscle muscle creates tension and tension is what allows us to do work so how do we vary the amount of tension that we produce well we've got many many muscle cells and as we know each muscle cell or each muscle fiber is innervated or supplied by a motor neuron so each muscle cell is innervated by exactly one motor neuron however one motor neuron can innervate multiple muscle fibers and this becomes important because we have groups of muscle fibers that are all innervated by the same motor neuron and we call this a motor unit so all the nerve fiber or sorry all the muscle fibers that are innervated by the same motor neuron are called a motor unit and so what happens is that we can recruit as many motor units as we need to get the job done so if we need to lift a heavy weight we'll lift or we'll recruit more of these motor units and we'll bring more of these muscle fibers online so they'll contract so that at any given time if you're using your muscles you may only have a certain number of these motor units that are operating and contracting so if you're doing something like one of these exercises where you have to you know hold your arms out to the side for long periods of time this is fairly easy to do because we have a fairly light weight out here and what's happening is there are a few motor units online at once and they'll fatigue out and they'll tac team it with another leather set of motor units and they'll just kind of keep tacting you so you can sit here without fatiguing too much if you are using fairly light weights or not exerting a lot of force simply because you have enough motor units that they can sort of rotate around now if you're lifting a heavier thing you will want to use more of these motor units so let's say you pick up a very light weight like this dry erase marker you don't have to use very much force at all so you may only need one or two motor units to pick this thing up as you pick up a slightly heavier weight then you're going to need to recruit maybe a few more motor units if you pick up a heavier weight still well now you're going to recruit more motor units and so that as you are increasing the amount of tension and generating more force then you will recruit more of these motor units