I understand how motor neurons work, but I would like clarification on what exactly is going on when a muscle contracts. For the sake of simplicity, let's just use the bicep as an example. If I do a bicep curl with a weight that is the maximum weight my bicep can lift, all the muscle fibers in my bicep will be activated and contract(more or less). If I remove the weight and do the same bicep curl, now only a small percentage of the muscle fibers in my bicep will be activated and contract, because that extra force isn't needed. But, the whole bicep muscle is still contracting! It must be, otherwise wouldn't the other muscle fibers staying at regular length prevent the bicep from contracting?
So the only thing I could think of is that the unactivated muscle fibers are pliant and DO contract, they just don't initiate the contraction, but instead go along for the ride. So if you looked at the level of sarcomeres, in the activated muscle fibers, the myosin heads pull on the actin, causing contraction. In the unactivated muscle fibers, myosin heads are NOT pulling on the actin, but the sarcomeres are still contracting, due to the forces from the activated muscle fibers. Is this a correct interpretation of what's going on or am I wrong?