This is a very short description of how a muscle can develop force:

What is a motor unit
A motor unit is a functional unit that contains a single nerve and all the muscle fibres innervated by the nerve. All muscle fibres are grouped together as motor units and have an average of 150 fibres pr motor neuron.

Hypertrophy
Larger muscle cells (that will say larger square diameter) can generate more power. That is the most commonly known way to increase power, though it is not desirable for cyclists. The problem is that a large muscle mass is heavy to carry and there is a dilution of mitochondrias. Thus, an increment of maximal strength made through hypertrophy will probably not result in a better overall cycling performance.

Nervous regulation of force
Basically there are two ways to control a muscle’s force. One way is to recruit more motor unit, which will activate more motor units. You can think of this as the brain tells the muscle to use a larger percentile of the muscle’s fibres to generate power. Motor units are recruited to in order of size. Small motor units are recruited before large motor units. This is called the size principle of recruitment. The second way to regulate force production is through rate coding. It is an increment of the frequency of impulse signals to the motor unit. When a motor unit is stimulated more frequently, the twitches begin to overlap each other, which will generate a larger force.

So now we know the basic physiology behind the mechanisms used to increase the force. It is either to build larger muscle mass, make a better recruitment of motor units or fire a higher frequency of stimuli to the motor neurons.