Contrary to what some may think pedalling is not an easy movement. It is not just a case of pushing as hard as you can on your cranks to climb effectively. Pedalling uses around 30 muscles. Some for moving forward, some just to maintain the position of the hips and body. Pedalling uses a number of joints, ankles, knees and hips. All of this consumes energy.
The ideal cadence is the one that lets you climb fastest over the whole route. Power and cadence need to be optimized to minimize energy expenditure. This depends on a number of factors including pedalling efficiency. While pedalling, two things consume energy: high power outputs and high cadence. There is a zone where power vs cadence results in the optimum energy use.
High power output serves no purpose if it doesn’t result in faster climbing or if it tires the rider before the col or finish. Heavy riders often produce more power than lighter riders. When climbing this extra power is consumed lifting the extra weight. The steeper the slope the more power is wasted. On the other hand higher cadences waste energy. Not all the power produced by leg movement is transmitted to the rear wheel. This is particularly true for novice riders or riders who have not done any specific cadence training and have wasteful “parasitic” movements in their pedalling
The choice of which gear to climb with on a particular slope is a compromise between a small gear, which reduces muscle, joint and tendon fatigue and a big gear which minimizes losses due to a high cadence. In general, for the same power output, experienced cyclists prefer higher cadences for climbing because they have a more effective and efficient pedalling technique. Through regular rides and specific training their muscles work in a more fluid, precise and efficient way. Higher cadence is still probably more costly but reduces strain and fatigue over distance and a number of climbs. However the same is not necessarily true for other riders. So there is not an ideal cadence for all cyclists for a given slope. One thing is sure, specific training either on the road, or rollers, can increase your cadence letting you push the same gears faster and more efficiently with an improvement in your endurance on longer distance events.