All these factors play significant roles in the training equation. More of any or all of them will improve the training effect as long as you can handle the training load. So this is highly individual. Keep in mind that training must be targeted toward the desired outcome. Think of the muscle fiber types engaged for the work you are doing
If you are planning training to improve basic aerobic capacity then it must be low to moderate intensity, long duration and frequent. This training engages the ST fibers which already have high endurance so to improve there aerobic capacity requires using them for a long time quite often so they remain in a glycogen depleted state to cause aerobic adaptions If you run 30 minutes 2 x per week thats is going to have minimal impact on improving this quality. This is why endurance athletes must train a high volume of Z1-2.
If you are hoping to improve your sport specific endurance (what we call Muscular Endurance), say skinning up hill fast for a skimo racer then you have train at that endurance limit as frequently as you can tolerate (and recover). This training engages FT fibers that are not well endurance trained so they fatigue faster and need longer to recover so this training will be less frequent to be effective. Too much too often and you loose aerobic capacity and eventually become overtrained. Because the FT fibers have less endurance they can not tolerate as much volume so this workout will be lowere duration and less frequent. The St fibers with their high endurance will not see much training effect from this workout because they don’t get fatigues in these shorter durations.
If you are hoping to training your maximum speed/power or strength like in uphill sprinting or powerful boulder problems, then you must engage the highest power FT fibers in those muscles. These fibers have almost no endurance so become fatigued very quickly and take several days to recover. You must be well rested for this type of training or you will only be able to recruit the lower powered FT and ST fibers which recover quicker. But, just as with the above two examples, the more of the training you can do the more gains you will see. It is just that you can’t train this every day like you can the ST fibers.
Make Sense?
Scott
