I have clients who do a lot of running, but when they try to do cycling or something outside of their normal program their legs start burning. Why?
One key aspect of training that people tend to forget is the neurological system. We must remember that all muscle contractions are stimulated through the nervous system in specific firing sequences. These neural pathways are formed through movement and become efficient after repetitive movement or a term we call practice or training. The brain creates the most efficient way of doing that particular motion at the expense of all other movements. Some of the physiological changes that occur when creating a new pathway are a higher heart rate and/or muscle fatigue. Although it seems the muscles are working the same way in both exercises, they are not. Without turning this into a functional anatomy course or one of biomechanics, we must look at the contact points and load distributions of each movement.
In a seated position like that on bicycle, much more hip flexing is occurring than if they were jogging or walking. If their feet are in clips, the hip flexors are working double time and it is also a different movement than without having your feet clipped. This musculature will fatigue quickly because it is not efficient at what it is doing. The paradox is that the better a person is at one thing, the worse they become at everything else. In fact, we V02 tested marathon runners on a standard bike test and they had to discontinue the test because of such high heart rates and muscle fatigue. These athletes were in amazing running condition but could not finish a standard bike test we give a de-conditioned person! Always remember that our bodies only get good at what we do. There is very little carry over from one mode of training to another. Can you believe that if you become strong at doing seated pulldowns in a gym, there is almost no carry over into the ability to do a pull-up.
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Mike DeMora, BCET