Here’s the deal with long slow running. Your muscles are able to use fatty acids to a much greater extent if the HR is kept low enough. When the HR gets higher and therefore the effort level is up too, the muscles can no longer supply the lion’s share of the energy aerobically. People become more dependent on anaerobic respiration. In order to go fast we all have to do this. There are some problems with becoming overly dependent on anaerobic energy delivery however. In events taking an hour or more-all triathlons-over 98% of the energy delivered is aerobically based. If we can develop this base, we will be able to go faster for longer periods of time.
If we never slow down enough to be able to depend on aerobic energy delivery, we’ll never develop greater efficiency at delivering energy aerobically. If we can’t be disciplined enough to slow down, pick up the pace, and let anaerobic energy delivery become a larger component of the total energy delivered, the lactic acid that is the product of anaerobic energy delivery will lower the pH in the working muscles. This lower pH will twist the enzymes that we need to deliver energy and contract our muscles. We become inhibited and can’t deliver energy aerobically or anaerobically without these enzymes functioning properly. We will eventually hit a wall. Considering this and the fact that we can only store enough glycogen, the only fuel source for anaerobic energy delivery for a relatively short period of time, makes becoming dependent on that fuel source a strategy that has no future.
You will find that running at low HRs will slow you down initially. Since you aren’t inhibiting aerobic energy delivery by producing too much lactic acid, you will be able to improve your ability to deliver a clean aerobic energy burn and maintain faster paces as a result.
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