In trained cyclists there's unlikely to be any useful adaptations below about 75% HRmax, and such a session may only be useful if you can ride for very high volumes. Additionally, the way that training data is presented about (e.g.) LA (and others) we simply don't always know what they mean by ~ 65 % HRmax. For e.g., is this the prescribed intensity (i.e., aim continually for that HR) or is this the mean average HR of the session. In the latter scenario i generally train for long endurance sessions at e.g., 75 - 80% HRmax, but my mean average is much lower than this as my HR decreases when e.g., descending or cycling through 'built up' areas. Thus my mean average does not always reflect the intensity of the session.
additionally, we know that LA (and many others) don't actually prescribe training by HR, but by power output. Trying to convert one to the other may cause issues.
Also, it's imperative to note, that when training hard that HR decreases (even though power output remains constant) due to certain physiological changes, which are immediately reversed upon rest. For e.g., in this case if you train long every day, your HR becomes very depressed even though there's been no change in fitness.
As most people don't train for e.g., 5/6 hrs per day, it's a waste of time for (most) trained cyclists to train at such low levels (assuming we're not including a depressed HR scenario etc). In general most cyclists have limited time to train, which decreases during the winter months (due to changes in lighting/riding in the dark) and as such we need to maintain a much higher level of intensity. There's no reason to not do certain intervals virtually all year round. in fact many elite cyclists will compete virtually all year round (e.g., track/cross over the winter period).
It's a mistake to ease up over the winter and neglect harder efforts. Your fitness decreases at a very rapid rate and you lose your summer gains quickly. It's best to keep working at most intensities all year round.
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