Hi, it’s my first post ever on any forum. Been following many of the conversations and this is one that raises some questions for me. There are some serious weights being carried here and I was just wondering about the adaptation effect you get - aerobic vs muscular endurance? Pace vs weight vs distance.
I don't claim to be an expert on this, but I like your question so I'll offer my thoughts on it. Carrying weight in a backpack is infinitely scale-able so it really covers the whole spectrum. (Just like farmer's carries with kettlebells -- I can carry 8kg in each hand and walk for an hour, or carry 48kg in each hand and walk for 100 feet. Same exercise with very different targets and effects!) I would say the aerobic benefits are maximized when the duration is long, the weight is light, the heart rate is MAF or below (180-age, and +/- adjustments). The strength-building (i.e. fast-twitch muscle fiber) muscular endurance benefits are maximized when the weight is heavy enough that it can only be maintained for short (5-15 minute) intervals, and in these conditions the HR will likely be higher than MAF, indicating it's also a glycolytic activity. The overall endurance (i.e. slow-twitch muscle fiber) muscular endurance benefits are maximized when the weight can be maintained for an hour or longer. Any weight or time will be great for building general load-bearing capability of the body -- the feet, the ankle/knee/hip joints, the trunk muscles, the back and shoulders. Smart progression in working up to weight and distance is important!
When I did my loaded-backpack training last year while preparing for a hiking trip, I found it to be hugely more beneficial (and enjoyable) to walk on uneven terrain rather than on a flat trail or road. There's really something powerful about the brain's connection with and control of the body in this condition - each step is different and challenges movement and balance, the load increases the challenge, and you are continuously navigating forward with purpose. Makes me want to get back out there and hike!
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