For me, 4 Kg = 30 sec per mile. Someone with some physics knowledge, please check my napkin math

Hi CoachB.

I think your computations of "here's how much energy is spent on vertical motion for a given weight" are sane. Notably, your 5% energy saving is exactly equivalent to the 5% weight cut (80kg / 84 kg) since the equation for vertical energy is going to be something like some_constant * weight.

So if you cut your weight by N%, then you'll spend N% less energy on vertical motion.

I'm skeptical when you translate "5% less energy spent on vertical motion" into "5% faster running". For instance, say 98% of your energy is already spent on horizontal motion, and only 2% is spent on vertical. The cutting the vertical by 5% will make it go down from 2% to 1.9%. So in this case a 5% cut in energy towards vertical motion causes only a 0.1% cut to total energy. But there are other factors at play - I think you'll need less energy per stride length as you get lighter, and also there'd be less friction while touching the ground.

Regardless, your number of 30s / 8.8lbs = 3.4 s/lb is not that far off of the 2-3 s/lb that others are mentioning in this thread.

Oh Please wrote:

CoachB wrote:

Yeah, I originally did the math with a 7:00 mile (because that's about where I'm at right now) and came up with 20 seconds. So, I'm looking at a loss of 12 Kg.....

Or I can just get in better shape.

Not sure it works that way. I ran a 39 flat 10k and 1:26 half at 195 pounds (88 kg), and I really seriously doubt that a loss of 10 pounds would have gained me 20s per mile on either of those races. That would be like a 1:21 flat half at 185 and I cannot entertain that at all!

I was roughly 6'0", 165lbs for most of college. Closer to 175 now, and have not been doing a lot of strength training. Running 30-40mpw, one workout a week, time trialed 15:02 for the full 5k this weekend. Ran 1:15 for a half this summer, as part of a 17 mile long run, again off of 30-40mpw, @ 175lbs. Weight isn't everything.

thicc boi wrote:

I was roughly 6'0", 165lbs for most of college. Closer to 175 now, and have not been doing a lot of strength training. Running 30-40mpw, one workout a week, time trialed 15:02 for the full 5k this weekend. Ran 1:15 for a half this summer, as part of a 17 mile long run, again off of 30-40mpw, @ 175lbs. Weight isn't everything.

Nobody thinks weight is everything. But odds are if you weighed a more reasonable 150lbs or so, you. would be running 14:30 instead of 15:00. When all things are equal, lugging around 15 lbs of fat doesn't help your times....

You mention vertical distance traveled... what would you say for someone who is a very efficient shuffler? There might be zero vertical distance traveled if someone is 100% efficient at shuffling. Only horizontal movement, except for picking up the foot/lower leg to move the lower leg forward. So maybe lower leg weight would make a difference, but nothing else (hypothetically of course)

adsfdasfasfsafadfa wrote:

thicc boi wrote:

I was roughly 6'0", 165lbs for most of college. Closer to 175 now, and have not been doing a lot of strength training. Running 30-40mpw, one workout a week, time trialed 15:02 for the full 5k this weekend. Ran 1:15 for a half this summer, as part of a 17 mile long run, again off of 30-40mpw, @ 175lbs. Weight isn't everything.

Nobody thinks weight is everything. But odds are if you weighed a more reasonable 150lbs or so, you. would be running 14:30 instead of 15:00. When all things are equal, lugging around 15 lbs of fat doesn't help your times....

Where do you get "lugging around 15 pounds of fat" from? That's eating disorder thinking.

10 pounds extra since his college days is probably 6 or 7 pounds of fat and 3-4 of extra muscle.