With the increase in altitude, pack weight, age or any other variable there are some other things that coincide with the increase in output. First is an increase in water loss due to a combination of perspiration and respiration from the increase in effort (an average hunter will burn between 10-15 calories per minute and data suggests that some may burn up to 7,000 calories or more per day)! This makes hydration super critical as a drop in hydration of even 3% may decrease muscle function, cause fatigue in muscles, especially in the diaphragm (critical for breathing) and abdominal muscles which are important for maintaining posture and stabilizing the trunk and any load you are carrying.

A significant threshold for change in slope was found at Load–Speed Index of 260 (p = 0.004), which corresponded to 47% VO₂max. About 47% VO₂max is very similar to ~45% VO₂max reported in the literature as the threshold above which time to fatigue decreases and cardiovascular drift increases.

David Boffey, Idan Harat, Yftach Gepner, Cheyanne L Frosti, Shany Funk, Jay R Hoffman, The Physiology and Biomechanics of Load Carriage Performance, Military Medicine, Volume 184, Issue 1-2, January-February 2019, Pages e83–e90, https://doi.org/10.1093/milmed/usy218