Many people are curious about running with a weight vest or loaded pack. First, unless you have excellent running form- do not attempt this. It is a very easy way to destroy your ankles, knees, hips and back. I have seen many soldiers, firefighters and weekend warriors limp in and out of my clinic with wrecked knees because they decided running with load was more important than learning how to run.
With that out of the way, Rusco and Bosco 1987 had 3 groups of college-age men. 1 wore a vest at 9-10% of bodyweight all day and during every workout, 1 wore it every day but only for every other workout and 1 never wore a vest.
After 4 weeks the unloaded group showed increase lactate threshold while the loaded groups showed decreased lactate threshold (p<0.05) and increased oxygen consumption (p<0.05). The loaded group showed increased run time to exhaustion, improved vertical velocity (as measured by stair running), and increased VO2 capacity during sub-maximal running. The authors theorized that the constant extra load for 4 weeks caused a shift toward fast twitch muscle cells and lactate metabolism.