Seems to be three separate issues here. Ankle weights, resisted running, and what your goals are.
I think it's been pretty much decided that ankle weights, specifically, are a bad idea. I don't know about the OP, but unfortunately I think many people may have gotten this idea from t3h deadly Bruce.
Sleds, vests and parachutes are a different deal. Obviously, as the military guys stated, people can and do run distances bearing weight or against resistance (headwind, for one).
The results indicate that commercial claims of marked increases in energy expenditure during running with hand/ankle weights are exaggerated. It appears that the small actual increases in energy expenditure, the potential for increased impact forces, and the relative discomfort of carrying weights discredit running with hand and/or ankle weights as a desirable exercise alternative.
Effects of extremity loading upon energy expenditure and running mechanics
I would not advise the use of conventional ankle weights, because they lower the mass of the limb that we are trying to move quickly. A proper swing phase in the stride cycle requires that the lower leg be folded tightly to the thigh before the thigh is advanced forward. Making the leg as short as possible right off the ground increases angular acceleration.
Further, biomechanical analysis indicates that sprinters generate seven times as much power at the hip than at the ankle. If the lower legs become too heavy relative to the thigh, as would be the case with ankle weights, they could retard the runner's stride frequency, just as a metronome slows as its weight shifts further from its fulcrum. In addition, the extra weight can cast open the lower leg before the thigh goes through its full range or flexion.
This is an interesting abstract from Sport Biomechanics on the subject of sleds and weight vests
So it can change your form. But that isn't the end of the story. This is where the issue of goals comes in, ie. why exactly you're running with resistance.
In this study, we compared sprint kinematics of sled towing and vest sprinting with the same relative loads. Twenty athletes performed 30-m sprints in three different conditions: (a) un-resisted, (b) sled towing, and (c) vest sprinting . . . There were significant differences between the two resisted conditions in trunk, thigh, and knee angles. We conclude that sled towing and vest sprinting have different effects on some kinematics and hence change the overload experienced by muscle groups.
I highlighted "during acceleration performance etc." because that's where the second study here may seem at first to contradict the other one. From the journal articles I've read, it seems the benefits of resisted sprints would be in the acceleration phase, ie. in acceleration rather than speed/maximal velocity. So, it seems that the specific benefits would change in longer distances. In other words, if you're not working on acceleration (or going distances beyond that), the benefits of sled/vest/rucksack would be to cardio and endurance rather than speed, which is, once again, where the goal issue comes in.
The effects of resisted sprint training on acceleration performance and kinematics in soccer, rugby union, and Australian football players . . . The results indicated that an 8-week RS training program (a) significantly improves acceleration and leg power (CMJ and 5BT) performance but is no more effective than an 8-week NRS training program, (b) significantly improves reactive strength (50DJ), and (c) has minimal impact on gait and upper- and lower-body kinematics during acceleration performance compared to an 8-week NRS training program
. These findings suggest that RS training [note, not ankle weights]
will not adversely affect acceleration kinematics and gait. Although apparently no more effective than NRS training
, this training modality provides an overload stimulus to acceleration mechanics and recruitment of the hip and knee extensors, resulting in greater application of horizontal power.
So, obviously, guys in the military are walking, jogging, and running longer distances than merely resistance sprints without their joints exploding. But your running kinematics, as opposed to acceleration kinematics, will be different (note, not horribly bad form = injury different). In other words, resisted running/jogging over distances will not make you run faster per se, although it will make you better at running/jogging over distances with resistance/weight and have endurance benefits. The issue then becomes if you specifically want/need to become faster, or if you want/are able to increase the endurance more through another method.
So for a guy in the military, it's probably a good thing to reap the benefits while training for something he specifically has to do, but theoretically it might not be the best approach for someone who a) wants to get better maximal velocity (vs. acceleration) or b) has another way to work on the endurance specific for what they need.
I thought the last line of the last article was especially interesting on the benefits of resisted sprinting vs. non-resistant. I heard anecdotally a while ago that the benefits of RS were hyped, and I read an article by Enamit that said parachute running/sprinting was vastly overrated.
This article has an interesting history on the sprint chute
The sprint chute was the brainchild of former Soviet sprint authority Ben Tabachnik, who in background, stature, and respect is the Arthur Lydiard of the sprint world. Tabachnik has authored a unique training manual called Soviet Training and Recovery Methods. In the book, co-authored by Rick Brunner, Tabachnik presents the speed chute as a unique means to intensify the training process physically, as well as metabolically and neurologically. Test results, apparently performed in a secluded stadium outside of Moscow, proved that the speed chute was superior to all other devices designed to improve maximal speed, start acceleration, and speed-endurance. Tabachnik notes a dramatic reduction of .2 to .4 seconds in 100 meter dash times, but he was also working with advanced athletes-not beginners.
He goes on to talk about the value of sleds and tires, which he says have better benefits to metabolic and strength goals than to speed.
Further, I am convinced that much of Tabachnik's work with the chutes had to be conducted in some of Russia's massive indoor training complexes or skating rinks, where there are no crosswinds to violently disrupt a runner's stride or alter his running path. Although such sudden shifts are appropriate for sports such as hockey and football, they are a disaster for single-direction activities such as sprinting, where athletes are traveling at six to eight meters per second and attempting to apply a force three to four times their body weight for 0.09 to .11 of a second while landing precariously on a three inch-wide spike plate or racing flat. A gust of wind will yank the sprinter all over the place, and such a traumatic oscillation, rather than tearing down a dynamic stereotype, will tear apart a runner's hips, knees, and ankles.
Tabachnik notes that the faster the athlete runs, the greater the drag. Herein is a problem, since the resistance is not uniform for any set length. Current research suggests that to achieve gains in maximum velocity athletes should not be slowed down more than 10% because, as the resistance becomes greater, the ground dynamics begin to change.
So, tl; dr, ankle weights= bad idea - vests/sleds/tires/chutes may benefit acceleration, but probably not much more than standard non-resistant sprint training - vests/sleds/tires etc. don't likely benefit maximum velocity, but they can help with strength and endurance- are they the method suited for the individual based on goals