Yea, you see lots of high jumpers doing half squats which is fine, you can learn to recruit the glutes and hams in a half squat very well. If the goal was primarily two leg jumping, I would use half squats ALOT, as an spp type squat as well. The low bar position saves the knees of my jumpers, ESPECIALLY triple jumpers. When they come in they have usually stalled or gone backwards in their jumps, have at least some degree of knee pain/issues, and have been using typical high jump/long jump strength training protocals for several years. The long leg/short torso build that is the majority in this case, take alot of stress off the knees with the lowered bar position, and get much more hamstring activity with it as well, even with a fair degree of knee travel (note that im referring to shoulder width, bar across upper rear delts, knees tracking over feet, stance).
Yes, if the strength training is NEW to the athlete, and the athlete is not a competitive jumper, or jumping extremely frequently, then the new stimulus of the strength training is going to weigh very heavily on their on court/field/track performance. This is not nearly the same thing as an olympic high jumper, jumping and doing jumping drills/practice, five days a week, and doing 4 sets of 5 high bar half squats twice a week.
The range of motion.... what has worked the best is starting at a larger range of motion, using the eccentric on one leg as well, and building a "base" first. As the season nears the box height increases to a rom more specific to the jump, then transitioning into bands as well as free weight, with a two leg eccentric, explosive single leg concentric on one leg. I can do them this way up to two days before the track meet and get pr;s in jumps. The parallel depth wouldnt be bad at first, you will develop some great single leg strength that way and thats pretty close to the offseason box height I use. I want them to be able to walk the leg out in front a decent ways, but also get a larger rom than they jump out of.
Youre exactly right, I like the bar on shoulders since it loads the posterior region much more than the pistol version, you can move the working leg out in front of the body, and you can get very strong at this exercise, the weights held in hand would cause you to fail at the shoulders or hands before you adequately challenged the jumping musculature. Putting the leg straight out in front of the body as in a pistol squat changes the dynamics of what im after as well, the non working leg will slightly trail the working leg in the slbs, finishing in front with the knee. (I have stacked plates under benches, etc. in commercial gym settings before with this exercise so thats one way around not having a box).
Yea, film the cleans, from the hang. Earlier I may have not done a very thorough job explaining what I meant when I was explaining how to get more hip into the olympic lifts, start by standing straight up, legs almost locked, descend into an rdl position, again, knees should be almost locked. Bar should be about mid thigh, shoulders as far in front of the bar as possible, then simply JUMP. What will happen is the knees will bend when the bar slides up higher on the thigh, but the hips rather than the quads are leading the lift. You sound like youre starting behind the bar, with knees already bent at the beginning of the hang position, this should fix that issue.