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I totally agree with you. You can play at any bodyweight (PAES) lol. But in my experience the body takes a far greater beating playing at a high bodyweight. Joints, ankles and knees especially take a huge pounding. Knees start to ache, ankles start to break. Spine gets shaken up too. Risk of injuries goes way up. Last summer when i was a lightweight (@165lb) i was pretty much unfit and yet i could play a full court game without much loss of performance. Actually that's not fully true, i had to pick and choose whether to play good D or O but the point is i could just depend on my low bodyweight to make up for my lack of fitness. But that's not something I want for this summer. I need to be properly fit -- regardless of playign bodyweight. Ideally i'll have low bodyfat and superb fitness, regardless of my eventual bodyweight whether it is 75kg or 85kg -- which one it is remains to be seen.

Maybe you have never been really lean... but all those symptoms you mention (and more) come right back when you get really really lean.  Leaning down to under 5% makes me feel like I have a cold and everything starts to hurt.  You will REALLY miss the lack of fat when you land on your tailbone on the basketball court.   You might jump higher; but for a contact sport I would be cautious about cutting too much fat.  Especially preseason.  It will go away during season a bit anyway.   You have added appreciable muscle (hopefully) since the last time you cut to 75kg....  Trying to get your weight down close to there again might result in some levels of leanness that you just don't want.   

Paper came out in nature a bit ago saying it was carcinogenic.  I would avoid it.

Lol, this thread is really silly.  Personally I really hate when the words timed electronically are used to describe something other than FAT timing.  It REALLY leads to confusion.  The 40yd dash is timed electronically, but it isn't FAT.  Now in bobsledding we have this strange standing past the line weird timing.... Since the base it on the footstrike do they end the clock when the foot lands past 60m or the torso crosses?  Wish we could stick to hand-times and electronic (FAT) times.  Would make things simpler. 

Saying they are the fastest requires you to describe what speed is.  200m athletes have the fastest average velocity.   Distance runners certainly are fast over the distances they cover.   In track we call the 100m winner the world fastest because he usually they invariably always hit the highest instantaneously velocity.    Bolt is tops at 12.3 m/s (or a 0.82 10m split).   That's what should be compared when we talk about fastest. 

By FAT reaction time measures the initial reaction (first twitch), this "electronic time" seems to start when your foot leaves the ground.  Way different.  One of the world-record beaters on that list (6.36) is Johnny Quinn who I've run against and he is a 10.6-10.9 guy.   I promise if you put a world class sprinter in this test they would all be sub 6.

wow great response, my physics knowledge is elementary here.

So that's my question, how are the forces involved in  a loaded depth jump different than an unloaded depth jump when the GRFs are the same?

GRF 100kg athlete dropping from 1m GRF who takes 0.15s to decelerate from 4.4m/s =
440/0.15 + 981 = 3914N

So the exact mass the athlete would need to add to produce the same GRF from only a 0.5m drop:
v = 3.1m/s

rearranging grf= mv/t +ma
grf/(v/t +a) = m

you get 27.5kg the athlete would have to add. But my question is, would the time it takes for the athlete to decelerate be the same?

Then furthermore as you mentioned, the time it takes to produce force by the athlete would certainly be less, increasing ground contact time, but not necessarily deceleration time, the time used to measure the GRF.

Good question.  First I need to remind you of one caveat.  We are calculating average GRF.  During the deceleration than instantaneous GRF increases and peaks and decreases.  You can measure the maximum amplitude using a force plate (and maximum might be most important for injuries).  You can imagine cases where you have the same GRF but different curves (eg you fall with almost straight knees and then bend vs a smoother landing where you bend nicely on impact) BUT I would bet for the most part athletes land in the same manner most of the time and average GRF is a pretty good substitute for our cases (could be wrong though - especially when you considering loaded landings may change peak force even if deceleration time is the same). 

So that partially answers your first question... even if the time to decelerate is the same MAYBE peak GRF is higher.  However... is time for athlete to decelerate the same?   Good question.  I don't really know.  For a depth drop maybe; you could make the case that altitude landings could be substituted with lower weighted landings (this might not be true but I haven't seen proof to contrary).   However, for a depth jump I would would argue that the time for athlete to decelerate would not be the same.   I would argue that you should think about two extremes - one is the ultimate reactive athlete who has springs (those perfect undamped springs you learn about in engineering courses) on his shoes and the other is the athlete who is completely inelastic.   The completely inelastic athlete lands and does not store any of landing force then initiates a jump.  His jump is the same as his standing vertical jump.  His deceleration time will be the same whether you add weight or height but his acceleration time will be longer (and less) if he is wearing load.   Essentially his deceleration and acceleration time are independent.   The athlete with springs is exactly the opposite; his landing is a completely elastic collision.   His deceleration time will equal his acceleration time.   He lands and his springs compress and store the force of his landing and then they decompress and return the force so he goes right up to the height he fell from (remember - frictionless).

Now, obviously you are not either of those examples.  You are somewhere in the middle.  You don't just want to be able to return energy; you want to be able to create it.  But you also want to use elasticity as much as possible so your jump is forceCreated + forceReturned = great jump.    If you have ever been trampolining you should know the feeling.  If you try to jump really high on the trampoline the experts will laugh at you are tell you not to "muscle it".  But in reality they are muscling it... The trampoline doesn't create energy it just returns it.  So when they go much higher than you they are using muscle to create force.... they skill is just to maximize the forceReturned part of the equation while also adding some of your own...

Given that I think you can see that if the goal of depth jumps is to practice at improving the forceReturned part of the equation we don't want a heavy load added.  To maximize force returned deceleration == acceleration because it is a return of force.  In reality we are using tendons to store force AND muscle to add force (because we aren't truly elastic) but we want to train ourselves to be elastic as much as possible; so any addition of weight which makes our acceleration longer would not be beneficial.  Now.... You could argue that weight is fine as long as you spend equal time decelerating and acceleration you are still training an elastic return and the elastic structures will improve and you will get better as using them...... However, I would imagine the elastic structures of a human have a pretty short time that they can store force and if you are too far outside of this you won't get much training benefit. 

edit: then another question is, is the GRF produced even important to consider in training for jumps?
How do quick depth jumps versus maximal height depth jumps differ in their GRFs?
I would think that quick depth jumps that emphasize minimal ground contact time would produce greater GRFs upon landing than depth jumps form the same height involving jumping as high as possible, but then how would the muscle-tendon unit be trained differently form those two types of jumps? Would there be any benefit to using a speed depth jump when it results in shorter jump height?

This is really the million dollar question isn't it?  The answer is I don't really know.  Physics is rock solid.  I can tell you what's right and what's wrong.  But physiology really isn't.  Human vary and what works and why it works isn't always so obvious.  That's why broscience creeps into training methodology so often.    We do know that humans are elastic.   On a basic level the GRF produced is VERY important to consider in training because GRF is what causes injury!  But as far as whether or not it causes the muscle/tendon to adapt and make one jump higher?  I don't really know. 

The difference with quick depth jumps vs. maximal height depth jumps (assuming the drop is the same) is that quick depth jumps may have greater GRF than maximal height but max-height never have greater GRF than quick depth jumps.

I would argue that if we believe depth jumps are important than the is certainly a benefit to using a minimal GCT depth jump even if it results in shorter height.   Consider that shortest GCT depth jump you can imagine.  Drop off the box, keep the knees straight and land and jump.  GRF will be high (a lot will be absorbed by your knees - hopefully you don't get injured) but some force will be stored in the muscle/tendon of your ankle.  The elasticity of your tendon will be trained.   Think of it as isolation for your ankle elasticity.  If this is a weakness of an athlete then this is beneficial.   In general the goal is to get the athlete to LEARN to be reactive so his total jump height will be better when he adds reactivity and power.   To force the athlete to do this we give him a goal (jump high) but keep GCT short so he he has to rely more on reactivity than on creating power.   Now, whether this LEARNING is just neural coordination or actual adaptation to the tendon or not... I don't know.   Whether this can actually be learned to much of a degree (would that athlete not have already figured out how to do it from just trying to jump high without a depth drop?)... I don't know....  But the training of short GCT does isolate the elastic ability of the athlete. 

One error I saw in the document I posted is the experiment they did with loaded depth jumps. They concluded that since athletes did not jump as high with a loaded depth jump it wasn't an effective exercise. Well we know that he ground reaction force will equal the mass of the athlete times gravity. The grf produced from a 3.5 drop landing can be matched with a loaded landing from a lesser drop in an infinite number of ways.

You are confusing absolute force with relative ground reaction force.   Read what you wrote.  If GRF = mass * gravity.... Then ground reaction force is the same no matter what because mass and gravity are constants....   When you are standing still (or on a scale) the normal force (GRF) is indeed mass * gravity, or your weight in pounds.   

The GRF we are interested in is the relative force during the landing (ie when you decelerate from x m/s to 0 m/s - after which GRF = ma again).   A force plate can approximate the instantaneous relative force during the landing but this of course depends on the manner of landing.   To calculate the average GRF during the landing you can use:    averageGRF = (mass * v )/ t  +  f  where v is landing speed and t is landing time.   

Now I'm rusty in physics but we do know that  distance = (1/2) * g * (t^2)   [ one half  A T squared ]  and that velocity = gt, so we can rearrange and get this formula :   V = sqrt ( 2 * d * g ) 

So take a 100 kg person and have them drop from 0.5 meters.    The velocity is 3.1 m/s.  If they drop from 1 meter it is 4.4 m/s.

So, the ground reaction force for the first and second case is 310/ t  + 980   and 440 / t + 980. 

Let's assume our athlete is quick and can jump up again in 0.3 seconds so and about half of that time is spent decelerating to zero, so a landing time of 0.15 seconds.

So our average GRF for each height is:  310/0.15 + 980 and 440/0.15 + 980 or 3046 N and 4109 N.   So the average ground reaction force is about 3 times bodyweight or 4 times bodyweight from the two heights if ground contact time is unchanged.

Now assume you only have a 0.5 meter box and you want to get the same GRF as the 1 meter box, so you strap weight onto your subject.

How much mass do we need to add?

I won't bore you solving it but rather say that:    ((100+35kg) * 3.1) / 0.15  + 135*9.8 = 4113 Newtons. 

So we have to add approximately 35 kilograms to our athlete to recreate the average GRF of a 1 meter drop with a 0.5 meter drop.   This is problematic because it might be hard to find a place to sufficiently load an athlete with 77 pounds where he can still have freedom of his limbs and movement AND because the athlete will almost surely not be able to jump up again in 0.15 seconds with 77 pounds of added weight. 

Now of course you can add more weight to compensate for the longer ground contacts but I believe the author is making the point that if ground contacts are much much longer the muscle will not be trained in the same weight.  Certainly could be so truth to that.   One interesting thing to do with these equations is play with force (gravity).  If you assume a nice band setup where you can manipulate force so you actually have gravity + bands you can probably maximize the training of the athlete to absorb force while maintaining small GCT.   

some n=1 personal advice: when we talk about "athleticism," in most of our cases we mean jumping and/or playing basketball or some other sport. in other words, a specific skill set that degrades with neglect. KF got good at SVJ by doing SVJ. sprinting 200m will help keep you in shape and build some of the physical characteristics that make for good jumping, but it will not really help you hold on to your basketball-specific jumping ability. this has been my stupidly obvious lesson of the past year: nothing can replace skill practice. so my advice is: sure, sprint a few times a week. hell, if nothing else it's just flat-out fun to do. but don't limit yourself to one day a week of jumping. i'm trying to practice jumping at least three days a week and even that's not enough, but it's what i can manage at the moment. those workouts don't have to be draining, either. 15-20 jumps doesn't exactly take a lot out of you.

For the most part I agree with this advice; but there is one caveat that's important to remember when it comes to sprinting.  Obviously the old concept of "base-work" mileage is pretty silly.  However, I think a lot of athletes/coaches go a little too far the other way in designing their sprint programs.  Sprinting any distance fast will have great benefits to body composition, general fitness, etc.   But if you want the benefits of sprinting to carry over to a different activity the most direct carryover is likely sprinting -> single leg jumping.   When going for the carryover most will follow the line of thinking that the effective length of the basketball court is about 20 meters (the farthest you run most of the time) and the approach of a single leg jump is a lot less.... so therefore it makes sense to practice a lot of 10m and 20m sprints.   This sounds correct but it is wrong.  Practicing non-resisted 10m and 20m sprints doesn't have much use (if your already an athlete) IMO.  The greatest gains will be made as you learn better acceleration technique and stride patterning (in other works learn how to run much differently than you do in basketball/jumping).   

The way I think of it is you are capable of taking off on a single leg at ~ 90-95% of your max v.  Any faster and you simply can't jump.  Same goes for more sports specific skills... ie catching a football, finishing a fast break.    This is why I think athletes will reap better carryover if they work on running fast rather than short sprints.  Max velocity work is almost most foreign to basketball players and as such you will be able to get gains faster.   Now this doesn't mean 200m necessarily.  But certainly longer runs 40-100m and flying runs would provide more carryover than they are given credit.  If you want anecdotal evidence look at Carl Lewis sprinting and jumping.   Poor acceleration and always last to 50m but incredible pickup and max velocity.  Also the second best long jumper of all time.   In fact jumpers are often serviceable members of 4x100m teams despite being poor in the open 100m.... their weakness is their acceleration NOT their max velocity. 

Are you sure you're not losing power landing on that mat? You bend quite a bit at the knee... and you're not really dorsiflexing too much (you're still pointing your toes down) but to be honest, I have no idea how much is enough and how much is not.

I'll try to film my depth jumps the next time I do them and compare them. I'll do it from 2 boxes as well.

I don't really get depth jumps, what is the advantage over consecutive jumps? 

Also, is the point to go as high as possible?  If so don't you want a higher box?  I tried these one time and it seems like the higher the box the higher you can jump right until the box height is a bit higher than you can jump....  is that generally true?