Yep, you can’t slowly work an inefficient motion up to speed, you have to be making a correct motion in the first place to play at that level.
Stamina shouldn’t really come into it, if your arm is fatiguing then you are probably doing something wrong or it would suggest to me the breakdown of your technique
so what’s the upper limit on down picking speed?
The point of practice is figuring out how to do the technique. Then, subsequently, you figure out how to do it immediately on command, every time, for whatever length of time you need, without getting confused or losing the coordination of it. Like juggling, let’s say. You could call that “stamina” but that makes people thinks it’s about athletic training and beating fatigue. It’s not, it’s coordination and making it automatic.
Here’s a segment of an interview I did where we discuss the downstroke thing specifically. In my case upstrokes were the first “fast” repeated pickstroke I could do because it’s similar to the table tapping tests we do in the Primer:
Why does this downstroke-only technique exist? Some say it sounds different than alternate picking, but do you believe that? 
It does sound different. At least to my ear, it’s not even a question.
100%, there’s a very distinctive flavor and feel with all downstrokes, esp when paired with some palm-muting.
very interesting. I guess I don’t really understand how to go about figuring it out… my table taps aren’t anywhere close to 235, for example. what do you make of Hetfield apparently losing his ability to down pick mop live as he’s getting older? makes me think endurance has to play some sort of role. he’s not exactly an old cripple.
40 years since forming Metallica. Not the model of a healthy life for a lot of it.
59 isn’t ‘old’ - but no one’s physical peak is within 20 or so years of that. Stuff wears out.
Not even the most perfect technique can stop the wear and tear of time.
You must be right, but how does the string “know” if it is being hit on the way down, or the way up? Or is it that the all-downstroke approach hits much harder than usual and really smashes the string, creating a different sound?
The string doesn’t know, but your pick does! It’s the difference in timbre between a downstroke and upstroke. The different sides of the pick hitting the string creates an alternating change in timbre. All downs, or ups, mean you get the same timbre, giving that machine gun like sound.
What many get wrong with the spider riff is learning it as straight 8ths and trying to ramp that picking motion up to speed. Sure recipe for failure. The 2 notes on the E and the single note on the A should be learned and played similar to a drum drag/ruff, not straight 8ths. This will work miracles for anyone trying to learn this. You will find it’s a different kind of consolidated motion. Troy’s answer is absolutely correct.
The other thing that has to have an effect is the velocity of the pick.
All downstrokes at a given tempo means it’s traveling at twice the speed of alternate picking as it passes through the string.
I notice this for sure, but there is a way you can get just the right angle where both strokes sound identical, on single notes, that is (this may be at odds with other factors though). I have pretty sensitive ears for this from my classical playing as we strive really hard to ensure each finger makes the exact same tone. So I am confident I am really hearing the difference, just like I’m pretty confident when I’ve attempted to get the tone consistent on up/down strokes, I’m really accomplishing that.
I’ve also got a lot of experience in metal playing, and I was definitely of the opinion that down strokes should be used whenever possible, just like Hetfield. I’d particularly notice differences if the rhythm part had some power chords (which almost always happens at some point in the phrase in that genre). Even if the tempo is fast, when more than one note is present in the part, upstroked chords will sound brighter since the higher note sounds first.
I think this is definitely part of it. Also, if rest strokes are being used, they have a more full sound by default too.
Anecdotally, I’ve noticed the players who do stringhopping can get a similar machine-gun like sound on every pickstroke, even though technically stringhopping is alternate picking — just not efficient alternate picking. So I think the idea that downstrokes and upstrokes have to sound different just because the pick is moving in different directions isn’t true. The machine gun is coming from some other aspect, like the similarity of the motion path or something.
It won’t, but it might know if it’s being hit with a trapped motion or an escaped motion. There is apparently some physics behind the idea of a trapped pickstroke having more bass response. I’m not familiar with why specifically. But you can sort of hear this on an acoustic guitar if you do a pronounced trapped pickstroke that rests. Note that whether it’s an upstroke or downstroke doesn’t matter. What matters is that the pick moves toward the guitar body and plays the string.
To my ears the effect is subtle and not what Hetfield is talking about when he talks about all downstrokes sounding more aggressive on a high gain amp with muting. I think he’s talking about the similarity of the pick attack, which, again, can be achieved with alternate picking if you make very similar motions in both directions, as stringhoppers often do.
These are good points. I can actually get a pretty similar sound if I do an exaggerated up and down Stringhopping motion. So it’s something in how the notes are being played and not whether it’s an up or downstroke then. Specifically what… I’m not sure
Do you think it has anything to do with having a free floating wrist and not planting the wrist with the entire arm on the body that might cause an escape path for the sound vibrations? This is something I only recently considered.
Truthfully I think all-downstrokes are phenomenally overrated just because they’re difficult if you don’t happen across the most efficient way of doing them. Just as with heavy strings it must sound better if it’s more difficult, right? IMO downstrokes became popular in the first place because they make things simpler, not because they sound better. This first became apparent to me watching a King’s X vid; the amazing Ty Tabor used alternate picking for the riff, and like everyone else, I never noticed.
I would disagree, as would most metal players I think. Actually, gypsy jazz is another genre that heavily emphasizes the use of downstrokes because of the noticeable difference in attack.
Just wanted to say I love King’s X and Ty!
They do double downs sure, but some don’t. Most of the newer players are choosing upstroke escape. But I would be willing to bet when they are bedroom playing, honing technique, they are doing it, it’s probably just not there to utilize when playing live. There is one ascended diminished 2 note per string phrase I do double down with a sweep string change as it just feels nice under my hands. Could I do it with alternate picking sure, I just like to play it double down. Is one harder than the other? No. Is one faster than the other I am sure if I really try to play it as fast as possible alternate picking would win, maybe not though, but to my hands I could probably flow it better with double downs. And it could really depend on how much endurance you have in the tank if you are mid solo, you might choose to alternate pick the phrase to conserve power for a later phrase.
If that’s the case, why would that be? To keep up the image of tradition, or because there’s a tangible difference in attack?
Back in the realm of metal (which I’m more comfortable with), the argument against downstrokes is also pretty much moot when you consider playing fast, successive chords comprised of two adjacent strings, which many modern metal players use. That just sounds strange alternate picked, not to mention awkward.