2 Added further references to support the claim of spinal hydration being an injury risk factor
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According to StrongLifts.com, the theory behind not working out within the first hour of waking is actually the opposite to the one you mention - it's because the spine is too well hydrated and heavy compression (e.g. from weight lifting) or bending movements could cause a disk to rupture.

Bottom line is that you should never do any kind of exercise during the first hour after waking up. Here’s why: when you’re lying horizontally for hours, like when you’re sleeping, your back has no compressive loading. As a result, water fills back into your discs to nourish them. That’s the nightly hydration.

Now imagine your discs are water balloons. If they’re half full and you push one side, the water will move around and return when you let go. But if they’re full of water, they can burst when you push. Well this is exactly what can happen to your spine if you do any kind of heavy bending first thing in the morning.

The good news as Dr Stuart McGill explained in his book "Low Back Disorders" is that after the first hour of being up, your spine dehydrates by about 90% of what it will for that day (this is why you’re taller in the morning). So the risk of lower back injuries plummets after that first hour.

Apparently it takes about an hour or so for the fluid to drain out once you get up, hence the suggestion for waiting an hour before exercising. However a good warm up can supposedly help speed up this process. Dr Stuart McGill's claim does seem to be backed up by at least one study on PubMed.

Personally, I lift weights most mornings within about 45 mins of waking. I normally spend some time stretching my back on the foam roller first and always do a good warm-up (e.g. dynamic stretches or light weight versions of the exercises I will be performing). Obviously it's important to listen to your body and if something doesn't feel right then stop, but I haven't had any problems so far (and I actually broke my back a few years ago so I'm quite cautious). YMMV.

According to StrongLifts.com, the theory behind not working out within the first hour of waking is actually the opposite to the one you mention - it's because the spine is too well hydrated and heavy compression (e.g. from weight lifting) or bending movements could cause a disk to rupture.

Bottom line is that you should never do any kind of exercise during the first hour after waking up. Here’s why: when you’re lying horizontally for hours, like when you’re sleeping, your back has no compressive loading. As a result, water fills back into your discs to nourish them. That’s the nightly hydration.

Now imagine your discs are water balloons. If they’re half full and you push one side, the water will move around and return when you let go. But if they’re full of water, they can burst when you push. Well this is exactly what can happen to your spine if you do any kind of heavy bending first thing in the morning.

Apparently it takes about an hour or so for the fluid to drain out once you get up, hence the suggestion for waiting an hour before exercising. However a good warm up can supposedly help speed up this process.

Personally, I lift weights most mornings within about 45 mins of waking. I normally spend some time stretching my back on the foam roller first and always do a good warm-up (e.g. dynamic stretches or light weight versions of the exercises I will be performing). Obviously it's important to listen to your body and if something doesn't feel right then stop, but I haven't had any problems so far (and I actually broke my back a few years ago so I'm quite cautious). YMMV.

According to StrongLifts.com, the theory behind not working out within the first hour of waking is actually the opposite to the one you mention - it's because the spine is too well hydrated and heavy compression (e.g. from weight lifting) or bending movements could cause a disk to rupture.

Bottom line is that you should never do any kind of exercise during the first hour after waking up. Here’s why: when you’re lying horizontally for hours, like when you’re sleeping, your back has no compressive loading. As a result, water fills back into your discs to nourish them. That’s the nightly hydration.

Now imagine your discs are water balloons. If they’re half full and you push one side, the water will move around and return when you let go. But if they’re full of water, they can burst when you push. Well this is exactly what can happen to your spine if you do any kind of heavy bending first thing in the morning.

The good news as Dr Stuart McGill explained in his book "Low Back Disorders" is that after the first hour of being up, your spine dehydrates by about 90% of what it will for that day (this is why you’re taller in the morning). So the risk of lower back injuries plummets after that first hour.

Apparently it takes about an hour or so for the fluid to drain out once you get up, hence the suggestion for waiting an hour before exercising. However a good warm up can supposedly help speed up this process. Dr Stuart McGill's claim does seem to be backed up by at least one study on PubMed.

Personally, I lift weights most mornings within about 45 mins of waking. I normally spend some time stretching my back on the foam roller first and always do a good warm-up (e.g. dynamic stretches or light weight versions of the exercises I will be performing). Obviously it's important to listen to your body and if something doesn't feel right then stop, but I haven't had any problems so far (and I actually broke my back a few years ago so I'm quite cautious). YMMV.

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source | link

According to StrongLifts.com, the theory behind not working out within the first hour of waking is actually the opposite to the one you mention - it's because the spine is too well hydrated and heavy compression (e.g. from weight lifting) or bending movements could cause a disk to rupture.

Bottom line is that you should never do any kind of exercise during the first hour after waking up. Here’s why: when you’re lying horizontally for hours, like when you’re sleeping, your back has no compressive loading. As a result, water fills back into your discs to nourish them. That’s the nightly hydration.

Now imagine your discs are water balloons. If they’re half full and you push one side, the water will move around and return when you let go. But if they’re full of water, they can burst when you push. Well this is exactly what can happen to your spine if you do any kind of heavy bending first thing in the morning.

Apparently it takes about an hour or so for the fluid to drain out once you get up, hence the suggestion for waiting an hour before exercising. However a good warm up can supposedly help speed up this process.

Personally, I lift weights most mornings within about 45 mins of waking. I normally spend some time stretching my back on the foam roller first and always do a good warm-up (e.g. dynamic stretches or light weight versions of the exercises I will be performing). Obviously it's important to listen to your body and if something doesn't feel right then stop, but I haven't had any problems so far (and I actually broke my back a few years ago so I'm quite cautious). YMMV.