Soft flasks or hydration bladders: which is best for running?

Running vest, backpack… whatever you call them, they’re an essential part of every trail and ultra runner’s gear. Some packs have bladders – also called reservoirs – that allow you to carry a large amount of liquid on your back. Some have soft flasks stored on the front, but which is best?

Answer: soft flasks. I hate bladders and you should, too. Here’s a few reasons why.

Hard to know how much is left

Most bladders carry two litres. Now imagine the following: you filled up at an aid station a few miles ago and have been taking sips along the way. Your pack is getting lighter, but how much water is left? Have you drank a litre? Maybe more?

Once you start running ultras, you should have a hydration strategy and plan your drinking. With soft flasks, you can easily gauge how much is left and there’s no guess work.

Bladders are hard to fill

One thing I always see while volunteering at races is how long it takes people to fill their water bladders.

You don’t want to be at an aid station for too long; you want to move through it as quickly as possible. With a bladder, you need to take your pack off, remove the seal, fill it up – typically a two-person job – then reverse the process. What a faff!

It’s faster to fill flasks

With soft flasks, your pack stays on you at all times. You can enter an aid station with both flasks ready, fill them up quickly and reinsert them as you’re leaving.

This saves so much time, especially if you’re doing a longer race with several aid stations. If you’re spending an extra couple of minutes fiddling with a bladder at each station, it soon adds up and creates a massive difference to make up for in the race.

Bladders hinder running uphill

A full bladder on your back works against gravity. If you are running up a hill, you want the weight to be at the front so you can lean forward. This keeps your centre of gravity perpendicular to the hill, which is the most efficient way of getting to the top.

A man running up a hill with a hydration pack and two soft flasks
It’s easier to run uphill with weight on the front

It’s easier to lean forward when you have the heaviest components of your vest – namely a couple of flasks – at the front. Whereas a bladder on the back does the opposite and pulls in the wrong direction.

The same is true for descending, too. You want to lean into downhills as much as possible and let gravity do some of the work. Having a load of liquid on your back shifts your weight backwards, increasing braking forces and slowing you down.

Can only carry one type of drink

You can only have one drink in a bladder. With a couple of soft flasks, you can obviously carry two different drinks at the same time.

Water in one flask, an electrolyte or carbohydrate-rich drink in the other, or maybe simply two of the same drink but different flavours for a bit of variety. It’s up to you.

Less strategy with bladders

Perhaps not so relevant here in the UK, but if you’re doing a race in warmer conditions and want to bring your core temperature down, you can pour the contents of your soft flask over your head.

Also, if you’re feeling very competitive and know how far to go until the next aid station, you can pour some liquid away. Racing snakes should be smart about how much water they’re carrying and not lugging unnecessary weight. It’s good to be as light as possible at all times.

Offloading water with a bladder is cumbersome in comparison. That’s if you even know much you’ve got left.

Bladders are a pain to clean

Cleaning a bladder is frustrating, fiddly and time consuming. The trickiest and most annoying bit is the tube, which can only be cleaned properly with a special brush. Even then, you can’t be sure it has been thoroughly washed, and good luck drying it when finished.

With a soft flask, you just unscrew the cap, give it a good scrub and leave it to dry. Simple.

So that’s several reasons why I think hydration bladders suck, especially in a racing scenario. Maybe I am missing something? If you disagree, leave a comment below and persuade me otherwise!

Picture of Dan Cross

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