JLL S400 Treadmill Review: The Bigger, Higher-Spec Folding JLL

The JLL S400 is the larger, better-specified sibling to the hugely popular JLL S300, and it is one of the best-selling folding treadmills on Amazon UK. It is aimed at home users who want more than a basic walking machine: a genuine motorised incline, the biggest running deck in the JLL range, and enough speed for jogging and light running, all in a frame that folds away between sessions. The headline verdict is that the S400 is a well-judged mid-priced treadmill that does the everyday things well, so long as you match its limits (a 120kg user cap, a deck best suited to average-height users, and a motor sized for fitness rather than marathon training) to how you actually plan to train.

What you are paying the step up over the cheaper S300 for is size and incline range. The S400 gives you the longest belt JLL fits to a folding treadmill and a 20-level powered incline, which together make a real difference if you want to walk on a gradient or stretch out into a jog. It is not a running-club machine, but as a do-everything home treadmill for walking, jogging and interval work it is a sensible, dependable choice.

Who it is for

The S400 makes most sense for home users who want a versatile machine for walking, jogging and the occasional run, and who value the powered incline. Regular exercise is one of the best things you can do for your heart, and the British Heart Foundation is clear that staying active is vital for your heart health. A treadmill that lets you train whatever the weather, at your own pace and on an incline, removes a lot of excuses. If you mainly want to walk and want to save money, the smaller JLL S300 or a budget treadmill may be enough. If you are a committed daily runner, look higher up the market.

Build and deck

The S400 feels sturdier than the budget machines it sits above, with a solid frame and a running belt that is noticeably roomier than most folding treadmills. At roughly 135cm long by 45cm wide, the deck is the largest JLL offers, and that extra length is what makes jogging on it comfortable rather than cramped. The 16-point cushioning system takes some of the impact out of each footstrike, which matters if you have cranky knees or plan to rack up the minutes.

The trade-off is that this is still a home-fitness treadmill, not a gym-grade one. The 120kg user cap and the size of the frame mean very heavy or very tall users are better served elsewhere. For an average-height user who wants to walk and jog, though, the deck strikes a good balance between usable space and a footprint that still folds away.

Performance and incline

In use, the S400 does the everyday jobs well. Up to jogging pace the belt runs smoothly and the motor holds its speed without complaint, and the cushioned deck is genuinely more forgiving than the hard belts on the cheapest treadmills. The 16km/h top speed is the sensible ceiling: plenty for brisk walking, jogging and running intervals, but not aimed at fast, sustained running. The motor is rated at 2.5HP continuous with a higher peak figure, which is the honest way to read it. Take the continuous number as the true measure of what it will comfortably sustain.

The standout feature is the 20-level motorised incline. Being able to raise the gradient from the console turns a flat walk into a genuine leg and lung workout, and it is the single biggest reason to choose the S400 over cheaper flat or manual-incline machines. Walking on an incline burns more energy and builds more strength than walking on the flat, which makes it one of the most useful settings on any treadmill.

Programmes, extras and folding

The console keeps things simple with a clear LCD display, 15 preset programmes and 3 custom slots so you can save your own sessions. It is not a flashy touchscreen, and there is no built-in interactive training platform, but the built-in Bluetooth, speakers, USB and AUX let you pipe in music or a podcast, which is all most home users actually want. If you like to follow videos, you will prop a tablet or phone on the console rather than rely on a screen.

Folding is handled well. The deck lifts and locks upright to roughly halve the footprint, and transport wheels let you roll it aside. It is worth being realistic though: at over 60kg this is a heavy machine, so decide where it will live before you build it, and expect assembly to take a while. Several owners note the instructions could be clearer, so patience during the build pays off. For guidance on getting the most from any treadmill once it is set up, see our tips on common treadmill mistakes to avoid.

Value

At its mid-market price the S400 is a fair rather than bargain buy. You are paying for the bigger deck, the powered incline and JLL's reliable warranty and support, all of which justify the step up over the S300 if you want to jog as well as walk. Budget treadmills undercut it, but they usually drop the incline, the deck size or the cushioning to hit their price. If you want a single home treadmill that walks, inclines and jogs without fuss, and you fit within its weight and height limits, the S400 earns its keep. For more options across every budget, see our best treadmills for home use guide, and keep the NHS physical activity guidelines in mind when planning how often to use it.