The Best Knee Ascenders for Aborists: HAAS Velox vs. SAKA vs. DIY

Ready, set, blast off!

So, you upgraded your tree-climbing ascent with a foot ascender, but for some reason your leg and upper body are still gassing out fast, and you're thinking, “I thought this gear was supposed to make life easier?!” Good news: you're extremely close to perfecting your ascent, and it sounds like the only thing missing is a knee ascender. In this guide, we run you through three different knee ascenders that will take your ascending method from good to exceptional.

What ascent technique came before the knee ascender?

Before knee ascenders caught on, most climbers relied on the single-leg pump or the frog walker technique.

The frog walker method uses a foot ascender, a jumar (better known as a hand ascender), and a foot loop tethered to the bottom of the hand ascender that attaches to the climber's boot, opposite the foot ascender. It beats the single-leg pump by bringing the other leg into play, but it still demands a fair bit of upper-body strength, especially when you're trying to haul ass up the rope. These days, you won't see many climbing arborists still running this technique.

What is a knee ascender?

In case you haven't come across one, a knee ascender is a chest ascender rigged with bungee so it “floats” at knee level and climbs the rope right along with you.

Start with the main component, a chest ascender. Traditionally, a chest ascender integrates into a chest harness or mounts directly onto a full-body harness, and its main job is to hold the climber vertical against the rope so you burn less energy on a long ascent. That works in the world of rope access, where climbers run two ascenders tethered back to the harness, but in arboriculture it doesn't translate. The reason: a fixed chest ascender interferes with how an arborist's mechanical device or hitch-based climbing system is meant to function.

So instead, a bungee is attached to the top, bottom, or both attachment points of the chest ascender. The bottom bungee connects to the boot via a foot loop or carabiner, and the bungee running above the ascender attaches to the chest harness or is adapted to the waist belt. The result floats around knee level, hence the name, knee ascender.

Ascenders… ASSEMBLE Building the Rope Walking System!

Put the foot ascender, knee ascender, and chest harness together, and you've built the rope walking system: arguably the easiest and most widely used ascent technique in tree climbing.

So which knee ascender is best for aborists?

They should all be the same, right? Yes and no. The concept is identical across every knee ascender; what changes are the small details. Here are the three routes most arborists take.

1. Weaver HAAS Velox Knee Ascender (Haul Ass Ascent System)

Invented by Michael Frankhauser and assembled by the folks at Weaver Arborist, the HAAS, short for Haul Ass Ascent System, may have been the first purpose-built commercial floating knee ascender developed specifically for modern arborist rope ascent. Other at-height industries use ascenders too, but the HAAS was built for tree work. The original commercial design used a hollow-braid tether spliced into an adjustable boot loop; that version is no longer available. In its place is the HAAS Velox, which streamlines the tether design that houses the tubing and bungee. The boot loop was upgraded to webbing with a cinching buckle, or you can opt for the HAAS Clip'N Step version. Best part? No major adjustments needed: rip it out of the bag, and it's ready to go.

2. SAKA (Self Advancing Knee Ascender)

The SAKA, Self Advancing Knee Ascender, was developed by Richard Mumford and manufactured through his company, Climbing Innovations. I was never able to get my hands on the original SAKA that everyone raved about, all due to a dispute between Weaver and Climbing Innovations over potential infringement. The legal back-and-forth dragged on for a while, and needless to say, the tree-climbing community was pretty upset with Weaver. The case eventually settled, and the SAKA lives on. It's now offered in two versions: one modeled after the original, and the SAKA Foldable. Both let you adjust bungee tension at the tethered point or route the bungee to the top of the ascender for a modified setup. A removable foot loop comes with the unit, or you can clip it straight to your boot if you're running boots like the SOLIDUR TreeClimb.

3. Build your own DIY knee ascender

This might be the go-to move, especially if you've bought into the DMM Tendon and Petzl Sequoia Plus concept of running bungee and tubing around the waist for fewer attachments at the chest. Start with a chest ascender such as the CAMP Turbochest or the Petzl Croll; any chest ascender you like will work, but those two are the most common in tree care. Building the tether, from the bottom of the ascender to the boot, can be as simple as small-diameter cordage attached to the ascender with a knot, plus a cinching knot for the carabiner at the other end. One heads-up: the knot at the lower attachment point can be bulky (depending on cordage diameter) and cause unwanted drag as the rope travels through the ascender. To clean it up, splice a small-diameter hollow braid to the bottom of the ascender and boom! You've got yourself a sexy setup. This DIY version takes up next to no space and stows easily when it's not in use.

Knee ascenders at a glance

Here's how the three options stack up side by side:

Final Verdict: Choosing the Best Knee Ascender

If you find yourself doing the dirt-bike-kickstart shuffle up the rope, it's time to invest in a knee ascender and save yourself the unnecessary fatigue. No single option is truly “better” than the rest; as I mentioned, they all run on the same concept. What changes are the features: size, adjustability, attachment points, and so on. Pick the one that fits your style and climb with ease!

Frequently asked questions about knee ascenders

Do I really need a knee ascender?

If you're relying on a single foot ascender and still gassing out, a knee ascender is the missing piece. It brings your second leg into the ascent and lets the bungee do the advancing, so you stop hauling yourself up the rope with your arms.

What's the difference between a foot ascender and a knee ascender?

A foot ascender mounts to your boot and grabs the rope as you step. A knee ascender is a bungee-tended chest ascender that floats at knee level and self-advances up the rope. Used together with a chest harness, they create the original rope walking system.

Can I use a knee ascender with a hitch or mechanical climbing system?

Yes, that's exactly why arborists float the ascender on bungee instead of fixing it to the chest. A fixed chest ascender interferes with a hitch-based or mechanical device, so the floating knee-ascender setup keeps your climbing system working the way it should.

HAAS or SAKA — which should I pick?

Both are excellent and work on the same principle. Reach for the HAAS Velox if you want a true plug-and-play, zero-setup system. Reach for the SAKA if you want more adjustability to dial in the bungee tension and stride to your body and climbing style.

What rope size do knee ascenders work with?

Most arborist knee ascenders run on common climbing-rope diameters, roughly 9–13 mm,  but always confirm the range against the manufacturer's specs before you climb.