What on earth is this study?

The researchers for (take a breath)  “Guidelines for Wrist Posture Based on Carpal Tunnel Pressure Thresholds” were seeking to create posture recommendations for the “design of tools and hand-intensive tasks.” These guidelines are limited to the scope of pressure caused by wrist positioning, rather than non-pressure causes of CTS.

With so much of our lives spent at our keyboards, the position of our wrists while we type is worth being aware of.  Keyboard ergonomics focuses primarily on how our keyboards can serve us, instead of us working to adapt our bodies to our keyboard. So we’re in good company with the journal of Human Factors. 

Method and Findings?

Carpal tunnel can develop from other sources beyond certain pressure thresholds being exceeded, but this study is focused only on pressure-induced CTS. Safe levels of pressure are under 10 mmHg. On the flip side, some “studies support a pressure threshold for nerve injury at or just below 30 mmHg.” These pressures were measured by inserting a catheter into the carpal tunnel.

One outstanding chart from the study showed the dramatic increase in pressure from extension over flexion.

Two illustrations, one of flexion: the hand bent downward fingers pointed at the ground, and one of extension, the hand pointed up.
(Just for reference, the terms are close and easily mistaken!)
It's... a banana?!

I mean, look at that spike in the curve! The pressure range in our carpal tunnel is intense at -50 degrees extension. Even more so when we compare it to the other side. Traditional row stagger keyboards explicitly put our wrists in extension, which seems egregious knowing we’ve had this data on the books since 2007. Almost 20 years!

With some participants not reaching the pressure threshold of 30 mmHg, it seems to suggest that individual variation is important. Take a look:

“A relatively normal distribution of wrist extension angles is shown in the left panels of Figure 3, with the exception of 9 individuals who did not reach the 30-mmHg pressure level…”

It’s great that we’re getting more detailed on identifying pressure thresholds that cause carpal tunnel, even if it’s not the entire population. Seeing the nuance here is valuable data!

Actionable steps

The cumulative studies coming together and then being further supported by this new study is a great leap forward in hand ergonomics. However, don’t worry too much if you use some of these hand positions and you haven’t seen any negative impact. 

But let’s hear from them which postures “prevent mean CTP from exceeding 30 mmHg in 75% of the population.”

“...wrist extension should not exceed 32.7°, flexion should not exceed 48.6°, and ulnar and radial deviation should not exceed 14.5° and 21.8°, respectively. These wrist angles may be interpreted as threshold limit values for wrist angles based on CTP.”
3 illustrations: one of radial deviation, where the hand is bent toward the thumb away from center, a second drawing of ulnar deviation where the hand is bent toward the pinky away from center, and the third drawing is the anatomy of the arm with the radius, ulna and humerus identified. The radius bone is connected to the thumb, and the ulna is connected to the pinky side of the hand. The humerus connects at the elbow to the radius and ulna.
Protip: the thumb is on the radius side, the ulna is on the pinky side.

Individual variation is something that definitely matters, and 75% of the population of the participants isn’t all of them.

Based on the data gathered from animal studies, having 30 mmHg pressure for just 2 hours caused swelling and other harm inside the nerves. The presumption here being that something similar could happen to human nerves as well under a long stretch of time.

If tinkering with your split keyboard setup has you intrigued, it’s worth a trial period for yourself to see how your hands feel when you change things up. And now, you have more specific guidance than avoiding a ‘nonneutral’ position. 

If the flexibility a split keyboard has brought you feels overwhelming and you don’t know where to start, get out your old grade school protractor to experiment for yourself with these guidelines as a helpful starting point.

The Nyquist LM

Read the paper here: https://www.researchgate.net/publication/6490035_Guidelines_for_Wrist_Posture_Based_on_Carpal_Tunnel_Pressure_Thresholds