In the field of ergonomic hardware design, the conversation often centers on "posture"—specifically, the angle of the wrist. However, the biomechanics of movement is often more clinically relevant than static posture.
Our use of keyboards and mice ultimately involves the tapping, pressing, or pinching motions of our fingertips. If the posture is "neutral" but the movement requires excessive force, the ergonomic benefit is lost.
Following our previous review of clinical evidence regarding vertical mice, we turn our attention to the mechanics of force. This report reviews the seminal 1998 study "Fingertip Loading and Carpal Tunnel Pressure: Differences between a Pinching and a Pressing Task" by Bach, Keir, and Rempel.
This study is particularly relevant to the "Vertical vs. Standard" mouse debate because it isolates the specific biomechanical difference between the two devices: the downward "Press" versus the lateral "Pinch."
Source: CDC Stacks / Journal of Orthopaedic Research
Study Methodology: Defining the "Neutral" Baseline
To accurately measure the impact of movement, the researchers first established a baseline posture. Notably, this baseline closely mirrors the geometry of modern vertical mice.
"Initially, the forearm and wrist posture associated with the lowest carpal tunnel pressure was determined for each subject. The subsequent tasks started from this posture, which was approximately a neutral position of the wrist with 45° of pronation. The subjects then performed two tasks: pressing a force transducer with the index finger and pinching the transducer between the index finger and thumb."
Plain Language Summary
The researchers did not test extreme or awkward angles. They situated the subjects' arms at 45° of pronation—a "handshake" angle that is widely considered ergonomic. Crucially, even within this "ideal" angle, the subjects were asked to perform two different actions:
- Pressing: Applying force downward (similar to a standard mouse click).
- Pinching: Applying force by squeezing the thumb and index finger together (similar to the stabilization grip required by a vertical mouse).
Results: The "Double Pressure" Phenomenon
The study measured Intracarpal Canal Pressure (ICCP) to determine how the median nerve responded to these two different tasks. The data revealed a significant divergence between pressing and pinching.
According to the original text:
"Mean pressure at the posture of lowest pressure (resting carpal tunnel pressure) was 5.5 ± 0.9 mm Hg... When the postures of press and pinch (i.e., unloaded) were assumed, the pressure rose to 7.8 mm Hg... and 14.2 ± 2.2 mm Hg... respectively."
"Carpal tunnel pressure increased with fingertip load (p = 0.0001). The pressures for a pinch grip were about twice as great as those found for finger pressing."
Data Analysis
The findings highlight a clear physiological cost to the "pinch" mechanic used in vertical mice:
- Postural Cost: Even before applying any force (unloaded), merely shaping the hand into a "pinch" posture raised nerve pressure to 14.2 mmHg, compared to just 7.8 mmHg for the "press" posture.
- Activity Cost: Once force was applied, the pressure generated by pinching was approximately 200% higher (twice as great) as the pressure generated by pressing, even when the external force was identical.
Discussion: Clinical Implications of Grip Style
The authors of the study concluded that the type of grip used is a major predictor of carpal tunnel pressure, potentially overriding the benefits of the wrist angle.
"On the basis of carpal tunnel pressure, pinching presents a greater probability of venous occlusion in the carpal tunnel than does finger pressing. Relatively low levels of fingertip force elevate pressures in the carpal tunnel. These findings have implications for the design of work tasks and tools."
What is Venous Occlusion?
"Venous occlusion" refers to the restriction or blockage of blood flow out of the nerve canal. The authors conclude that the "pinch" mechanic—even at low force levels—creates a much higher risk of this blood flow restriction than a standard "press."
Conclusion
This literature review suggests that ergonomic evaluation must look beyond simple static "postures".
While the "handshake" position (45° pronation) is theoretically sound for skeletal alignment, the data from Bach et al. (1998) indicates that if this position requires a pinch grip to maintain, the internal pressure on the median nerve may double.
For users with existing Carpal Tunnel Syndrome, the clinical goal should be to achieve a neutral angle without introducing the static load of a pinch grip.
References
Bach, J. M., Keir, P. J., & Rempel, D. M. (1998). "Fingertip loading and carpal tunnel pressure: differences between a pinching and a pressing task." Journal of Orthopaedic Research, 16(1), 112–115.
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