When designing mid-air interactions, a common complain between users is the tiredness in the arms after using the system for a while. At the CHI 2014 conference in Toronto, we will presenting our research on how to measure arm fatigue in an non-obtrusive and quantitative manner. Our metric is called consumed endurance, and build’s on Rohmert’s biomechanical studies of endurance time of the upper limbs . Endurance time is defined as the maximum amount of time that a muscle can maintain a contraction level before needing rest. Our metric, called Consumed Endurance (CE), is defined as the ratio between the interaction time and the available endurance time.
In order to calculate CE in real time we used a Microsoft Kinect (the same device that started the mass exploration of mid-air interactions) to track the user arms in real time. Based on the arm positions and established measured of length, weight and maximum forces for the 50th percentile male and female , we calculate the forces involved at each moment.
One of the most important questions we had to answer, was whether CE actually measures arm fatigue. Given the lack of clarity in relation to which EMG metric is the adequate for the shoulder muscles  and its reliability , we opted to validate CE against a widely used and researched subjective metric called Borg CR10 . Our results showed that CE presents a “very strong to perfect association” with CE, which means CE actually does measure arm fatigue (at least in the conditions of our validation).
The following video shows how CE can be used for 1) evaluation mid-air interaction designs and 2) to guide the design of new mid-air user-interfaces.
- Hincapié-Ramos, J.D., Guo, X., Moghadasian P., and Irani, P. 2014. Consumed Endurance: A Metric to Quantify Arm Fatigue of Mid-Air Interactions. In Proc. CHI 2014, May 2014, Toronto, Canada. [pdf (1.0MB)]
- Consumed Endurance (CE) Workbench [download]
 – Rohmert, W. 1960. Ermittlung von erholungspausen fur statische arbeit des menschen. European Journal of Applied Physiology and Occupational Physiology 18. 123–164.
 – Freivalds, A. 2004. Biomechanics of the Upper Limbs: Mechanics, Modeling, and Musculoskeletal Injuries. 1 ED. CRC Press.
 – Troiano, A., Naddeo, F., Sosso, E., Camarota, G., Merletti, R., and Mesin, L. 2008. Assessment of force and fatigue in isometric contractions of the upper trapezius muscle by surface EMG signal and perceived exertion scale. Gait & Posture, 28(2), 179-186.
 – Öberg, T., Sandsjö, L., and Kadefords, R. 1994. Subjective and objective evaluation of shoulder muscle fatigue. Ergonomics, 37(8), 1323-1333.
 – Borg, G. 1998. Borg’s Perceived Exertion and Pain Scales. Human Kinetics.