title: Grasp and index finger reach zone during one-handed smartphone rear interaction: effects of task type, phone width and hand length.
abstract: Recently, some smartphones have introduced index finger interaction functions on the rear surface. The current study investigated the effects of task type, phone width, and hand length on grasp, index finger reach zone, discomfort, and muscle activation during such interaction. We considered five interaction tasks (neutral, comfortable, maximum, vertical, and horizontal strokes), two device widths (60 and 90 mm) and three hand lengths. Horizontal (vertical) strokes deviated from the horizontal axis in the range from -10.8° to -13.5° (81.6-88.4°). Maximum strokes appeared to be excessive as these caused 43.8% greater discomfort than did neutral strokes. The 90-mm width also appeared to be excessive as it resulted in 12.3% increased discomfort relative to the 60-mm width. The small-hand group reported 11.9-18.2% higher discomfort ratings, and the percent maximum voluntary exertion of their flexor digitorum superficialis muscle, pertaining to index finger flexion, was also 6.4% higher. These findings should be considered to make smartphone rear interaction more comfortable. Practitioner Summary: Among neutral, comfortable, maximum, horizontal, and vertical index finger strokes on smartphone rear surfaces, maximum vs. neutral strokes caused 43.8% greater discomfort. Horizontal (vertical) strokes deviated from the horizontal (vertical) axis. Discomfort increased by 12.3% with 90-mm- vs. 60-mm-wide devices. Rear interaction regions of five commercialised smartphones should be lowered 20 to 30 mm for more comfortable rear interaction.
authors.affiliation: a Department of Human and Systems Engineering , UNIST , Ulsan , Republic of Korea.
authors.forename: Songil
authors.initials: S
authors.lastname: Lee
authors.affiliation: a Department of Human and Systems Engineering , UNIST , Ulsan , Republic of Korea.
authors.forename: Gyouhyung
authors.initials: G
authors.lastname: Kyung
authors.affiliation: b Ergonomics Team , Hyundai Motor Company , Gyeonggi-Do , Republic of Korea.
authors.forename: Jungyong
authors.initials: J
authors.lastname: Lee
authors.affiliation: c School of Mechanical and Aerospace Engineering , Nanyang Technological University , Singapore.
authors.forename: Seung Ki
authors.initials: SK
authors.lastname: Moon
authors.affiliation: d Department of Business Administration , Gwangju University , Gwangju , Republic of Korea.
authors.forename: Kyoung Jong
authors.initials: KJ
authors.lastname: Park
mesh_terms: D001696:Biomechanical Phenomena; D005260:Female; D005385:Fingers; D006225:Hand; D018737:Hand Strength; D006801:Humans; D008297:Male; D018482:Muscle, Skeletal; D016059:Range of Motion, Articular; D000068997:Smartphone; D055815:Young Adult
keywords: %MVE; One-handed smartphone interaction; index finger interaction; rear interaction