Justin G. Young

Touch-screen tablet user configurations and case-supported tilt affect head and neck flexion angles.

OBJECTIVE The aim of this study was to determine how head and neck postures vary when using two media tablet (slate) computers in four common user configurations. METHODS Fifteen experienced media tablet users completed a set of simulated tasks with two media tablets in four typical user configurations. The four configurations were: on the lap and held with the users hands, on the lap and in a case, on a table and in a case, and on a table and in a case set at a high angle for watching movies. An infra-red LED marker based motion analysis system measured head/neck postures. RESULTS Head and neck flexion significantly varied across the four configurations and across the two tablets tested. Head and neck flexion angles during tablet use were greater, in general, than angles previously reported for desktop and notebook computing. Postural differences between tablets were driven by case designs, which provided significantly different tilt angles, while postural differences between configurations were driven by gaze and viewing angles. CONCLUSION Head and neck posture during tablet computing can be improved by placing the tablet higher to avoid low gaze angles (i.e. on a table rather than on the lap) and through the use of a case that provides optimal viewing angles.

Wrist and shoulder posture and muscle activity during touch-screen tablet use: Effects of usage configuration, tablet type, and interacting hand

BACKGROUND Due to its rapid growth in popularity, there is an imminent need for ergonomic evaluation of the touch-screen tablet computing form-factor. OBJECTIVE The aim of this study was to assess postures of the shoulders and wrists and their associated muscle activity during touch-screen tablet use. METHODS Fifteen experienced adult tablet users completed a set of simulated software tasks on two media tablets in a total of seven user configurations. Configurations consisted of a combination of a support condition (held with one hand, two hands or in a case), a location (on the lap or table surface), and a software task (web browsing, email, and game). Shoulder postures were measured by using an infra-red LED marker based motion analysis system, wrist postures by electro-goniometry, and shoulder (upper trapezius and anterior deltoid) and forearm (flexor carpi radialis, flexor carp ulnaris, and extensor radialis) muscle activity by surface electromyography. RESULTS Postures and muscle activity for the wrist significantly varied across configurations and between hands, but not across the two tablets tested. Wrist extension was high for all configurations and particularly for the dominant hand when a tablet was placed on the lap (mean=38°). Software tasks involving the virtual keyboard (e-mailing) corresponded to higher wrist extensor muscle activity (50th percentile=9.5% MVC) and wrist flexion/extension acceleration (mean=322°/s2). High levels of wrist radial deviation were observed for the non-dominant hand when it was used to tilt and hold the tablet (mean=13°). Observed differences in posture and muscle activity of the shoulder were driven by tablet location. CONCLUSION Touch-screen tablet users are exposed to extreme wrist postures that are less neutral than other computing technologies and may be at greater risk of developing musculoskeletal symptoms. Tablets should be placed in cases or stands that adjust the tilt of the screen rather than supporting and tilting the tablet with only one hand.

Effects of handle orientation, gloves, handle friction and elbow posture on maximum horizontal pull and push forces.

Biomechanical models were evaluated for effects of handle orientation, handle material, gloves and arm posture on maximal pull/push force. Eight healthy subjects performed maximum pull/push exertions on handles with two different orientations and two different surface materials, using bare hand and two types of glove as well as two arm postures. The empirical data supported the proposed biomechanical models: Pull/push forces for the bare hand on a rubber handle decreased 10% when the handle was parallel to the pull/push direction, compared with when perpendicular to it. For parallel handles, pull/push forces further decreased with decreasing hand–handle friction coefficient (simulated by different handle materials and gloves). Pull force exerted by the bare hand was 29% greater when the elbow was extended than when flexed. Pull force was greater than push force (with bare hand and flexed elbow). The biomechanical models suggest that friction between the hand and handle limits pull/push forces for parallel handles. Elbow strength may be responsible for decreased pull force for the flexed elbow posture and decreased force for pull compared with push in the postures examined. Statement of Relevance: Biomechanical models presented in this paper provide insights for causes of upper extremity strength limitations during pull/push tasks. Findings in this paper can be used directly in the design of workstation and objects to reduce fatigue and risk of musculoskeletal disorders.

Hand-Handhold Coupling: Effect of Handle Shape, Orientation, and Friction on Breakaway Strength

Objective: The aim was to determine the maximum force that can be exerted on an object before it is pulled or slips from the grasp of the hand (“breakaway strength”) for fixed overhead handholds of varying orientation, shape, and friction. Background: Many studies have quantified hand strength by having participants squeeze, pull on, or create torque on an object or handle, but few studies have measured breakaway strength directly. Method: In two experiments, hand strength was measured as both overhead breakaway strength for handholds typical of fixed industrial ladders and as maximum isometric grip strength measured using a common Jamar grip dynamometer. Results: Breakaway strength was greatest for a fixed horizontal cylinder (“high friction”; 668 ± 40 N and 691 ± 132 N for Experiments 1 and 2, respectively), then for a horizontal cylinder that simulated low surface friction (“low friction”; 552 ± 104 N), then for a vertical cylinder (435 ± 27 N), and finally, for a vertical rectangular-shaped rail (337 ± 24 N). Participants are capable of supporting only their own body weight with one hand when grasping the fixed horizontal cylinder. Breakaway strength for both the high- and low-friction horizontal cylinders was significantly greater than isometric grip strength (1.58 ± 0.25 and 1.26 ± 0.19 times, respectively). Conclusion: Results support the hypothesis that hand-handhold coupling is composed of active (isometric or eccentric finger flexion) and passive (frictional) components. Traditional isometric grip strength alone does not predict the strength of a couple between a hand and a handhold well. Application: This research shows that handhold shape, orientation, and friction are important in the safe design of grab rails or ladders.

Evaluating the effect of four different pointing device designs on upper extremity posture and muscle activity during mousing tasks

The goal of this study was to evaluate the effect of different types of computer pointing devices and placements on posture and muscle activity of the hand and arm. A repeated measures laboratory study with 12 adults (6 females, 6 males) was conducted. Participants completed two mouse-intensive tasks while using a conventional mouse, a trackball, a stand-alone touchpad, and a rollermouse. A motion analysis system and an electromyography system monitored right upper extremity postures and muscle activity, respectively. The rollermouse condition was associated with a more neutral hand posture (lower inter-fingertip spread and greater finger flexion) along with significantly lower forearm extensor muscle activity. The touchpad and rollermouse, which were centrally located, were associated with significantly more neutral shoulder postures, reduced ulnar deviation, and lower forearm extensor muscle activities than other types of pointing devices. Users reported the most difficulty using the trackball and touchpad. Rollermouse was not more difficult to use than any other devices. These results show that computer pointing device design and location elicit significantly different postures and forearm muscle activities during use, especially for the hand posture metrics.

The Effect of Handhold Orientation, Size, and Wearing Gloves on Hand-Handhold Breakaway Strength

Objective: The aim of this study was to quantify the effect of handhold orientation, size (diameter), and wearing a glove on the maximum breakaway strength between a hand and handhold. Background: Manual breakaway strength is known to be greatly reduced for vertical compared with horizontal handholds, but oblique orientations have yet to be studied. Method: For this study, 12 young adults (6 female) attempted to hold on to fixed overhead cylindrical handholds with one hand in low-speed simulated falls as forces on the handhold were recorded in two experimental designs. Breakaway strength was measured for (a) three different-sized cylinders in four orientations while the participants were using the dominant hand and (b) a single-sized cylinder in four orientations while the participants were bare-handed or wearing a glove on the nondominant hand. Results: Handhold orientation (p < .001), handhold diameter (p < .001), and wearing gloves (p < .001) significantly affected breakaway strength. Breakaway strength increased 75% to 94% as the orientation of the handhold was moved from vertical to horizontal. Breakaway strength decreased 8% to 13% for large-diameter (51-mm) handholds as compared with smaller diameters (22 mm to 32 mm), depending on orientation. Gloves may increase or decrease the ability to hang on depending on interface friction; greater friction increased breakaway force. Conclusion: Handles oriented perpendicular to the pull direction and high-friction gloves provide the greatest breakaway strength. Smaller handhold diameters than predicted by grip strength afford greater capability in these orientations. Application: These insights can be used to design handholds that increase the ability to support one’s body weight and reduce the effort needed to pull or lift heavy items.

Force distribution at the hand/handle interface for grip and pull tasks

The purpose of this experiment was to investigate how surface pressure distribution at the hand/handle interface changes during gripping and pulling and to enhance understanding of coupling between hands and grasped objects. Two pressure-sensing arrays were wrapped around a 3.18 cm diameter cylindrical handle which was fixed to a six-axis load cell. Six male subjects grasped the fixed overhead instrumented handle and performed two exertion types: downward pull exertions of 30, 60, and 90 percent of their measured grip strength and a maximal isometric grip exertion. Consistent with previous studies, the greatest pressure was exerted on the distal segments of the phalanges and at the base of the thumb and palm during maximum isometric gripping. However, when pulling, pressure on the palm (underside of the handle) was negligible. Most pull pressure was distributed over the fingers unevenly in a bimodal distribution, with the greatest pressures occurring on the proximal side of the hand followed by the fingertips. This supports the hypothesis that friction acts through the soft tissues of the fingers and creates an increased normal force in the direction of proximal segments (i.e. “belt friction”). When the surface of the handle is lumped into 5 equal zones, the observed bimodal trend is lost, with the greatest average pressure observed at the top of the handle corresponding to the pull direction. This discrepancy highlights the importance of resolution when making conclusions about applied hand forces. Future research is needed to investigate pressures on handles of different size and surface friction to develop comprehensive models of hand/object coupling.

The natural angle between the hand and handle and the effect of handle orientation on wrist radial/ulnar deviation during maximal push exertions.

The purpose of this experiment was to quantify the natural angle between the hand and a handle, and to investigate three design factors: handle rotation, handle tilt and between-handle width on the natural angle as well as resultant wrist radial/ulnar deviation (‘RUD’) for pushing tasks. Photographs taken of the right upper limb of 31 participants (14 women and 17 men) performing maximal seated push exertions on different handles were analysed. Natural hand/handle angle and RUD were assessed. It was found that all of the three design factors significantly affected natural handle angle and wrist RUD, but participant gender did not. The natural angle between the hand and the cylindrical handle was 65 ± 7°. Wrist deviation was reduced for handles that were rotated 0° (horizontal) and at the narrow width (31 cm). Handles that were tilted forward 15° reduced radial deviation consistently (12–13°) across handle conditions. Practitioner summary: Manual materials handling (MMH) tasks involving pushing have been related to increased risk of musculoskeletal injury. This study shows that handle orientation influences hand and wrist posture during pushing, and suggests that the design of push handles on carts and other MMH aids can be improved by adjusting their orientation to fit the natural interface between the hand and handle.

Thumb Motor Performance is Greater for Two-Handed Grip Compared to Single-Handed Grip on a Mobile Phone

The aim of this study was to determine if thumb motor performance varied between singlehanded and two-handed grip for thumb tapping tasks on a mobile phone. A secondary aim was to determine if differences in phone movement variation and thumb flexion could account for variations in motor performance across the two grip configurations. Ten right-handed participants (5 males, 5 females) completed reciprocal thumb tapping tasks on an Apple iPhone 3® in a single-handed and a two-handed grip configuration while an active-marker motion capture system measured 3D kinematics of the thumb and phone. The results show that thumb motor performance was significantly greater for the two-handed grip configuration due to less phone movement variation compared to the single-handed grip configuration. Thumb flexion did not significantly vary across configurations. These data suggest that increasing support for the phone such as by using a two-handed grip could lead to increases in tapping performance. For example, increased performance would be expected when using phones that include a landscape mode and are wide enough to allow a stable two-handed grip.

Gestural Workspaces for Computer Interaction Configuration and Performance

While the hardware challenges facing integration of hand-gestural controls into mainstream computer interfaces appear to be shrinking, design challenges related to the physical and mental burden required of gesture interfaces remain. This study aims to determine the effect of the gestural workspace configuration (vertical, horizontal, and jointspace) and affordance of arm support on speed, accuracy, and performance when performing pointing tasks on a computer. Seventeen participants played a computer card game (Solitaire) and then completed Fitts’ serial clicking tasks to evaluate performance in five gestural workspace configurations implemented using a Microsoft Kinect®. Gestural input configurations were also compared to a traditional mouse input. A traditional mouse performed better than gestural controls in all aspects of performance. Among gestural workspaces, the vertical configuration performed significantly better in throughput and accuracy outcomes. Limitations of the gestural tracking system and large variation in individual performance may hinder establishment of generalized design recommendations at this point; however, gestural workspace configurations with direct mapping to onscreen movements and the presence of supporting surfaces appear to increase user performance and reduce perceived difficulty.