Towards functionally individualised designed footwear recommendation for overuse injury prevention: a scoping review.

Injury prevention is essential in running due to the risk of overuse injury development. Tailoring running shoes to individual needs may be a promising strategy to reduce this risk. Novel manufacturing processes allow the production of individualised running shoes that incorporate features that meet individual biomechanical and experiential needs. However, specific ways to individualise footwear to reduce injury risk are poorly understood. Therefore, this scoping review provides an overview of (1) footwear design features that have the potential for individualisation; and (2) the literature on the differential responses to footwear design features between selected groups of individuals. These purposes focus exclusively on reducing the risk of overuse injuries. We included studies in the English language on adults that analysed: (1) potential interaction effects between footwear design features and subgroups of runners or covariates (e.g., age, sex) for running-related biomechanical risk factors or injury incidences; (2) footwear comfort perception for a systematically modified footwear design feature. Most of the included articles (n = 107) analysed male runners. Female runners may be more susceptible to footwear-induced changes and overuse injury development; future research should target more heterogonous sampling. Several footwear design features (e.g., midsole characteristics, upper, outsole profile) show potential for individualisation. However, the literature addressing individualised footwear solutions and the potential to reduce biomechanical risk factors is limited. Future studies should leverage more extensive data collections considering relevant covariates and subgroups while systematically modifying isolated footwear design features to inform footwear individualisation.

Surface Stiffness and Footwear Affect the Loading Stimulus for Lower Extremity Muscles When Running.

ABSTRACT: Willwacher, S, Fischer, KM, Rohr, E, Trudeau, MB, Hamill, J, and Brüggemann, G-P. Surface stiffness and footwear affect the loading stimulus for lower extremity muscles when running. J Strength Cond Res 36(1): 82-89, 2022-Running in minimal footwear or barefoot can improve foot muscle strength. Muscles spanning the foot and ankle joints have the potential to improve performance and to reduce overuse injury risk. Surface stiffness or footwear use could modify the intensity of training stimuli acting on lower extremity joints during running. The purpose of this study was to systematically investigate external ankle, knee, and hip joint moments during shod and barefoot running while considering the stiffness of the running surface. Two footwear conditions (barefoot and neutral running shoe) and 4 surface conditions (Tartan, Tartan + Ethylene Vinyl Acetate [EVA] foam, Tartan + artificial turf, Tartan + EVA foam + artificial turf) were tested at 3.5 m·s-1. Repeated measures analysis of variance revealed that barefoot running in general and running barefoot on harder surfaces increased and decreased ankle (between +5 and +26%) and knee (between 0 and -11%) joint moments, respectively. Averaged over all surfaces, running barefoot was characterized by a 6.8° more plantarflexed foot strike pattern compared with running shod. Foot strike patterns were more plantarflexed on harder surfaces; the effects, however, were less than 3°. Most surface effects were stronger in barefoot compared with shod running. Surface stiffness may be used to modulate the loading intensity of lower extremity muscles (in particular extrinsic and intrinsic foot muscles) during running. These results need to be considered when coaches advise barefoot running as a method to improve the strength of extrinsic and intrinsic foot muscles or when trying to reduce knee joint loading.

The Influence of Prolonged Running and Footwear on Lower Extremity Joint Stiffness.

INTRODUCTION: The purpose of this study was to compare leg, sagittal plane knee and ankle, and frontal plane ankle stiffness over the course of a prolonged treadmill run in neutral and stability footwear. METHODS: Thirteen male habitual rearfoot runners completed two biomechanical testing sessions in which they ran for 21 min at their preferred running speed in a neutral shoe, then changed either into the same neutral shoe or a stability shoe and ran a further 21 min on a force-instrumented treadmill. Three-dimensional kinematics and kinetics were recorded at the beginning and end of each 21-min interval. RESULTS: No differences were observed in leg stiffness between footwear conditions throughout the run (P > 0.05). Knee stiffness increased during the first 21 min (P = 0.009), whereas ankle stiffness reduced at minute 21 (P = 0.004) and minute 44 (P = 0.006). These changes were modulated by an increase in ankle joint compliance and knee joint moments. No differences were observed between footwear conditions for leg and sagittal plane lower extremity joint stiffness (P > 0.05). During the second half of the run, frontal plane ankle stiffness increased in the stability shoe but decreased in the neutral shoe (P = 0.019), attributed to reduced eversion range of motion caused by the added medial post. CONCLUSIONS: These results suggest that over the course of a prolonged treadmill run, shock attenuation strategies change, which may affect the knee joint.

The habitual motion path theory: Evidence from cartilage volume reductions in the knee joint after 75 minutes of running.

The habitual motion path theory predicts that humans tend to maintain their habitual motion path (HMP) during locomotion. The HMP is the path of least resistance of the joints defined by an individual's musculoskeletal anatomy and passive tissue properties. Here we tested whether participants with higher HMP deviation and whether using footwear that increases HMP deviation during running show higher reductions of knee joint articular cartilage volume after 75 minutes of running. We quantified knee joint articular cartilage volumes before and after the run using a 3.0-Tesla MRI. We performed a 3D movement analysis of runners in order to quantify their HMP from a two-legged squat motion and the deviation from the HMP when running in different footwear conditions. We found significantly more cartilage volume reductions in the medial knee compartment and patella for participants with higher HMP deviation. We also found higher cartilage volume reductions on the medial tibia when runners wore a shoe that maximized their HMP deviation compared with the shoe that minmized their HMP deviation. Runners might benefit from reducing their HMP deviation and from selecting footwear by quantifying HMP deviation in order to minimize joint cartilage loading in sub-areas of the knee.

Influence of neutral and stability athletic footwear on lower extremity coordination variability during a prolonged treadmill run in male rearfoot runners.

The purpose of this study was to investigate changes in coordination variability (CV) over the course of a prolonged treadmill run and the influence of stability and neutral footwear on CV. Fourteen male habitually rearfoot runners completed two 42 min prolonged running sessions while three-dimensional kinematics and kinetics were recorded. During the first 21 min, participants ran in a neutral shoe (baseline run), then changed into either another neutral shoe of the same construction but another colour or a stability shoe and ran a further 21 min (intervention run). A modified vector coding technique was used to compute thigh-leg, leg-rearfoot and rearfoot-forefoot segment CV. Following the baseline run, thigh flexion/extension-leg flexion/extension, rearfoot inversion/eversion-forefoot plantar flexion/dorsiflexion and rearfoot inversion/eversion-forefoot adduction/abduction CV increased (p < .05). During the intervention run, CV was higher in the neutral shoe compared with the stability shoe for thigh flexion/extension-leg flexion/extension and leg flexion/extension-rearfoot inversion/eversion couplings (p < .05). Lower extremity CV increased or was maintained during a prolonged treadmill run in healthy male rearfoot runners, likely to distribute stresses among the tissues as muscles begin to fatigue. CV increased to a greater extent in neutral compared with stability footwear which may be a result of: (1) the stability shoe acting as a perturbation to the runner and their response is to regulate CV, or; (2) stability footwear provides greater support and consequently, runners do not need to explore additional degrees of freedom to reduce stresses applied to the tissues throughout a prolonged run.

Does footwear affect articular cartilage volume change after a prolonged run?

The aim of this study was to investigate knee intra-articular cartilage volume changes after a prolonged running bout in three footwear conditions. Twelve participants performed 75-minute running bouts in the three footwear conditions. Before and after each running bout, magnetic resonance imaging (MRI) scans were obtained using a high-resolution 3.0 Tesla MRI. Three-dimensional reconstruction of the cartilage plates of the patella, the femur, and the tibia was created to quantify cartilage volume change due to the 75-minute running bout. Three-dimensional biomechanical data were also collected using an integrated motion capture and force treadmill system. There were no statistically significant differences among shoe conditions for all anatomical regions. However, significant cartilage volume reductions at all anatomical sites were observed after the 75-minute running bout in each footwear condition. These data suggest that the intra-articular knee cartilage undergoes a significant reduction in cartilage volume during a prolonged run that may indicate an increase in joint loading. There was a considerable variation in cartilage volume between participants across footwear conditions indicating an individual cartilage volume response to footwear. An individualistic approach to footwear recommendations may help in minimizing this change in cartilage.

Estimating Tibial Stress throughout the Duration of a Treadmill Run.

INTRODUCTION: Stress fractures of the tibia are a problematic injury among runners of all levels. Quantifying tibial stress using a modeling approach provides an alternative to invasive assessments that may be used to detect changes in tibial stress during running. This study aimed to assess the repeatability of a tibial stress model and to use this model to quantify changes in tibial stress that occur throughout the course of a 40-min prolonged treadmill run. METHODS: Synchronized force and kinematic data were collected during prolonged treadmill running from 14 recreational male rearfoot runners on two separate occasions. During each session, participants ran at their preferred speed for two consecutive 20-min runs, separated by a 2-min pause. The tibia was modeled as a hollow ellipse and bending moments and stresses at the distal third of the tibia were estimated using beam theory combined with inverse dynamics and musculoskeletal modeling. RESULTS: Intraclass correlation coefficients indicated good-to-excellent repeatability for peak stress values between sessions. Peak anterior and posterior stresses increased after 20 min of prolonged treadmill running and were 15% and 12% greater, respectively, after 40 min of running compared with the start of the run. CONCLUSION: The hollow elliptical tibial model presented is a repeatable tool that can be utilized to assess within-participant changes in peak tibial stress during running. The increased stresses observed during a prolonged treadmill run may have implications for the development of tibial stress fracture.

Calcaneus range of motion underestimated by markers on running shoe heel.

BACKGROUND: The measurement of rearfoot kinematics by placing reflective markers on the shoe heel assumes its motion is identical to the foot's motion. Studies have compared foot and shoe kinematics during running but with conflicting results. The primary purpose of this study was to compare shoe and calcaneus three-dimensional range of motion during running. A secondary purpose was to determine the effect of a less rigid heel counter on tibia motion. RESEARCH QUESTION: Do markers placed on the shoe heel accurately represent calcaneus kinematics during running? METHODS: Three-dimensional coordinate data were collected on 14 subjects (M/F: 9/5) who ran on an instrumented treadmill at 3.35 m/s under four conditions: modified/intact neutral shoes, and modified/intact support shoes. Shoes were modified by placing holes through the heel to allow for shoe heel and calcaneus coordinate data to be collected simultaneously via reflective markers on the shoe and on the skin of the heel within the shoe. Calcaneus, shoe heel, and tibia ROM were calculated from 0 to 50% stance phase and compared across shoe conditions. RESULTS: Calcaneal frontal plane ROM was significantly greater than neutral and support shoe heel ROM (p < 0.001). Calcaneus ROM was also significantly greater than shoe heel ROM in the transverse (p < 0.001) and sagittal (p < 0.001) planes. No change in tibial transverse plane ROM was observed (p = 0.346) across shoe heel conditions. SIGNIFICANCE: Shoe markers significantly underestimated calcaneus ROM across all planes of motion. These findings suggest calcaneus kinematics cannot be accurately measured with markers placed solely on the shoe heel. Additionally, the required modifications to the shoe's heel had no effect on tibia ROM in the transverse plane.

The Preferred Movement Path Paradigm: Influence of Running Shoes on Joint Movement.

PURPOSE: (A) To quantify differences in lower extremity joint kinematics for groups of runners subjected to different running footwear conditions, and (B) to quantify differences in lower extremity joint kinematics on an individual basis for runners subjected to different running footwear conditions. METHODS: Three-dimensional ankle and knee joint kinematics were collected for 35 heel-toe runners when wearing three different running shoes and when running barefoot. Absolute mean differences in ankle and knee joint kinematics were computed between running shoe conditions. The percentage of individual runners who displayed differences below a 2°, 3°, and 5° threshold were also calculated. RESULTS: The results indicate that the mean kinematics of the ankle and knee joints were similar between running shoe conditions. Aside from ankle dorsiflexion and knee flexion, the percentage of runners maintaining their movement path between running shoes (i.e., less than 3°) was in the order of magnitude of about 80% to 100%. Many runners showed ankle and knee joint kinematics that differed between a conventional running shoe and barefoot by more than 3°, especially for ankle dorsiflexion and knee flexion. CONCLUSIONS: Many runners stay in the same movement path (the preferred movement path) when running in various different footwear conditions. The percentage of runners maintaining their preferred movement path depends on the magnitude of the change introduced by the footwear condition.

Two-handed grip on a mobile phone affords greater thumb motor performance, decreased variability, and a more extended thumb posture than a one-handed grip.

Holding a mobile computing device with two hands may affect thumb motor performance, joint postures, and device stability compared to holding the device and tapping the touchscreen with the thumb of the holding hand. We tested the hypotheses that holding a touchscreen mobile phone with two hands lead to increased thumb motor performance, different thumb postures, and decreased device movement relative to using one hand. Ten right-handed participants completed reciprocal thumb tapping tasks between emulated keys on a smartphone in either a one- (portrait) or two-handed (landscape) grip configuration. Effective index of performance measured from Fitts' Law was 9% greater (p < 0.001), movement time 7% faster (p < 0.001), and taps were 4% more precise (p < 0.016) for the two-handed grip. Tapping with a two-handed grip involved significantly different wrist and thumb postures than a one-handed grip. Variability of the computing device's movement was 36-63% lower for the two-handed grip compared to the one-handed grip condition (p < 0.001). The support for our hypotheses suggests that a two-handed grip results in increased performance and more extended wrist and thumb postures than a single-handed grip. Device designs that allow two-handed grips may afford increased performance relative to a one-handed grip.

Increased Athletic Performance in Lighter Basketball Shoes: Shoe or Psychology Effect?

PURPOSE: To determine the effect of shoe mass on performance in basketball-specific movements and how this affects changes if an athlete is aware or not of the shoe's mass relative to other shoes. METHODS: In an experimental design, 22 male participants were assigned to 2 groups. In the "aware" group, differences in the mass of the shoes were disclosed, while participants in the other group were blinded to the mass of shoes. For both groups lateral shuffle-cut and vertical-jump performances were quantified in 3 different basketball-shoe conditions (light, 352 ± 18.4 g; medium, 510 ± 17 g; heavy, 637 ± 17.7 g). A mixed ANOVA compared mean shuffle-cut and vertical-jump performances across shoes and groups. For blinded participants, perceived shoe-weight ratings were collected and compared across shoe conditions using a Friedman 2-way ANOVA. RESULTS: In the aware group, performance in the light shoes was significantly increased by 2% (vertical jump 2%, P < .001; shuffle cut 2.1%, P < .001) compared with the heavy shoes. In the blind group, participants were unable to perceive the shoe-weight variation between conditions, and there were no significant differences in vertical-jump and shuffle-cut performance across shoes. CONCLUSIONS: Differences in performance of the aware participants were most likely due to psychological effects such as positive and negative expectancies toward the light and heavy shoes, respectively. These results underline the importance for coaches and shoe manufacturers to communicate the performance-enhancing benefits of products or other interventions to athletes to optimize their performance outcome.

Assessing Footwear Effects from Principal Features of Plantar Loading during Running.

PURPOSE: The effects of footwear on the musculoskeletal system are commonly assessed by interpreting the resultant force at the foot during the stance phase of running. However, this approach overlooks loading patterns across the entire foot. An alternative technique for assessing foot loading across different footwear conditions is possible using comprehensive analysis tools that extract different foot loading features, thus enhancing the functional interpretation of the differences across different interventions. The purpose of this article was to use pattern recognition techniques to develop and use a novel comprehensive method for assessing the effects of different footwear interventions on plantar loading. METHODS: A principal component analysis was used to extract different loading features from the stance phase of running, and a support vector machine (SVM) was used to determine whether and how these loading features were different across three shoe conditions. RESULTS: The results revealed distinct loading features at the foot during the stance phase of running. The loading features determined from the principal component analysis allowed successful classification of all three shoe conditions using the SVM. Several differences were found in the location and timing of the loading across each pairwise shoe comparison using the output from the SVM. CONCLUSIONS: The analysis approach proposed can successfully be used to compare different loading patterns with a much greater resolution than has been reported previously. This study has several important applications. One such application is that it would not be relevant for a user to select a shoe or for a manufacturer to alter a shoe's construction if the classification across shoe conditions would not have been significant.

A data-driven design evaluation tool for handheld device soft keyboards.

Thumb interaction is a primary technique used to operate small handheld devices such as smartphones. Despite the different techniques involved in operating a handheld device compared to a personal computer, the keyboard layouts for both devices are similar. A handheld device keyboard that considers the physical capabilities of the thumb may improve user experience. We developed and applied a design evaluation tool for different geometries of the QWERTY keyboard using a performance evaluation model. The model utilizes previously collected data on thumb motor performance and posture for different tap locations and thumb movement directions. We calculated a performance index (PITOT, 0 is worst and 2 is best) for 663 designs consisting in different combinations of three variables: the keyboard's radius of curvature (R) (mm), orientation (O) (°), and vertical location on the screen (L). The current standard keyboard performed poorly (PITOT = 0.28) compared to other designs considered. Keyboard location (L) contributed to the greatest variability in performance out of the three design variables, suggesting that designers should modify this variable first. Performance was greatest for designs in the middle keyboard location. In addition, having a slightly upward curve (R = -20 mm) and orientated perpendicular to the thumb's long axis (O = -20°) improved performance to PITOT = 1.97. Poorest performances were associated with placement of the keyboard's spacebar in the bottom right corner of the screen (e.g., the worst was for R = 20 mm, O = 40°, L =  Bottom (PITOT = 0.09)). While this evaluation tool can be used in the design process as an ergonomic reference to promote user motor performance, other design variables such as visual access and usability still remain unexplored.

Effects of forearm and palm supports on the upper extremity during computer mouse use.

The use of forearm and palm supports has been associated with lower neck and shoulder muscle activity as well as reduced musculoskeletal discomfort during keyboard use, however, few studies have investigated their effect during computer mouse use. Eight men and eight women completed several computer mousing tasks in six arm support conditions: Forearm Support, Flat Palm Support, Raised Palm Support, Forearm + Flat Palm Support, Forearm + Raised Palm Support, and No Support. Concurrently, an infrared three-dimensional motion analysis system measured postures, six-degree-of-freedom force-torque sensors measured applied forces & torques, and surface electromyography measured muscle activity. The use of forearm support compared to the no support condition was significantly associated with less shoulder muscle activity & torque, and the raised palm support was associated with less wrist extension. Forearm supports reduced shoulder flexion torque by 90% compared to no support. The use of either support also resulted in lower applied forces to the mouse pad. Participants reported less musculoskeletal discomfort when using a support. These results provide recommendations for office workstation setup and inform ergonomists of effective ways to reduce musculoskeletal exposures.

Joint contribution to fingertip movement during a number entry task: an application of Jacobian matrix.

Upper extremity kinematics during keyboard use is associated with musculoskeletal health among computer users; however, specific kinematics patterns are unclear. This study aimed to determine the dynamic roles of the shoulder, elbow, wrist and metacarpophalangeal (MCP) joints during a number entry task. Six subjects typed in phone numbers using their right index finger on a stand-alone numeric keypad. The contribution of each joint of the upper extremity to the fingertip movement during the task was calculated from the joint angle trajectory and the Jacobian matrix of a nine-degree-of-freedom kinematic representation of the finger, hand, forearm and upper arm. The results indicated that in the vertical direction where the greatest fingertip movement occurred, the MCP, wrist, elbow (including forearm) and shoulder joint contributed 10.2%, 55.6%, 27.7% and 6.5%, respectively, to the downward motion of the index finger averaged across subjects. The results demonstrated that the wrist and elbow contribute the most to the fingertip vertical movement, indicating that they play a major role in the keying motion and have a dynamic load beyond maintaining posture.

Tablet Keyboard Configuration Affects Performance, Discomfort and Task Difficulty for Thumb Typing in a Two-Handed Grip.

When holding a tablet computer with two hands, the touch keyboard configuration imposes postural constraints on the user because of the need to simultaneously hold the device and type with the thumbs. Designers have provided users with several possible keyboard configurations (device orientation, keyboard layout and location). However, potential differences in performance, usability and postures among these configurations have not been explored. We hypothesize that (1) the narrower standard keyboard layout in the portrait orientation leads to lower self-reported discomfort and less reach than the landscape orientation; (2) a split keyboard layout results in better overall outcomes compared to the standard layout; and (3) the conventional bottom keyboard location leads to the best outcomes overall compared to other locations. A repeated measures laboratory experiment of 12 tablet owners measured typing speed, discomfort, task difficulty, and thumb/wrist joint postures using an active marker system during typing tasks for different combinations of device orientation (portrait and landscape), keyboard layout (standard and split), and keyboard location (bottom, middle, top). The narrower standard keyboard with the device in the portrait orientation was associated with less discomfort (least squares mean (and S.E.) 2.9±0.6) than the landscape orientation (4.5±0.7). Additionally, the split keyboard decreased the amount of reaching required by the thumb in the landscape orientation as defined by a reduced range of motion and less MCP extension, which may have led to reduced discomfort (2.7±0.6) compared to the standard layout (4.5±0.7). However, typing speed was greater for the standard layout (127±5 char./min.) compared to the split layout (113±4 char./min.) regardless of device orientation and keyboard location. Usage guidelines and designers can incorporate these findings to optimize keyboard design parameters and form factors that promote user performance and usability for thumb interaction.

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.

Thumb motor performance varies with thumb and wrist posture during single-handed mobile phone use.

Design features of mobile computing technology such as device size and key location may affect thumb motor performance during single-handed use. Since single-handed use requires the thumb posture to vary with key location, we hypothesize that motor performance is associated with thumb and wrist joint postures. A repeated measures laboratory experiment of 10 right-handed participants measured thumb and wrist joint postures during reciprocal tapping tasks between two keys for different key pairs among 12 emulated keys. Fitts' effective index of performance and joint postures at contact with each key were averaged across trials for each key. Thumb motor performance varied for different keys, with poorest performances being associated with excessive thumb flexion such as when tapping on keys closest to the base of the thumb in the bottom right corner of the phone. Motor performance was greatest when the thumb was in a typical resting posture, neither significantly flexed nor fully extended with slight CMC joint abduction and supination, such as when tapping on keys located in the top right and middle left areas on the phone. Grip was also significantly affected by key location, with the most extreme differences being between the top left and bottom right corners of the phone. These results suggest that keypad designs aimed at promoting performance for single-handed use should avoid placing frequently used functions and keys close to the base of the thumb and instead should consider key locations that require a thumb posture away from its limits in flexion/extension, as these postures promote motor performance.

Thumb motor performance varies by movement orientation, direction, and device size during single-handed mobile phone use.

OBJECTIVE: The aim of this study was to determine if thumb motor performance metrics varied by movement orientation, direction, and device size during single-handed use of a mobile phone device. BACKGROUND: With the increased use of mobile phones, understanding how design factors affect and improve performance can provide better design guidelines. METHOD: A repeated measures laboratory experiment of 20 right-handed participants measured the thumb tip's 3-D position relative to a phone during reciprocal tapping tasks across four phone designs and four thumb tip movement orientations. Each movement orientation included two movement directions: an "outward" direction consisting in CMC (carpometacarpal) joint flexion or abduction movements and an "inward" direction consisting in CMC joint extension or adduction movements. Calculated metrics of the thumb's motor performance were Fitts' effective width and index of performance. RESULTS: Index of performance varied significantly across phones, with performance being generally better for the smaller devices. Performance was also significantly higher for adduction-abduction movement orientations compared to flexion-extension, and for "outward" compared to "inward" movement directions. CONCLUSION: For single-handed device use, adduction-abduction-type movements on smaller phones lead to better thumb performance. APPLICATION: The results from this study can be used to design new mobile phone devices and keypad interfaces that optimize specific thumb motions to improve the user-interface experience during single-handed use.

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 user's 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.