Force fluctuations while pressing and moving against high- and low-friction touch screen surfaces.

The purpose of this study was to identify the influence of a high- and low-friction surface on the ability to maintain a steady downward force during an index finger pressing and moving task. Fifteen right-handed subjects (24-48 years) performed a static force pressing task and a hybrid pressing and moving task on the surface of an iPad mini while holding a steady 2-N force on high- and low-friction surfaces. Variability of force was quantified as the standard deviation (SD) of normal force (F z) and shear force (F xy) across friction conditions and tasks. The SD of F z was 227 % greater during the hybrid task as compared to the static task (p < .001) and was 19 % greater for the high- versus low-friction condition (p = .033). There were positive correlations between SD of F z and F xy during the hybrid force/motion tasks on the high- and low-friction conditions (r (2) = 0.5 and 0.86, respectively), suggesting significant associations between normal and shear forces for this hybrid task. The correlation between the SD of F z for static and hybrid tasks was r (2) = 0.44, indicating that the common practice of examining the control of static tasks may not sufficiently explain performance during hybrid tasks, at least for the young subjects tested in the current study. As activities of daily living frequently require hybrid force/motion tasks (e.g., writing, doing the dishes, and cleaning counters), the results of this study emphasize the need to study motor performance during hybrid tasks in addition to static force tasks.

Reliability of a Seven-Segment Foot Model with Medial and Lateral Midfoot and Forefoot Segments During Walking Gait.

In-vitro and invasive in-vivo studies have reported relatively independent motion in the medial and lateral forefoot segments during gait. However, most current surface-based models have not defined medial and lateral forefoot or midfoot segments. The purpose of the current study was to determine the reliability of a 7-segment foot model that includes medial and lateral midfoot and forefoot segments during walking gait. Three-dimensional positions of marker clusters located on the leg and 6 foot segments were tracked as 10 participants completed 5 walking trials. To examine the reliability of the foot model, coefficients of multiple correlation (CMC) were calculated across the trials for each participant. Three-dimensional stance time series and range of motion (ROM) during stance were also calculated for each functional articulation. CMCs for all of the functional articulations were ≥ 0.80. Overall, the rearfoot complex (leg-calcaneus segments) was the most reliable articulation and the medial midfoot complex (calcaneus-navicular segments) was the least reliable. With respect to ROM, reliability was greatest for plantarflexion/dorsiflexion and least for abduction/adduction. Further, the stance ROM and time-series patterns results between the current study and previous invasive in-vivo studies that have assessed actual bone motion were generally consistent.

The effect of boundary shape and minima selection on single limb stance postural stability.

The effect of time-to-boundary minima selection and stability limit definition was investigated during eyes open and eyes closed condition single-limb stance postural stability. Anteroposterior and mediolateral time-to-boundary were computed using the mean and standard deviation (SD) of all time-to-boundary minima during a trial, and the mean and SD of only the 10 absolute time-to-boundary minima. Time-to-boundary with rectangular, trapezoidal, and multisegmented polygon defined stability limits were also calculated. Spearman's rank correlation coefficient test results revealed significant medium-large correlations between anteroposterior and mediolateral time-to-boundary scores calculated using both the mean and SD of the 10 absolute time-to-boundary minima and of all the time-to-boundary minima. Friedman test results revealed significant mediolateral time-to-boundary differences between boundary shape definitions. Follow-up Wilcoxon signed rank test results revealed significant differences between the rectangular boundary shape and both the trapezoidal and multisegmented polygon shapes during the eyes open and eyes closed conditions when both the mean and the SD of the time-to-boundary minima were used to represent postural stability. Significant differences were also revealed between the trapezoidal and multisegmented polygon definitions during the eyes open condition when the SD of the time-to-boundary minima was used to represent postural stability. Based on these findings, the overall results (i.e., stable versus unstable participants or groups) of studies computing postural stability using different minima selection can be compared. With respect to boundary shape, the trapezoid or multisegmented polygon shapes may be more appropriate than the rectangular shape as they more closely represent the anatomical shape of the stance foot.

Normalizing hip muscle strength: establishing body-size-independent measurements.

OBJECTIVE: To investigate the effectiveness of computing body-size-independent hip strength measures using muscle-specific allometric scaling and ratio standard normalization methods. DESIGN: Cross-sectional study. SETTING: University laboratory. PARTICIPANTS: A convenience sample of healthy participants (N=113; 42 men, 71 women). INTERVENTIONS: Not applicable. MAIN OUTCOME MEASURES: Anthropometric measurements of the leg and thigh were obtained, and maximal hip strength was tested (medial and lateral rotation, abduction, adduction, flexion, extension). Strength was measured isometrically as force (kg) and then converted to torque (Nm). RESULTS: The allometric scaling analysis resulted in exponents for normalizing body mass (BM) in each muscle group assessed. In addition, a 6-muscle average exponent was also computed (bavg) for force (men, .554; women, .335) and torque (men, .792; women, .482). The nonsignificant results of the linear regression analysis revealed that normalizing hip strength to BM(bavg) (hip strength/BM(bavg)) effectively removed the influence of BM on force and torque. However, sex should be factored into analyses of allometric scaling because men have higher b-values than women for both force and torque. The linear regression analyses also demonstrated that force normalized to BM (P=.162-.895) and torque normalized to BM × Height (P=.146-.889) were body-size-independent measures. Force normalized to BM°·67 (P=.001-.191) and body mass index (BMI) (P=<.001-.066), and torque normalized to BM (P=.004-.415) and BMI (P<.001) were significantly related to BM and therefore were not body-size independent. CONCLUSIONS: Normalizing force and torque to BM(bavg) is the most effective method of removing body-size dependence and allowing comparisons of persons with differing body sizes.

The relationship among foot posture, core and lower extremity muscle function, and postural stability.

CONTEXT: Identification of impaired balance as a risk factor for lower extremity injury regardless of injury history has led to subsequent investigation of variables that may adversely affect balance in healthy individuals. OBJECTIVES: To investigate the relationship among core and lower extremity muscle function, foot posture, and balance. DESIGN: Descriptive laboratory study. SETTING: Musculoskeletal injury biomechanics laboratory. PATIENTS OR OTHER PARTICIPANTS: A total of 108 individuals (40 men, 68 women; age = 22.8 ± 4.7 years, height = 168.5 ± 10.4 cm, mass = 69.9 ± 13.3 kg) participated in the study. MAIN OUTCOME MEASURE(S): Core endurance was assessed during 1 time-to-failure trial, and isometric hip and ankle strength were assessed using a handheld dynamometer and isokinetic dynamometer, respectively. Foot structure was quantified using the digital photographic measurement method. Single-limb-stance time to boundary was assessed using a force plate during an eyes-closed condition. Hierarchical multiple regression analyses were performed to predict balance using lower extremity strength, foot posture, and core endurance. RESULTS: Foot posture (ß = -0.22, P = .03) and ankle-inversion strength (ß = -0.29, P = .006) predicted mediolateral balance. Increasing arch posture and ankle-inversion strength were associated with decreased mediolateral single-limb-stance balance. CONCLUSIONS: Increasing arch height was associated with decreased mediolateral control of single-limb stance. The relationship between time to boundary and injury risk, however, has not been explored. Therefore, the relationship between increasing arch height and injury due to postural instability cannot be determined from this study. If authors of future prospective studies identify a relationship between decreased time to boundary and increased injury risk, foot structure may be an important variable to assess during preparticipation physical examinations. The relationship between increasing ankle-inversion strength and decreased balance may require additional study to further elucidate the relationship between ankle strength and balance.