New species in the Sitalcina sura species group (Opiliones, Laniatores, Phalangodidae), with evidence for a biogeographic link between California desert canyons and Arizona sky islands.

The western United States is home to numerous narrowly endemic harvestman taxa (Arachnida, Opiliones), including members of the genus Sitalcina Banks, 1911. Sitalcina is comprised of three species groups, including the monospecific Sitalcina californica and Sitalcina lobata groups, and the Sitalcina sura group with eight described species. All species in the Sitalcina sura group have very small geographic distributions, with group members distributed like disjunct "beads on a string" from Monterey south to southern California and southeast to the sky-island mountain ranges of southern Arizona. Here, molecular phylogenetic and species delimitation analyses were conducted for all described species in the Sitalcina sura group, plus several newly discovered populations. Species trees were reconstructed using multispecies coalescent methods implemented in *BEAST, and species delimitation was accomplished using Bayes Factor Delimitation (BFD). Based on quantitative species delimitation results supported by consideration of morphological characters, two new species (Sitalcina oasiensis sp. n., Sitalcina ubicki sp. n.) are described. We also provide a description of the previously unknown male of Sitalcina borregoensis Briggs, 1968. Molecular phylogenetic evidence strongly supports distinctive desert versus coastal clades, with desert canyon taxa from southern California more closely related to Arizona taxa than to geographically proximate California coastal taxa. We hypothesize that southern ancestry and plate tectonics have played a role in the diversification history of this animal lineage, similar to sclerophyllous plant taxa of the Madro-Tertiary Geoflora. Molecular clock analyses for the Sitalcina sura group are generally consistent with these hypotheses. We also propose that additional Sitalcina species await discovery in the desert canyons of southern California and northern Baja, and the mountains of northwestern mainland Mexico.

Stabilisation times after transitions to standing from different working postures.

Transitioning to standing after maintaining working postures may result in imbalance and could elicit a fall. The objective of this study was to quantify the magnitude of imbalance using a stabilisation time metric. Forty-five male participants completed three replications of conditions created by one of four working postures (bent at waist, squat, forward kneel, reclined kneel) and three durations within posture. Participants transitioned to quiet standing at a self-selected pace. Stabilisation time, based on changes in centre of pressure velocity, was used to indicate the initiation of steady state while standing. Stabilisation time was significantly affected by static postures but not duration within posture. The largest stabilisation times resulted from transitions initiated from a bent at waist posture. The smallest were associated with the kneeling postures, which were not significantly different from each other. Findings may lead to recommendations for redesign of tasks, particularly in high-risk environments such as construction. Statement of Relevance: Task performance on the jobsite often requires individuals to maintain non-erect postures. This study suggests that working posture affects stabilisation during transition to a standing position. Bending at the waist and squatting resulted in longer stabilisation times, whereas both kneeling postures evaluated resulted in greater imbalance but for a shorter duration.

Investigating gait adjustments and body sway while walking across wooden scaffold boards.

The flexible wooden boards are still being used on the scaffolds at some construction worksites in China. We examined if the board dimension, acclimation phase and construction work experience influence workers' walking strategies on these boards. Among the 40 construction workers recruited, half of them had more than 1 year of construction work experience. The participants' body sway and the gait adjustments were captured and analysed. Our analysis showed the width and thickness of the boards had significant effects on walking speed. There is a significant interaction between construction experience and board width on stride width, whereas the interaction between construction experience and acclimation phase affects step length and walking speed. The body sway at the C6/C7 and L5/S1 locations are significantly affected by most of the factors. We also found that once the participants acclimated to the board condition, the sway variability increased. Practitioner Summary: This study investigated gait and postural adjustments of workers with different construction experience while walking over wooden scaffold boards, which is one of the activities highly associated with falling accidents in China.

Factors affecting time-to-contact calculations during quiet standing.

Time-to-contact (TtC) is an alternative measure of postural stability to center of pressure (CoP) velocity. TtC is based on both spatial and temporal aspects of CoP displacement, definition of the boundary shape, and quantity of minima analyzed. Three boundary shapes and three minima selection methods were used to compute TtC during bipedal quiet standing. The results suggest that there is a strong correlation between TtC values obtained using each of the calculation methods (r ≥ .73) and mean CoP velocity (r ≥ -.70). TtC was significantly affected by boundary shape and minima selection method. This limits the ability to compare absolute values, but relative levels of stability computed using TtC can be compared due to strong correlations. Given the task parameters studied, mean CoP velocity may even be adequate to assess levels of stability. Future studies are needed to examine the generalizability of these findings for different groups and task parameters.

Methodological considerations of existing techniques for determining stabilization times following a multi-planar transition.

Postural stabilization is required following perturbations or after transitioning to standing. The current research evaluated two available algorithms that utilize within-trial data to quantify standing following multi-planar transitions. Forty-five participants began each trial by assuming a static forward kneeling posture that ended with an auditory signal prompting transition to standing. Data from two force plates was collected at 100Hz for 20s starting with the transition. With one algorithm, using windows of various lengths, stabilization time was defined as when mean center of pressure (CoP) velocity of the current window was less than that for the mean of all subsequent windows. This algorithm produced significantly different stabilization times (1.3-6.9s) depending on the window length. In a second algorithm, a negative exponential mathematical model was fit to data within each trial (R(2)=0.93). This approach was easily implemented and produced results (mean=2.1s) with lower variability (SD=0.9s). Though approaches exist that adequately determine stabilization times in well-constrained uni-planar movements, there are limitations to generalizability. The negative exponential mathematical model evaluated in this study provides a promising method for systematically determining stabilization times for multi-planar movements.

Assessment of differenced center of pressure time series improves detection of age-related changes in postural coordination.

Center of Pressure (CoP) time series exhibit non-stationarity. Most CoP analyses assume a stationary signal, which could lead to measurement inaccuracy. Despite this, few researchers have reported the incidence of CoP non-stationarity or employed procedures to mitigate non-stationarity prior to time-series analysis. Differencing is a pre-processing technique that reduces non-stationarity, though it has only recently been used with CoP data. This study sought to report the incidence of CoP non-stationarity in a sample data set and determine whether differencing mitigated any CoP non-stationarity that was detected. In addition, researchers examined whether analysis of differenced CoP improved the ability to detect age-related changes in postural coordination.

Effects of common working postures on balance control during the stabilisation phase of transitioning to standing.

Standing after maintaining working postures may result in imbalance and could elicit a fall. The objective of this study was to assess the magnitude of this imbalance. Forty-five male participants completed three replications of conditions created by four static postures and three durations within posture. Participants transitioned to quiet standing at a self-selected pace. Body segment location and displacement of the centre of pressure (COP) were recorded using a motion capture system and two forceplates, respectively. Balance control measures were calculated during the stabilisation phase. All balance control measures were significantly affected by static posture but not duration within posture. Bending over at waist generally caused the smallest changes in balance control measures, whereas the reclined kneeling posture resulted in the largest. Findings may lead to recommendations for redesign of tasks to reduce the use of certain working postures, particularly in high-risk environments such as construction. STATEMENT OF RELEVANCE: Task performance on the jobsite often requires individuals to maintain non-erect postures. This study suggests that the working posture chosen affects stabilisation during a transition to a standing position. Bending at the waist or squatting seems to have less of an affect on balance control measures, whereas both types of kneeling postures evaluated resulted in greater imbalance.

Balance control during lateral load transfers over a slippery surface.

Few studies have measured balance control during manual material handling, and even fewer with environmental cofactors. This study examined the effect of different surface frictions during a stationary manual material handling task. Thirty-six healthy participants completed 180° lateral transfer tasks of a load over high- and low-friction surfaces (µ = 0.86 and µ = 0.16, respectively). Balance measures, stance kinematics and lower extremity muscle activities were measured. Success during the novel slippery surface dichotomised our population, allowing us to investigate beneficial techniques to lateral load transfers over the slippery surface. Stance width reduction by 8 cm and 15° of additional external foot rotation towards the load were used to counter the imbalance created by the slippery surface. There was no clear alteration to lower extremity muscular control to adapt to a slippery surface. Changes in stance seemed to be used successfully to counter a slippery surface during lateral load transfers. STATEMENT OF RELEVANCE: Industries requiring manual material handling where slippery conditions are potentially present have a noticeable increase in injuries. This study suggests stance configuration, more so than any other measure of balance control, differentiates vulnerability to imbalance during material handling over a slippery surface.

The effect of load weight on balance control during lateral box transfers.

Few studies have endeavoured to measure balance control during manual material handling. This study examined the effects of load weight during a stationary manual material handling task. In total, 36 healthy participants completed 180° lateral transfer tasks of a loaded (5% of body weight) and an unloaded box. The projection of the centre of mass onto the base of support, as measured via a passive-marker 3-D motion analysis system, was used to quantify balance control. Muscle activities of lower extremity muscles were also measured. When moving the loaded box, individuals ventured ≥ 1 cm closer to the edges of the base of support and increased centre of mass movement up to 14%. In addition, muscle electromyographic activity on both sides of the shank increased. In summary, during loaded configurations, vulnerability to loss of balance was increased and individuals appeared to adapt by increasing co-contraction of the shank muscles suggesting increased ankle stiffness. STATEMENT OF RELEVANCE: Industries requiring manual material handling have a particularly high rate of injuries due to falls. This study suggests that larger load weights during lateral material handling tasks adversely affect balance control and may create a vulnerability to imbalance throughout the entire manoeuvre.

Effects of simulated occupational task parameters on balance.

The effects of single-handed load holding, length of the base of support, and standing surface condition (narrow and wide construction beams) on balance were investigated in twenty-three healthy men between the ages of 18 and 55 years old. Balance during quiet standing was evaluated from postural sway measurements derived from center of pressure (COP) displacement. These measurements included the range or maximal displacement of the COP in the anteroposterior (AP) and mediolateral (ML) directions, the elliptical area, and mean sway velocity. Holding a load in the hand did not significantly affect postural sway measures (p > 0.05), although the effect of surface condition was significant on all COP measures (p < 0.001). Lengthening the base of support did not affect the ranges or elliptical area, but increased the mean velocity of sway (p = 0.001). Changes in the dimensional characteristics of the surface condition and length of base of support affected postural sway, possibly by requiring adjustments to balance and motor control strategies. Further research is required to determine if these changes are detrimental to maintaining balance and increase the risk of falls for workers in similar environments.

The anatomy of a slip: Kinetic and kinematic characteristics of slip and non-slip matched trials.

To improve understanding of slip propagation mechanisms, one could compare features of early stance phase during slips and non-slips. This study investigated the similarities and differences in kinematics and utilized COF of paired trials, defined as a matched pair of slip and non-slip trials produced by the same participant walking on the same floor condition at the same walking speed condition. Twenty-two participants produced 47 matched trial pairs while walking at 1.5, 1.8 and 2.1m/s, over a forceplate with an available COF ranging from 0.12 to 0.21. Heel displacement was captured with an infrared motion tracking system and utilized COF was derived from ground reaction forces. ANOVA revealed no significant differences between the slip and non-slip groups in horizontal heel velocity just prior to heel strike or for heel velocity or slip distance during the 20ms period following heel strike. Significant differences were found between the groups in utilized COF and horizontal heel velocity at 25 and 30ms following heel strike. Differences in heel kinematics and kinetics during early stance phase between the slip and non-slip trials are discussed. The results differ from several previous studies, likely due to methodological differences, as the present study was conducted on marginally slippery surfaces, as opposed to very low COF conditions with thick contaminant layers.

Estimation of forces exerted by the fingers using standardised surface electromyography from the forearm.

Determination and integration of human force capabilities and requirements is an essential component of ergonomic evaluation. With regard to hand-intensive tasks, direct force measurements can be cumbersome and intrusive. Here, the use of surface electromyography (EMG) was evaluated. EMG was obtained from three standardised electrode sites on the forearms of 30 individuals. Linear regression models were generated to estimate finger force levels from normalised electromyographic measures, while forces were generated in several finger couplings. The results suggest that standardised procedures for obtaining electromyographic data and simple linear models can be used to accurately estimate finger forces during a variety of finger exertions in fixed postures, although the level of accuracy depends on the type of model. Such models begin to overcome the limitations of direct finger strength measurements of individuals.

Rearward movement of the heel at heel strike.

This paper describes the observation of rearward movement (RM) of the heel following heel strike occurring during normal gait. Thirty-one participants recruited as part of a larger study on slip kinematics walked the length of an 8-m runway at a speed of 1.5 m/s. Several floor surfaces, presented dry and with contaminant, were used for the purpose of eliciting a wide range of small slip distances. The normal force applied to a forceplate mounted in the runway was used to identify heel strike, as well as to calculate the utilized coefficient of friction during early stance phase. A motion analysis system tracked the displacement of two heel-mounted markers, and the data were used to derive kinematic variables related to the heel strike event. Results showed that RMs occurred in 18.1% of 494 trials, with a mean rearward displacement of 5.02 (+/-3.68) mm. When present, RMs occurred in close temporal proximity to heel strike, typically completing RM within 40 ms of the heel strike event. When divided into groups by age, older participants (>40 years) were more than twice as likely to have RMs as younger participants. When grouped by height or weight, differences in the proportion of trials with RMs were small. In trials where RMs were observed, forward slip distances were significantly less than for trials with no RMs, 2.17 (+/-3.87) mm vs. 12.58 (+/-10.71) mm, respectively. The time until the heel stopped moving during the post-heel strike period was not significantly different between RM and non-RM trials. Further investigation of this gait feature may improve understanding of normal gait patterns and may have implications for future slipmeter development.

Association of subjective ratings of slipperiness to heel displacement following contact with the floor.

Falls precipitated by slipping are listed among the leading causes of occupational injuries. Several factors may influence the risk of slips and falls, including perception of surface conditions. The current research examined the relationship between perceptions of slipperiness and slip distance at heel strike. The investigation compared objective and subjective measures for 31 participants ranging in age from 19 to 67 years old. Subjective slipperiness ratings for several floor surface and walking velocity conditions were obtained. Small slips were not generally perceived, but a uniform slip distance threshold could not be identified due to the large variability in ratings. The Pearson correlation coefficient between slip distance and subjective ratings for slides (30-100mm) was r=-0.17. Results indicate that subjective ratings should be used cautiously as a measure of slipperiness, partially due to possible underestimation of slipperiness and the variability in perceptions.

The use of a heel-mounted accelerometer as an adjunct measure of slip distance.

A human-centered measure of floor slipperiness could be useful as an adjunct to conventional tribologic measures. This paper reports on the development and evaluation of a measure of slip distance based on variables derived from the signal of a heel-mounted accelerometer. Twenty-one participants walked on a laboratory runway under several surface slipperiness conditions at three walking speeds during a protocol designed to produce a wide range of slip distances at heel strike. Analysis of variance showed significant effects of slip distance (no-slip, micro-slip and slide), walking speed (1.52, 1.78 and 2.13 m/s) and their interactions on peak forward acceleration, peak vertical acceleration and deceleration time of the heel following heel strike in 704 trials. Regression analysis of slip distance and deceleration time showed the strongest relationship with R2=0.511. Large individual variation in the strength of this relationship was observed. The heel-mounted accelerometer may have utility as an adjunct measure in the evaluation of floor slipperiness, particularly for field applications where direct measurement may not be feasible.

Measurement and prediction of single and multi-digit finger strength.

Hand and finger strength has direct application in the design of human-machine interfaces involving the whole hand or single digits. Limited finger strength data is available, however, particularly for practical situations such as pinching and poking. A study was conducted in which strength in a variety of couplings was collected from 100 participants, in order to enhance and supplement the existing literature. Differences between couplings, gender, and age groups were evaluated. Strength was significantly higher for multi-digit couplings as compared with single digit couplings (p < 0.05). In addition, female strength was approximately 70% of male strength across all couplings. No significant differences were found between three age groups ranging from 18 to 40 + years old. Multiple regression models were used to determine whether finger strength could be predicted from other strength measures and anthropometry. Regression results suggest that finger strength can be predicted with only moderate accuracy using these variables (R2-adj: 0.45 - 0.64; standard error: 12 - 19 N). Such models are easy to implement, however, and begin to overcome the limitations of direct finger strength measurements.