The relation between falls risk and movement variability in Parkinson's disease.

Falls are a major health concern for older adults with Parkinson's disease (PD). This study was designed to examine differences in falls risk and its relation to changes in the average and variability (i.e. intra-individual variability) of reaction time (RT), finger tapping, standing balance and walking between healthy older adults and persons with PD. Thirty-nine adults with PD (70.0 ± 8.1 years) and 29 healthy older adults (66.8 ± 10.4 years) participated in this study. Falls risk (using the physiological profile assessment), gait, RT, balance and tapping responses were assessed for all persons. Results demonstrated that individuals with PD exhibited a greater risk of falling coupled with a general slowing of motor function covering declines in walking, RT and finger tapping. In addition, the movement responses of the PD group were more variable than the healthy older adults. Correlation results revealed group differences with regards to the neuromotor measures which were significantly correlated with falls risk. For the PD group, gait measures were highly correlated with their falls risk while, for the healthy older adults, falls risk was linked to balance measures even though PD persons had increased sway. Overall, persons with PD were at greater falls risk, moved slower and with increased variability compared to the healthy older adults. Further, while there are some similarities between the two groups in terms of those measures related to falls risk, there were also several differences which highlight that persons with PD can have different risk factors for falling compared to healthy adults of similar age.

Anterior cruciate ligament reconstructed individuals demonstrate slower reactions during a dynamic postural task.

INTRODUCTION: To determine whether individuals with a history of anterior cruciate ligament reconstruction (ACLR) exhibit altered neuromotor function compared to healthy controls. It was hypothesized that the ACLR group would have slower postural responses compared to healthy individuals of similar age. MATERIALS AND METHODS: Sixteen adults with a unilateral ACLR and 16 matched healthy controls participated. General assessments of neuromotor function were gathered and included measures of reaction time (both seated and postural conditions), walking ability, balance, ankle ROM, proprioception, knee joint laxity, patellar tendon reflex latency, and quadriceps strength. Data were analyzed using mixed generalized linear models with between-subject (ie, controls, ACLR) and within-subject factors (ie, affected, unaffected limb). RESULTS: Individuals with an ACLR exhibited a significant slowing of their postural reaction times compared to the control individuals. The ACLR group was slower under both the simple (ACLR: 484 ± 6.17 ms, control: 399 ± 1.95 ms) and choice reaction time conditions (ACLR: 550 ± 43 ms, control: 445 ± 43 ms). No other group differences were found in any of the other measures. CONCLUSION: Overall, ACLR individuals had a reduced ability to respond quickly under more challenging postural conditions (ie, stepping response). This finding would indicate that the impact of an ACLR is not purely mechanical and restricted to the joint. Rather, injury and reconstruction of the ACL impact neural mechanisms, altering individuals' ability to respond under challenging balance tasks.

Coordination dynamics of (a)symmetrically loaded gait.

Asymmetries in the resonant frequency of limbs/effectors lead to changes in coordination dynamics, including deviations in relative phase at ϕ = 0 or π rad and reduced stability. These effects have been successfully modeled by the extended Haken-Kelso-Bunz (HKB) coupled oscillator model (Kelso et al. in Attention and performance XIII. Erlbaum, Hillsdale, pp 139-169, 1990), and supported in laboratory tasks of rhythmic limb motions. Efforts to apply the HKB model to walking have supported the predicted deviations in phase, but not the expected decreases in coordination stability. The lack of stability effects arising from asymmetries may be due to the stabilizing influence of a treadmill or may be obscured by the balance requirements and ground impacts in gait. This study examined these possibilities by investigating walking overground with ankle weights of 3 or 6 kg to create asymmetries between the legs, as well as symmetrical loads. Participants walked without a metronome and separately with a metronome to control speed and cadence. Coordination dynamics between the legs were quantified through mean and standard deviation (SD) of ϕ, while individual leg local dynamic stability was calculated as maximum Lyapunov exponent (λ (MAX)). Irrespective of the condition, asymmetrical loads led to deviations in phase from antiphase with the loaded leg lagging behind the other, and both SDϕ and λ (MAX) increased (i.e., stability decreased). Symmetrical loads had no effect on phase deviations, but decreased stability. Overall, these findings indicate that the HKB model captures coordination dynamics in walking, but also highlights limitations in modeling the influence of loads on an individual limb.

Prospective versus predictive control in timing of hitting a falling ball.

Debate exists as to whether humans use prospective or predictive control to intercept an object falling under gravity (Baurès et al. in Vis Res 47:2982-2991, 2007; Zago et al. in Vis Res 48:1532-1538, 2008). Prospective control involves using continuous information to regulate action. τ, the ratio of the size of the gap to the rate of gap closure, has been proposed as the information used in guiding interceptive actions prospectively (Lee in Ecol Psychol 10:221-250, 1998). This form of control is expected to generate movement modulation, where variability decreases over the course of an action based upon more accurate timing information. In contrast, predictive control assumes that a pre-programmed movement is triggered at an appropriate criterion timing variable. For a falling object it is commonly argued that an internal model of gravitational acceleration is used to predict the motion of the object and determine movement initiation. This form of control predicts fixed duration movements initiated at consistent time-to-contact (TTC), either across conditions (constant criterion operational timing) or within conditions (variable criterion operational timing). The current study sought to test predictive and prospective control hypotheses by disrupting continuous visual information of a falling ball and examining consistency in movement initiation and duration, and evidence for movement modulation. Participants (n = 12) batted a ball dropped from three different heights (1, 1.3 and 1.5 m), under both full-vision and partial occlusion conditions. In the occlusion condition, only the initial ball drop and the final 200 ms of ball flight to the interception point could be observed. The initiation of the swing did not occur at a consistent TTC, τ, or any other timing variable across drop heights, in contrast with previous research. However, movement onset was not impacted by occluding the ball flight for 280-380 ms. This finding indicates that humans did not need to be continuously coupled to vision of the ball to initiate the swing accurately, but instead could use predictive control based on acceleration timing information (TTC2). However, other results provide evidence for movement modulation, a characteristic of prospective control. Strong correlations between movement initiation and duration and reduced timing variability from swing onset to arrival at the interception point, both support compensatory variability. An analysis of modulation within the swing revealed that early in the swing, the movement acceleration was strongly correlated to the required mean velocity at swing onset and that later in the swing, the movement acceleration was again strongly correlated with the current required mean velocity. Rather than a consistent movement initiated at the same time, these findings show that the swing was variable but modulated for meeting the demands of each trial. A prospective model of coupling τ (bat-ball) with τ (ball-target) was found to provide a very strong linear fit for an average of 69% of the movement duration. These findings provide evidence for predictive control based on TTC2 information in initiating the swing and prospective control based on τ in guiding the bat to intercept the ball.

Electromyographical Assessments of Recommended Neck and Trunk Positions for Dental Hygienists.

Purpose: Dental professionals are recommended to limit neck and trunk flexion to within 20° of a neutral (0°) body posture, however empirical support for the recommendations is lacking. The purpose of this study was to determine whether there are differences in muscle workload between a range of neck and trunk postures in a population of dental hygiene students.Methods: Fifteen first semester senior dental hygiene students with no history of neck and trunk injury volunteered to participate. Surface electromyography was used to record muscle activity from two neck extensors muscles, cervical erector spinae (CES) and upper trapezius (UT), and two trunk extensor muscles, thoracic erector spinae (TES) and iliocostalis lumboruni (IL). Participants performed ten conditions, including five neck flexion angles (0°, 10°, 20°, 30°, 40°) and five trunk flexion angles (0°,10°, 20°, 30°, 40°). For each trial, posture was checked with a goniometer and maintained for 20s. Muscle activity for each muscle was normalized to the individual's maximum voluntary isometric contraction (MVIC).Results: Activity of the CES was significantly lower in the neutral position than all flexed neck positions. Activation of the UT increased with neck flexion but required 30° of flexion to differ significantly from the neutral position. Activity of the TES required 20° of trunk flexion to differ significantly from neutral and IL activity in the neutral position was significantly lower than all other trunk flexion conditions.Conclusion: Even small amounts of neck or trunk flexion (10°), within the recommended range (≤ 20°), can significantly increase the workload for some muscles in an oral health care provider.

Online Search Behavior Related to COVID-19 Vaccines: Infodemiology Study.

BACKGROUND: Vaccination against COVID-19 is an important public health strategy to address the ongoing pandemic. Examination of online search behavior related to COVID-19 vaccines can provide insights into the public's awareness, concerns, and interest regarding COVID-19 vaccination. OBJECTIVE: The aim of this study is to describe online search behavior related to COVID-19 vaccines during the start of public vaccination efforts in the United States. METHODS: We examined Google Trends data from January 1, 2021, through March 16, 2021, to determine the relative search volume for vaccine-related searches on the internet. We also examined search query log data for COVID-19 vaccine-related searches and identified 5 categories of searches: (1) general or other information, (2) vaccine availability, (3) vaccine manufacturer, (4) vaccine side-effects and safety, and (5) vaccine myths and conspiracy beliefs. In this paper, we report on the proportion and trends for these different categories of vaccine-related searches. RESULTS: In the first quarter of 2021, the proportion of all web-based search queries related to COVID-19 vaccines increased from approximately 10% to nearly 50% of all COVID-19-related queries (P<.001). A majority of COVID-19 vaccine queries addressed vaccine availability, and there was a particularly notable increase in the proportion of queries that included the name of a specific pharmacy (from 6% to 27%; P=.01). Queries related to vaccine safety and side-effects (<5% of total queries) or specific vaccine-related myths (<1% of total queries) were uncommon, and the relative frequency of both types of searches decreased during the study period. CONCLUSIONS: This study demonstrates an increase in online search behavior related to COVID-19 vaccination in early 2021 along with an increase in the proportion of searches related to vaccine availability at pharmacies. These findings are consistent with an increase in public interest and intention to get vaccinated during the initial phase of public COVID-19 vaccination efforts.

Grip width and forearm orientation effects on muscle activity during the lat pull-down.

Based on electromyographic (EMG) studies, an anterior (in front of the face) wide grip with a pronated forearm has been recommended as the optimal lat pull-down (LPD) variation for strengthening the latissimus dorsi (LD) (Signorile, JF, Zink, A, and Szwed, S. J Strength Cond Res 16: 539-546, 2002; Wills, R, Signorile, J, Perry, A, Tremblay, L, and Kwiatkowski, K. Med Sci Sports Exerc 26: S20, 1994). However, it is not clear whether this finding was because of grip width or forearm orientation. This study aimed to resolve this issue by comparing wide-pronated, wide-supinated, narrow-pronated, and narrow-supinated grips of an anterior LPD. Twelve healthy men performed the 4 grip variations using an experimentally determined load of 70% of 1 repetition maximum. Two trials of 5 repetitions were analyzed for each grip type. Participants maintained a cadence of 2-second concentric and 2-second eccentric phases. The grip widths were normalized for each individual by using a wide grip that corresponded to their carrying width and a narrow grip that matched their biacromial diameter. Surface EMG of the LD, middle trapezius (MT), and biceps brachii (BB) was recorded, and the root mean square of the EMG was normalized, using a maximum isometric voluntary contraction. Repeated-measures analysis of variance for each muscle revealed that a pronated grip elicited greater LD activity than a supinated grip (p < 0.05), but had no influence of grip type on the MT and BB muscles. Based on these findings, an anterior LPD with pronated grip is recommended for maximally activating the LD, irrespective of the grip width (carrying width or biacromial diameter).

The Use of Augmented Information for Reducing Anterior Cruciate Ligament Injury Risk During Jump Landings: A Systematic Review.

CONTEXT:: A comprehensive systematic review of the literature on the use of augmented information in anterior cruciate ligament (ACL) injury-prevention programs to improve jump-landing technique was conducted. The use of motor-learning concepts could provide more robust means of preventing ACL injuries. OBJECTIVE:: To systematically summarize the effectiveness of augmented information in improving the biomechanical factors associated with an increased risk for ACL injury. DATA SOURCES:: Articles were retrieved using the electronic databases of PubMed, MEDLINE, CINAHL, and Google Scholar and 3 lines of truncated search words: (a) lower extremity, knee, ACL, and anterior cruciate ligament; (b) prevention, injury prevention, and prehab; and (c) augmented information, augmented feedback, feedback, cue, and instruction. We also performed a hand search of the reference lists of the screened articles. DATA EXTRACTION:: We independently assessed the methodologic quality using the Cochrane Group on Screening and Diagnostic Test Methods list. Articles were placed in 1 of 3 augmented-information categories: prescriptive, feedback, or transition. Articles were also categorized based on whether the information likely encouraged an internal or external focus of attention. DATA SYNTHESIS:: The searches identified a total of 353 studies, of which 18 were included. Most researchers found that augmented information could lead to technique changes to reduce the biomechanical risk factors associated with ACL injury. The average methodologic quality of the studies was 11.8 out of 17, with a range from 8 to 15. The authors of only 7 studies examined retention of the improved techniques. CONCLUSIONS:: The evidence suggests that augmented information can be used to significantly improve the biomechanical indicators associated with ACL injury and to enhance current ACL injury-prevention programs. Combined prescriptive and feedback information that encouraged both internal and external foci led to the largest retention effect sizes.

Coordination stability between the legs is reduced after anterior cruciate ligament reconstruction.

BACKGROUND: The study was designed to examine coordination differences in walking between individuals with an anterior cruciate ligament reconstruction compared with healthy matched controls. Predictions from the extended Haken, Kelso, and Bunz coupled oscillator model were tested in these populations. METHODS: Seventeen persons with anterior cruciate ligament reconstruction and 17 matched controls participated in the study. Sagittal plane angular knee displacement was recorded using electrogoniometers over the lateral right and left knee joints while participants walked at five walking speeds overground. Coordination pattern and stability between the knees were quantified by mean and standard deviation of relative phase, respectively. FINDINGS: Mean relative phase was not influenced by walking speed or group. For both groups, coordination stability was maximal when individual's walked at their preferred gait speed. However, the anterior cruciate ligament reconstruction group demonstrated reduced coordination stability compared with healthy controls across the five speeds. Multiple regression analyses found that people with anterior cruciate ligament reconstruction who deviated more from antiphase coordination had decreased coordination stability. INTERPRETATION: Anterior cruciate ligament reconstruction results in decreased coordination stability, indicative of reduced coupling strength between the legs. This change in gait coordination, which has not previously been found in the literature, may contribute to the increased rate of re-injury and degeneration in individuals who have had this reconstructive surgery. Application of a motor control model enhances our understanding of the influence of an injury on coordination during gait.

On No-KR tests in motor learning, retention and transfer.

In this paper we examine the role of no-knowledge of results (KR) tests in motor learning, retention and transfer. A distinction is drawn between studies of KR and studies of other issues in motor learning that use a No-KR test. Arguments are presented against the currently received position that a No-KR test is an essential protocol to examine motor learning, although a No-KR test is most relevant in the few contexts where the outcome information is not naturally available. Perspectives are provided on the limited though proper role of No-KR tests in motor learning, retention and transfer and these are illustrated through experimental examples. It is proposed that the current restrictive interpretation of KR as only augmented outcome information is narrowing not only the context of KR research but also the study of motor learning in general.

The Effect of Magnification Loupes on Dental Hygienists' Posture while Exploring.

Purpose: The purpose of this study was to determine the effects of dental magnification loupes on posture during instrumentation.Methods: A convenience sample of 27 right-handed dental hygienists, with no prior history of injuries or disabilities of the head, neck, or trunk region, enrolled in the study. Baseline posture calibration was taken and tri-axial accelerometers were placed on four locations of the head and trunk (occipital region of head; cervical vertebrae C5; thoracic vertebrae T5; lumbar vertebrae L1) to measure acceleration and the orientation of the body to gravity. Participants were randomly assigned to wear self-supplied magnification loupes during either the first or second half of the session. Dental chair mounted typodonts, prepared with artificial calculus, were used to represent a simulated oral environment. Participants were asked to explore all areas of the mouth using an ODU 11/12 explorer. Mean accelerations of the three axes were used to compute average forward/backward (AP) and side to side (ML) tilt of each accelerometer recorded during the instrumentation sessions. An end-user opinion survey was completed by each participant at the conclusion of the session.Results: No statistically significant differences in posture were revealed between the sessions with the participants wearing their loupes and not wearing loupes. However, data from the end-user survey indicate that 74% of all the participants strongly agreed that magnification loupes made exploring easier and 67% strongly agreed that they felt that magnification loupes improved their posture.Conclusion: While the majority of participants perceived that their magnification loupes enhanced their posture and made exploring easier, data from this study provided little evidence to suggest that wearing loupes leads to improved body orientation. Future research needs to examine the declination angle of ergonomic loupes and its relationship to neck and trunk flexion.

Effects of Instrument Handle Design on Dental Hygienists' Forearm Muscle Activity During Scaling.

Purpose: The purpose of this study was to determine the effects of 4 different commercially available instrument handle designs (A. 16 grams and 12.7 mm diameter, B. 23 grams and 11.1 mm diameter, C. 21 grams and 7.9 mm diameter and D. 18 grams and 6.35 mm diameter) on the muscle activity of four forearm muscles during a simulated scaling experience.Methods: A convenience sample of 27 (n=27) dental hygienists used a Columbia 13/14 curet with four different instrument handles to scale artificial calculus from typodont teeth. Each participant's muscle activity was measured using surface electromyography (sEMG).Results: Similar muscle activity was generated when scaling with instruments at 16, 18, and 21 grams with varying diameter handles. Instrument B generated significantly more muscle activity when compared to each of the other instrument handle designs (p=0.001, p=0.002, p=0.039). The lower left quadrant displayed significantly less muscle activity during scaling than the upper and lower right quadrants (p=0.026, p=0.000), although no significant interaction effect was found with instruments within quadrants. Most participants (62.96%) preferred instrument A, which was rated more comfortable based on weight when compared to the other instruments tested.Conclusions: Instrument handle design has an effect on forearm muscle activity when scaling in a simulated environment. The heaviest instrument with a relatively large diameter (B 11.1 mm and 23 g) generated significantly more overall mean muscle activity compared to the other three instruments. Similar amounts of muscle activity were produced by instruments weighing between 16 and 21 g. Participants' instrument preferences were more affected by handle diameter than weight. Results support the need for further research to determine the impact of these findings on muscle load related to risk of musculoskeletal disorders in a real-world setting.

The impact of speed and time on gait dynamics.

To determine the effects of speed on gait previous studies have examined young adults walking at different speeds; however, the small number of strides may have influenced the results. The aim of this study was to investigate the immediate and long-term impact of continuous slow walking on the mean, variability and structure of stride-to-stride measures. Fourteen young adults walked at a constant pace on a treadmill at three speeds (preferred walking speed (PWS), 90% and 80% PWS) for 30 min each. Spatiotemporal gait parameters were computed over six successive 5-min intervals. Walking slower significantly decreased stride length, while stride period and width increased. Additionally, stride period and width variability increased. Signal regularity of stride width increased and decreased in stride period. Persistence of stride period and width increased significantly at slower speeds. While several measures changed during 30min of walking, only stride period variability and signal regularity revealed a significant speed and time interaction. Healthy young adults walking at slower than preferred speeds demonstrated greater persistence and signal regularity of stride period while spatiotemporal changes such as increased stride width and period variability arose. These results suggest that different control processes are involved in adapting to the slower speeds.

Plantar cooling does not affect standing balance: A systematic review and meta-analysis.

The purpose of this systematic review with meta-analysis was to critically appraise and synthesize the literature to determine if cooling the plantar surface of the foot is an effective method of inducing postural control alterations in healthy individuals. Specific variables that were evaluated within the meta-analysis included (1) the duration of the plantar cooling intervention, (2) the stance type during postural control assessment, and (3) the role of visual input. A computerized search of four electronic databases from inception to March 2015 was performed to identify studies which examined the effects of plantar cooling on postural control in healthy individuals. A critical appraisal of the methodological quality of the included studies was performed using an appraisal criteria instrument previously used for assessing biomechanics studies. The literature search yielded 88 references of which 7 were deemed relevant and included in the systematic review and meta-analysis. A single, overall random-effects model meta-analysis of all study variables were performed; as well as, separate meta-analyses for each study variable. Across the 7 studies, the overall effect was 0.028 (95% CI=-0.177, 0.234; p=0.78) indicating that cooling the plantar surface had a very small effect on postural control. Similar results were obtained when data were examined based on study quality, plantar cooling intervention duration, stance type during postural control assessment, and the presence of visual input. Therefore, cooling the plantar surface of the foot had a very weak, non-significant effect on standing balance despite the anesthetic effects identified in each study.

Walking at the preferred stride frequency minimizes muscle activity.

This study determined whether walking at the preferred stride frequency minimizes muscle activity compared with other cadences at the same speed. Anthropometric measurements were recorded from 10 subjects and used to estimate their predicted resonant stride frequency. The preferred walking speed and stride frequency were determined from freely adopted walking on a treadmill. For the experimental trials the treadmill was set at each individual's preferred walking speed. Participants walked for 6 min at eight cadences prescribed by an auditory metronome: preferred stride frequency and -35, -25, -15, 0, +15, +25, +35% of predicted resonant stride frequency. Oxygen consumption was measured via gas analysis. Muscle activity of the right leg gastrocnemius (GA), tibialis anterior (TA), biceps femoris (BF) and rectus femoris (RF) muscles was recorded via electromyography (EMG). On average, participants preferred to walk with a stride frequency .07 Hz lower than their predicted resonant stride frequency, however a strong positive correlation was observed between these variables. Stride frequency had a significant and large quadratic effect on VO2 (RLR(2)=.76), and activity of the GA (RLR(2)=.66), TA (RLR(2)=.83), BF (RLR(2)=.70) and RF (RLR(2)=.78) muscles. VO2, GA and TA activity were all minimal at the preferred stride frequency and increased for faster or slower cadences. BF and RF activity were minimal across a broad range of slow frequencies including the preferred stride frequency and increased for faster frequencies. The preferred stride frequency that humans readily adopt during walking minimizes the activation of the GA, TA, BF and RF muscles, which in turn minimizes the overall metabolic cost.

Bracing the trunk and neck in young adults leads to a more aged-like gait.

Older individuals typically walk at slower speeds, with shorter step lengths, greater step widths and spend a larger proportion of the gait cycle in double stance. Changes in neck and trunk mobility may underlie some of the changes in walking seen with increasing age. Consequently, this study was designed to assess whether externally increasing trunk/neck stiffness in young adults leads to similar changes in gait pattern observed with aging. Twelve young adults (20-29 years), sixteen old adults (60-69 years) and fifteen older adults (70-79 years) walked across a 20' pressure sensitive GAITRite© instrumented walkway at their preferred speed. The young adults also walked under three bracing conditions: (1) Neck braced, (2) Trunk braced, and (3) Neck and Trunk braced. The results revealed that the old and older age groups walked significantly slower, with a shorter step length and with a narrower base of support (p's<0.05) compared to the young adults. In young adults, combined neck and trunk bracing led to reduced walking speed, shorter step length, wider base of support and a larger proportion of the gait cycle spent in double stance (p's<0.05). The walking speed and step length of older adults remained less than fully braced young adults (p's<0.05). Overall these results indicate that artificially stiffening the trunk and neck of young individuals leads to systematic gait changes similar to aging. Consequently, age-related changes in mobility of the neck and torso may in part contribute to the decrements in walking seen for older adults.

Task-effector asymmetries in a rhythmic continuation task.

Variability in rhythmic movements has been interpreted as a signature of internal or peripheral noise processes. Grounded in an oscillator interpretation, this study hypothesized that period variability and drift arises from the asymmetry between target period and the limb's intrinsic dynamics. Participants synchronized to 7 target periods, swinging 1 of 3 pendulums in a continuation paradigm; 3 periods were longer, 3 shorter, and 1 identical to the preferred period. Results supported 5 predictions: Drift toward the preferred period was observed that scaled with the asymmetry. Variability was lowest for symmetry conditions and increased with the asymmetry. Variability decreased concomitant with the approach toward the preferred period. Periods exponentially approached the preferred period with positive autocorrelations up to 10 cycles.

Comparison of Corded and Cordless Handpieces on Forearm Muscle Activity, Procedure Time and Ease of Use during Simulated Tooth Polishing.

PURPOSE: Dental professionals suffer from a high prevalence of work-related musculoskeletal disorders (MSD). Dental hygienists in particular have a high prevalence of pain in the forearms and hands. The objective of this study was to compare 1 cordless handpiece to 2 corded handpieces during simulated tooth polishing in terms of the muscle loads (recorded as electromyography (EMG) activity), duration of polishing procedure, and dental hygienist opinion about ease of use. METHODS: EMG was used to quantify muscle electrical activity of 4 forearm muscles during simulated dental polishing with 2 corded handpieces (HP-A and HP-B) and 1 cordless handpiece (HP-C). A convenience sample of 30 dental hygienists (23 to 57 years of age) with 1 to 20+ years of clinical practice experience completed the study. Each participant spent approximately 5 minutes polishing 3 predetermined teeth in each of the 4 quadrants. The sequence of the handpieces was randomly assigned. At the end of the study, participants completed a subjective end user evaluation of handpiece preference. RESULTS: Muscle activity levels of 10th, 50th and 90th percentiles did not differ significantly between the 3 handpieces tested (p>0.05). However, total muscle workload (integrated EMG) was lowest for the cordless handpiece (HP-C), but this was only significantly less than HP-A (p<0.05). Polishing using the cordless handpiece (HP-C) (M=257 seconds, SD=112 seconds) took significantly less time than either the HP-A corded (M=290 seconds, SD=137 seconds) or HP-B corded handpiece (M=290 seconds, SD=126 seconds) (p<0.05). Overall, 50% of the study participants preferred the cordless handpiece, 37% preferred HP-A and 13% preferred HP-B (p<0.05). CONCLUSION: Use of the cordless handpiece reduced the duration of polishing, which in turn led to less total muscle activity, but not muscle intensity. Overall, dental hygienists preferred the cordless handpiece.

Walking at the preferred stride frequency maximizes local dynamic stability of knee motion.

Healthy humans display a preference for walking at a stride frequency dependent on the inertial properties of their legs. Walking at preferred stride frequency (PSF) is predicted to maximize local dynamic stability, whereby sensitivity to intrinsic perturbations arising from natural variability inherent in biological motion is minimized. Previous studies testing this prediction have employed different variability measures, but none have directly quantified local dynamic stability by computing maximum finite-time Lyapunov exponent (λ(Max)), which quantifies the rate of divergence of nearby trajectories in state space. Here, ten healthy adults walked 45 m overground while sagittal motion of both knees was recorded via electrogoniometers. An auditory metronome prescribed 7 different frequencies relative to each individual's PSF (PSF; ±5, ±10, ±15 strides/min). Stride frequencies were performed under both freely adopted speed (FS) and controlled speed (CS: set at the speed of PSF trials) conditions. Local dynamic stability was maximal (λ(Max) was minimal) at the PSF, becoming less stable for higher and lower stride frequencies. This occurred under both FS and CS conditions, although controlling speed further reduced local dynamic stability at non-preferred stride frequencies. In contrast, measures of variability revealed effects of stride frequency and speed conditions that were distinct from λ(Max). In particular, movement regularity computed by approximate entropy (ApEn) increased for slower walking speeds, appearing to depend on speed rather than stride frequency. The cadence freely adopted by humans has the benefit of maximizing local dynamic stability, which can be interpreted as humans tuning to their resonant frequency of walking.