Changes In Center Of Mass Research Articles

Assessing the additivity of mass distribution-based isotopic shifts in high-resolution cyclic ion mobility separations

Recent advancements in high-resolution ion mobility spectrometry-mass spectrometry (IMS-MS) have enabled the separation of isotopologues and isotopomers based on their mass distribution-based isotopic shifts (i.e., changes in center of mass and moments of inertia). To better understand the fundamental nature of these isotopic shifts, we investigated whether they were additive in nature by introducing varying isotopic substitutions (e.g., 13C, 2H/D, and 81Br) through either hydrogen deuterium exchange or permethylation. From there, we measured the relative arrival times between light and heavy isotopologues with high-resolution cyclic ion mobility separations. Globally, we observed that the isotopic shifts were approximately additive in nature regardless of the molecule system or specific isomer studied. Furthermore, we discovered that additivity occurs in the isotopic shifts irrespective of the absolute shift, potentially indicating this observation may be more global in nature. We believe that our findings will serve to better understand the fundamental nature of mass distribution-based isotopic shifts and will inform theoretical ion mobility calculations in the future.

Center of mass change in the biomechanics citation network

ABSTRACTThis study examined the hypothesis that the network of citation in biomechanics journals would change over the last decade and specifically the centrality of the journal Sports Biomechanics. Top 20 cited biomechanics journals identified using the ‘Journal Citation Relationships’ feature from Journal Citation Reports were extracted for 12 ‘seed’ biomechanics journals in 2011, 2016, and 2021. From 2011 till 2021 the total number of top cited biomechanics journals in Journal Citation Reports decreased (17%) from 90 to 74, while the citations to these journals nearly increased 5-fold (8,051 to 39,574). Top Journal Citation Reports cited biomechanics journals changed over time, indicating variability in the centrality of many journals in the biomechanics citation network based on this prestigious database. Over time there were decreasing citation of medical journals, as well as increasing citation of open access mega journals and the journal Sports Biomechanics. Citation network analysis provides insight into the changing structure of knowledge creation in biomechanics, with recent trends towards applied research particularly on wearable sensors published in mega journals.

Rapid-cycle Thouless pumping in a one-dimensional optical lattice

An adiabatic cycle around a degeneracy point in the parameter space of a one-dimensional band insulator is known to result in an integer valued noiseless particle transport in the thermodynamic limit. Recently, it was shown that in the case of an infinite bipartite lattice the adiabatic Thouless protocol can be continuously deformed into a fine tuned finite-frequency cycle preserving the properties of noiseless quantized transport. In this paper, we numerically investigate the implementation of such an ideal rapid-cycle Thouless pumping protocol in a one-dimensional optical lattice. It is shown that the rapidity will cause first order corrections due to next-to-nearest-neighbour hopping and second order corrections due to the addition of a harmonic potential. Lastly, the quantization of the change in center of mass of the particle distribution is investigated, and shown to have corrections in the first order of the potential curvature.

Impact of daily soft-tissue image guidance to prostate on pelvic lymph node (PLN) irradiation for prostate patients receiving SBRT.

PurposeTo determine the impact of using fiducial match for daily image‐guidance on pelvic lymph node (PLN) coverage for prostate cancer patients receiving stereotactic body radiation therapy (SBRT).MethodsThirty patients underwent SBRT treatment to the prostate and PLN from 2014 to 2016. Each patient received either 800cGy × 5 or 500cGy × 5 to the prostate and 500cGy × 5 to the PLN. A 5 mm clinical target volume (CTV)‐to‐planning target volume (PTV) margin around the PLN was used for planning. Two registrations with planning computed tomography (PCT) for each of the daily cone beam CTs (CBCTs) were performed: a rigid registration to fiducials and to the bony anatomy. The average translational difference between fiducial and bony match as well as percentage of fractions with differences > 5mm were calculated. Changes in bladder and rectal volume as well as center‐of‐mass (COM) position from simulation parameters, and their correlation with translational difference were also evaluated. The dosimetric impact of the translational differences was calculated by shifting the plan isocenter.ResultsThe average translational difference between fiducial and bony match was 0.06 ± 0.82, 2.1 ± 4.1, −2.8 ± 4.3, and 5.5 ± 4.2 mm for lateral, vertical, longitudinal, and vector directions. The average change in bladder and rectal volume from simulation was −67.2 ± 163.04 cc (−12 ± 52%) and −1.6 ± 18.75 (−2 ± 30%) cc. The average change in COM of bladder from the simulation position was 0.34 ± 2.49, 4.4 ± 8.1, and −3.9 ± 7.5 mm along the LR, AP, and SI directions. The corresponding COM change for the rectum was 0.17 ± 1.9, 1.34 ± 3.5, and −0.6 ± 5.2 mm.ConclusionsThe 5 mm margin covered ~75% of fractions receiving PLN irradiation with SBRT, daily CBCT and fiducial‐guided setup. The dosimetric impact on PLN coverage was significant in 19% of fractions or 25% of patients. A larger translational shift was due to variation in rectal volume and changes in COM position of the bladder and rectum. A consistent bladder positioning and/or rectum filling compared with presimulation volume were essential for adequate coverage of PLN in a hypofractionated treatment regime.

Acute and Chronic Kinematic Effect of a Resistance Tubing Training Device on Youth Baseball and Softball Swings

Objective: Baseball and softball athletes often use swing training implements in preparation for an at bat, or to develop strength/ power. The purpose of the present study is to examine the kinematic effects of a resistance tubing training device on youth baseball and softball swings both acutely, and after a 4-week intervention. Methods: Twenty youth baseball and softball athletes participated. Ten completed the 4-week intervention and returned for follow up testing. Kinematic data were collected using an electromagnetic motion capture system on baseline swings. Participants then swung with the resistance tubing device, and then took it off, and subsequent swings were recorded. Participants completed a 4-week intervention using the swing trainer, and then reported back for follow-up testing. All collected swings were taken off a tee with the instructions to hit line drives up the middle of the cage. Data were analyzed for center of mass (COM) positioning over base of support (BOS), hand velocity, and hand path. Results: Repeated measures analysis of variance revealed no significant changes in COM over BOS, hand velocity, or hand path between time points: baseline, acute, and follow-up. Conclusion: The absence of significant kinematic changes means the resistance tubing swing training device could be used as a preparation tool for at-bats without the negative performance indicators reported in previous research on weighted implements.

Effects of Kinesiology Tape on Non-linear Center of Mass Dispersion During the Y Balance Test.

Static taping of the ankle or knee joint is a common method of reducing risk of injury by providing mechanical stability. An alternative taping technique employs kinesiology tape, which has the additional benefit of improving functionality by stimulating proprioception. There is substantial disagreement whether kinesiology tape shows significant differences in proprioception and postural stability as compared to rigid/static tape when applied at the lower limb. The current study investigated the effects of kinesiology tape and static tape during a Y Balance Test on center of mass as an indicator for postural stability. Forty-one individuals, free of injury, performed the Y Balance Test under the three conditions; no tape, kinesiology tape, and static tape applied at the lower limb to the quadriceps, triceps surae and ankle joint. All participants completed the Y Balance Test to determine whether any significant differences could be observed using center of mass movement as a surrogate measure for balance and proprioception. The Minkowski-Bouligand and box-counting fractal dimension analyses were used as measures of the dynamic changes in the center of mass whilst undertaking the Y Balance Test. Statistical analyses included the Kruskal Wallis test to allow for non-normally distributed data and a Bonferroni corrected pairwise T-test as a post hoc test to ascertain pairwise differences between the three taping conditions. Significance was set at 0.05. The fractal analyses of the dynamic changes in center of mass showed significant differences between the control and both the static tape and kinesiology tape groups (p = 0.021 and 0.009, respectively). The current study developed a novel measure of dynamic changes in the center of mass during a set movement that indicated real-time processing effects during a balance task associated with the type of taping used to enhance postural stability.

Effects of Simulated Low Vision on Postural Adjustment to Changes in Center of Mass in Older Adults

ABSTRACTThis study examined how instrumental activities of daily living (IADL) performed by older adults under low-vision simulation conditions affect postural adjustments to changes in center of mass (COM). Ten participants with normal vision performed seven activities under two conditions, normal vision, and simulated macular degeneration (MD). Postural adjustment to changes in COM and time to complete activities were recorded. Low vision was compared to normal vision using Wilcoxon signed rank and t tests. Differences between the two conditions were statistically significant for postural adjustments to change in COM and time. Postural adjustments and time to perform IADLs are greater under simulated low vision conditions versus normal vision. These preliminary findings support research with older adults with MD, who may be at risk when making movement transitions like descending or ascending stairs, stepping in and out of a tub, stooping, or reaching from one surface to another.

Effect of variations in atelectasis on tumor displacement during radiation therapy for locally advanced lung cancer

PurposeAtelectasis (AT), or collapsed lung, is frequently associated with central lung tumors. We investigated the variation of atelectasis volumes during radiation therapy and analyzed the effect of AT volume changes on the reproducibility of the primary tumor (PT) position. Methods and materialsTwelve patients with lung cancer who had AT and 10 patients without AT underwent repeated 4-dimensional fan beam computed tomography (CT) scans during radiation therapy per protocols that were approved by the institutional review board. Interfraction volume changes of AT and PT were correlated with PT displacements relative to bony anatomy using both a bounding box (BB) method and change in center of mass (COM). Linear regression modeling was used to determine whether PT and AT volume changes were independently associated with PT displacement. PT displacement was compared between patients with and without AT. ResultsThe mean initial AT volume on the planning CT was 189 cm3 (37-513 cm3), and the mean PT volume was 93 cm3 (12-176 cm3). During radiation therapy, AT and PT volumes decreased on average 136.7 cm3 (20-369 cm3) for AT and 40 cm3 (−7 to 131 cm3) for PT. Eighty-three percent of patients with AT had at least one unidirectional PT shift that was greater than 0.5 cm outside of the initial BB during treatment. In patients with AT, the maximum PT COM shift was ≥0.5 cm in all patients and >1 cm in 58% of patients (0.5-2.4 cm). Changes in PT and AT volumes were independently associated with PT displacement (P < .01), and the correlation was smaller with COM (R2 = 0.58) compared with the BB method (R2 = 0.80). The median root mean squared PT displacement with the BB method was significantly less for patients without AT (0.45 cm) compared with those with AT (0.8cm, P = .002). ConclusionsChanges in AT and PT volumes during radiation treatment were significantly associated with PT displacements that often exceeded standard setup margins. Repeated 3-dimensional imaging is recommended in patients with AT to evaluate for PT displacements during treatment.

Comparing the Effectiveness of a Short-Term Vertical Jump vs. Weightlifting Program on Athletic Power Development

Teo, SYM, Newton, MJ, Newton, RU, Dempsey, AR, and Fairchild, TJ. Comparing the effectiveness of a short-term vertical jump vs. weightlifting program on athletic power development. J Strength Cond Res 30(10): 2741-2748, 2016-Efficient training of neuromuscular power and the translation of this power to sport-specific tasks is a key objective in the preparation of athletes involved in team-based sports. The purpose of this study was to compare changes in center of mass (COM) neuromuscular power and performance of sport-specific tasks after short-term (6-week) training adopting either Olympic-style weightlifting (WL) exercises or vertical jump (VJ) exercises. Twenty-six recreationally active men (18-30 years; height: 178.7 ± 8.3 cm; mass: 78.6 ± 12.2 kg) were randomly allocated to either a WL or VJ training group and performance during the countermovement jump (CMJ), squat jump (SJ), depth jump (DJ), 20-m sprint, and the 5-0-5 agility test-assessed pre and posttraining. Despite the WL group demonstrating larger increases in peak power output during the CMJ (WL group: 10% increase, d = 0.701; VJ group: 5.78% increase, d = 0.328) and SJ (WL group: 12.73% increase, d = 0.854; VJ group: 7.27% increase, d = 0.382), no significant between-group differences were observed in any outcome measure studied. There was a significant main effect of time observed for the 3 VJs (CMJ, SJ, and DJ), 0- to 5-m and 0- to 20-m sprint times, and the 5-0-5 agility test time, which were all shown to improve after the training (all main effects of time p < 0.01). Irrespective of the training approach adopted by coaches or athletes, addition of either WL or VJ training for development of power can improve performance in tasks associated with team-based sports, even in athletes with limited preseason training periods.

Motion detection and amelioration in a dedicated cardiac solid-state CZT SPECT device.

A solid-state cadmium zinc tellurium (CZT) dedicated multipinhole cardiac camera which acquires all views simultaneously has been introduced for myocardial SPECT acquisition. We report a method to detect and ameliorate patient motion artifacts in myocardial perfusion imaging (MPI) studies recorded with this device. To detect motion, a myocardial phantom study was recorded, and at mid-scan, the phantom was moved stepwise along each of 6 orthogonal directions, causing MPI artifacts. Using QPS software (Cedars-Sinai) and an in-house normal database, displacements giving artifactual perfusion defects (total perfusion deficit score, TPD, >5%) were all 1.5cm or greater (11.2±1.3% for 1.5cm). List mode data were reframed into 10-s steps, and the norm of the changes in center of mass among the 19 projections (32×32 matrix, pixel size 2.46mm) was used as a motion index. Rejection of misregistered data gave artifact-free reconstructions (TPD=1.0±0.8%) in phantom scans and reduced blur in a rest/stress clinical study. Blur on the patient's stress scan was consistent with increased motion compared to rest (motion index of 4.4 vs. 3.0 pixels, respectively). For CZT cameras that acquire data from multiple views simultaneously, motion during MPI can cause clinically significant artifacts. Reframing acquisitions into discrete time intervals enables the detection of motion and its amelioration, improving diagnostic accuracy.

SU‐F‐J‐127: Multi‐Institutional Evaluation of Setup, Organ Deformation, Precision Dosimetry in Total Marrow Irradiation

Purpose:Totals Marrow Irradiation (TMI) is a highly focused radiation delivery to the human skeleton structure therefore requiring a high amount of precision and accuracy for a quality treatment. Not much is known on how the patient position varies across multiple treatment fractions and how that positioning impacts the dose delivery. Currently TMI is studied as an international collaboration with multiple centers around the world; however, many of these centers used different pretreatment techniques. The goal of this work is to measure the accuracy of patient positioning, its impact on dose delivery and compare the impact of each technique for multiple institutions.Methods:Using Tomotherapy pretreatment MVCTs and the planning KVCTs measurements are made of the 3D setup uncertainties of the TMI treatment. Then, using the dose and plan files of the treatment impact of patient position on dose can be measured. Measurement of organ deformation and center of mass change were done using the Velocity AI program from Varian. We are looking at four the boney targets (skull, spine, pelvis, and femur) and three key sensitive tissues (eyes, lungs, kidneys).Results:Position measurements have been made for 3 different institutions using 3 different pre‐treatment techniques. Comparing the translation motion we can observe the greatest change in the Y and Z direction of patient set up. For intra‐fractional motion the shoulder and clavicle represent the greatest potential for motion and therefore most likely to have a dose change.Conclusion:All centers use different techniques for their treatment and this study shows that these techniques do not produce the same pretreatment results. We hope to expand this study further. Currently we have 3 centers participating in this study with more centers joining every day.

Sensor-Based Interactive Balance Training with Visual Joint Movement Feedback for Improving Postural Stability in Diabetics with Peripheral Neuropathy: A Randomized Controlled Trial

Background: Individuals with diabetic peripheral neuropathy (DPN) have deficits in sensory and motor skills leading to inadequate proprioceptive feedback, impaired postural balance and higher fall risk. Objective: This study investigated the effect of sensor-based interactive balance training on postural stability and daily physical activity in older adults with diabetes. Methods: Thirty-nine older adults with DPN were enrolled (age 63.7 ± 8.2 years, BMI 30.6 ± 6, 54% females) and randomized to either an intervention (IG) or a control (CG) group. The IG received sensor-based interactive exercise training tailored for people with diabetes (twice a week for 4 weeks). The exercises focused on shifting weight and crossing virtual obstacles. Body-worn sensors were implemented to acquire kinematic data and provide real-time joint visual feedback during the training. Outcome measurements included changes in center of mass (CoM) sway, ankle and hip joint sway measured during a balance test while the eyes were open and closed at baseline and after the intervention. Daily physical activities were also measured during a 48-hour period at baseline and at follow-up. Analysis of covariance was performed for the post-training outcome comparison. Results: Compared with the CG, the patients in the IG showed a significantly reduced CoM sway (58.31%; p = 0.009), ankle sway (62.7%; p = 0.008) and hip joint sway (72.4%; p = 0.017) during the balance test with open eyes. The ankle sway was also significantly reduced in the IG group (58.8%; p = 0.037) during measurements while the eyes were closed. The number of steps walked showed a substantial but nonsignificant increase (+27.68%; p = 0.064) in the IG following training. Conclusion: The results of this randomized controlled trial demonstrate that people with DPN can significantly improve their postural balance with diabetes-specific, tailored, sensor-based exercise training. The results promote the use of wearable technology in exercise training; however, future studies comparing this technology with commercially available systems are required to evaluate the benefit of interactive visual joint movement feedback.

Developing a measure of muscular power during a functional task for older adults.

BackgroundMuscular power is an important aspect of many activities of daily living and declines at a faster rate than other fitness parameters (i.e. muscular strength and endurance). Assessing muscular power among older adults is problematic as many of the popular tests are contraindicated among older adults and field tests to assess muscular power among older adults have not been validated among older adults. Therefore, the aim of the present investigation was to determine the validity and reliability of a field test to measure of muscular power during a functional movement among community-dwelling older adults (≥ 65 years).MethodsTwenty community-dwelling older adults (71.6 ± 5.6) volunteered to have their muscular power assessed during repeated sit-to-stand (STS) tasks. Each participant performed 10 STS with 60 s rest between trials. Muscular power was assessed during this functional movement with the Tendo as well as change in center of mass (COM) over time using cinematography.ResultsRelative power measured by Tendo was 5.34 ± 1.67 W/kg and values for COM were 5.39 ± 1.73 W/kg (p = .86). Cronbach’s alpha for Tendo muscular power for repeated trials was .98.ConclusionsTendo is a simple field method of determining muscular power among older adults and validation is essential. Results from this investigation support Tendo as a valid and reliable method for determining muscular power during a STS task among older adults. Clinicians may use this tool to evaluate and assess progress in older adults’ power and physical functioning.

Interactive balance training integrating sensor-based visual feedback of movement performance: a pilot study in older adults.

BackgroundWearable sensor technology can accurately measure body motion and provide incentive feedback during exercising. The aim of this pilot study was to evaluate the effectiveness and user experience of a balance training program in older adults integrating data from wearable sensors into a human-computer interface designed for interactive training.MethodsSenior living community residents (mean age 84.6) with confirmed fall risk were randomized to an intervention (IG, n = 17) or control group (CG, n = 16). The IG underwent 4 weeks (twice a week) of balance training including weight shifting and virtual obstacle crossing tasks with visual/auditory real-time joint movement feedback using wearable sensors. The CG received no intervention. Outcome measures included changes in center of mass (CoM) sway, ankle and hip joint sway measured during eyes open (EO) and eyes closed (EC) balance test at baseline and post-intervention. Ankle-hip postural coordination was quantified by a reciprocal compensatory index (RCI). Physical performance was quantified by the Alternate-Step-Test (AST), Timed-up-and-go (TUG), and gait assessment. User experience was measured by a standardized questionnaire.ResultsAfter the intervention sway of CoM, hip, and ankle were reduced in the IG compared to the CG during both EO and EC condition (p = .007-.042). Improvement was obtained for AST (p = .037), TUG (p = .024), fast gait speed (p = . 010), but not normal gait speed (p = .264). Effect sizes were moderate for all outcomes. RCI did not change significantly. Users expressed a positive training experience including fun, safety, and helpfulness of sensor-feedback.ConclusionsResults of this proof-of-concept study suggest that older adults at risk of falling can benefit from the balance training program. Study findings may help to inform future exercise interventions integrating wearable sensors for guided game-based training in home- and community environments. Future studies should evaluate the added value of the proposed sensor-based training paradigm compared to traditional balance training programs and commercial exergames.Trial registrationhttp://www.clinicaltrials.govNCT02043834.Electronic supplementary materialThe online version of this article (doi:10.1186/1743-0003-11-164) contains supplementary material, which is available to authorized users.

Identifiability of physical parameters in systems with limited sensors

In this paper, a method for estimating physical parameters using limited sensors is investigated. As a case study, measurements from an IMU are used for estimating the change in mass and the change in center of mass of a ship. The roll motion is studied and an instrumental variable method estimating the parameters of a transfer function from the tangential acceleration to the angular velocity is presented. It is shown that only a subset of the unknown parameters are identifiable simultaneously. A multi-stage identification approach is presented as a remedy for this. A limited simulation study is also presented to show the properties of the estimator. This shows that the method is indeed promising but that more work is needed to reduce the variance of the estimator.

Numerical-experimental analysis of the static roll stability of a demonstrator for the development of a rollover prevention device for dump semi-trailers

When raising the dump body of a tipping vehicle the overall centre of mass changes its position, and the presence of factors such as a sloping ground or an off-centred load could lead to instability situations. In this respect, a prototype of rollover prevention system for dump semi-trailers has been developed. At the very instant at which the vehicle loses its stability, this device is able to lower the dump body to its horizontal rest position in order to prevent the rollover. A dump semi-trailer demonstrator has been built so as to verify the performance of the prototype. Then, a numerical-experimental analysis of the static roll stability of the demonstrator has been carried out by means finite element calculations and stability tests. From this initial analysis, it will be possible to study and solve different problems associated with the installation of the rollover prevention device in a real vehicle.

Dynamics of inter-modality re-weighting during human postural control

Flexible and stable postural control requires adaptation to changing environmental conditions, a process which requires re-weighting of multisensory stimuli. Recent studies, as well as predictions from a computational model, have indicated a reciprocal re-weighting relationship between modalities when a sensory stimulus changes amplitude. As one modality is down-weighted, another is up-weighted to compensate (and vice versa). The purpose of this study was to investigate the dynamics of intra- and inter-modality re-weighting process by examining postural responses to manipulation of proprioception and visual modalities simultaneously. Twenty-two young adults were placed in a visual cave and stood on a variable-pitch platform for thirteen trials of 250s apiece. The platform was rotated at constant frequency of 0.4Hz and amplitudes of 0.3 (low) or 1.5 (high) degrees. Platform amplitude was manipulated in two conditions: low-to-high or high-to-low. The visual stimulus was displayed at constant frequency of 0.35Hz and amplitude of 0.08 degrees. The results showed both fast and slow changes in center of mass (CoM) response to the switch in platform amplitude. On both timescales, CoM response changed in a reciprocal manner relative to platform amplitude. When the platform amplitude increased (low-to-high condition), CoM response decreased relative to the platform and increased relative to the visual stimulus, indicating both intra-modality and inter-modality sensory re-weighting. In the high-to-low condition, however, there was no change in CoM response relative to visual stimulus, indicating that re-weighting may also be dependent on the absolute level of gain. Sway variability at frequencies other than the stimulus frequency also showed a reciprocal relationship with CoM gain relative to platform. Overall, these results indicate that dynamics of multisensory re-weighting is clearly more complicated than the schemes proposed by current adaptive models of human postural control.

Optimal impedance control based on body inertia for a hydraulically driven hexapod robot walking on uneven and extremely soft terrain

AbstractThis paper presents the implementation of impedance control for a hydraulically driven hexapod robot named COMET‐IV, which can walk on uneven and extremely soft terrain. To achieve the dynamic behavior of the hexapod robot, changes in center of mass and body attitude must be taken into consideration during the walking periods. Indirect force control via impedance control is used to address these issues. Two different impedance control schemes are developed and implemented: single‐leg impedance control and center of mass‐‐based impedance control. In the case of single‐leg impedance control, we derive the necessary impedance and adjust parameters (mass, damping, and stiffness) according to the robot legs' configuration. For center of mass–based impedance control, we use the sum of the forces of the support legs as a control input (represented by the body's current center of mass) for the derived impedance control and adjust parameters based on the robot body's configuration. The virtual forces from the robot body's moment of inertia are adapted to achieve optimal control via a linear quadratic regulator method for the proposed indirect attitude control. In addition, a compliant switching mechanism is designed to ensure that the implementation of the controller is applicable to the tripod sequences of force‐based walking modules. Evaluation and verification tests were conducted in the laboratory and the actual field with uneven terrain and extremely soft surfaces. © 2011 Wiley Periodicals, Inc.

Perception of affordances for standing on an inclined surface depends on height of center of mass

We investigated whether perception of affordances for standing on an inclined surface depended on the height of center of mass of the perceiver-actor. Participants adjusted the angle of inclination of a surface until they felt that it was just barely possible for them to stand on that surface. They performed this task while wearing a backpack apparatus to which masses were attached in one of three configurations-high-mass, low-mass, and no-mass. Moreover, participants performed this task by viewing the inclined surface or by probing it with a hand-held rod (while blindfolded). Perception of affordances for standing on the inclined surface reflected the changes in center of mass brought on by the weighted backpack apparatus (the perceptual boundary occurred at a smaller angle of inclination in the high-mass condition than in the low-mass condition and in the no-mass condition). Moreover, perception of this affordance reflected such changes both when the surface was viewed and when the surface was probed with a hand-held rod (while blindfolded). The results highlight that perception of affordances is dynamic and task-dependent and suggest that the stimulation patterns that support perception of affordances are invariant and modality-independent.

Postural sway and joint kinematics during quiet standing are affected by lumbar extensor fatigue

The purpose of this study was to investigate changes in postural sway and strategy elicited by lumbar extensor muscle fatigue. Specifically, changes in center of mass (COM), center of pressure (COP), and joint kinematics during quiet standing were determined, as well as selected cross correlations between these variables that are indicative of movement strategy. Twelve healthy male participants stood quietly both before and after exercises that fatigued the lumbar extensors. Whole-body movement and ground reaction force data were recorded and used to calculate mean body posture and variability of COM, COP, and joint kinematics during quiet standing. Three main findings emerged. First, participants adopted a slight forward lean post-fatigue as evidenced by an anterior shift of the COM and COP. Second, post-fatigue increases in joint angle variability were observed at multiple joints including joints distal to the fatigued musculature. Despite these increases, anterior–posterior (AP) ankle angle correlated well with AP COM position, suggesting the body still behaved similar to an inverted pendulum. Third, global measures of sway based on COM and COP were not necessarily indicative of changes in individual joint kinematics. Thus, in trying to advance our understanding of how localized fatigue affects movement patterns and the postural control system, it appears that joint kinematics and/or multivariate measures of postural sway are necessary.