Particle and vapor emissions from vat polymerization desktop-scale 3-dimensional printers.

Little is known about emissions and exposure potential from vat polymerization additive manufacturing, a process that uses light-activated polymerization of a resin to build an object. Five vat polymerization printers (three stereolithography (SLA) and two digital light processing (DLP) were evaluated individually in a 12.85 m3 chamber. Aerosols (number, size) and total volatile organic compounds (TVOC) were measured using real-time monitors. Carbonyl vapors and particulate matter were collected for offline analysis using impingers and filters, respectively. During printing, particle emission yields (#/g printed) ranged from 1.3 ± 0.3 to 2.8 ± 2.6 x 108 (SLA printers) and from 3.3 ± 1.5 to 9.2 ± 3.0 x 108 (DLP printers). Yields for number of particles with sizes 5.6 to 560 nm (#/g printed) were 0.8 ± 0.1 to 2.1 ± 0.9 x 1010 and from 1.1 ± 0.3 to 4.0 ± 1.2 x 1010 for SLA and DLP printers, respectively. TVOC yield values (µg/g printed) ranged from 161 ± 47 to 322 ± 229 (SLA printers) and from 1281 ± 313 to 1931 ± 234 (DLP printers). Geometric mean mobility particle sizes were 41.1-45.1 nm for SLA printers and 15.3-28.8 nm for DLP printers. Mean particle and TVOC yields were statistically significantly higher and mean particle sizes were significantly smaller for DLP printers compared with SLA printers (p < 0.05). Energy dispersive X-ray analysis of individual particles qualitatively identified potential occupational carcinogens (chromium, nickel) as well as reactive metals implicated in generation of reactive oxygen species (iron, zinc). Lung deposition modeling indicates that about 15-37% of emitted particles would deposit in the pulmonary region (alveoli). Benzaldehyde (1.0-2.3 ppb) and acetone (0.7-18.0 ppb) were quantified in emissions from four of the printers and 4-oxopentanal (0.07 ppb) was detectable in the emissions from one printer. Vat polymerization printers emitted nanoscale particles that contained potential carcinogens, sensitizers, and reactive metals as well as carbonyl compound vapors. Differences in emissions between SLA and DLP printers indicate that the underlying technology is an important factor when considering exposure reduction strategies such as engineering controls.

Ventilation rates in recently constructed U.S. school classrooms.

Low ventilation rates (VRs) in schools have been associated with absenteeism, poorer academic performance, and teacher dissatisfaction. We measured VRs in 37 recently constructed or renovated and mechanically ventilated U.S. schools, including LEED and EnergyStar-certified buildings, using CO2 and the steady-state, build-up, decay, and transient mass balance methods. The transient mass balance method better matched conditions (specifically, changes in occupancy) and minimized biases seen in the other methods. During the school day, air change rates (ACRs) averaged 2.0±1.3 hour-1 , and only 22% of classrooms met recommended minimum ventilation rates. HVAC systems were shut off at the school day close, and ACRs dropped to 0.21±0.19 hour-1 . VRs did not differ by building type, although cost-cutting and comfort measures resulted in low VRs and potentially impaired IAQ. VRs were lower in schools that used unit ventilators or radiant heating, in smaller schools and in larger classrooms. The steady-state, build-up, and decay methods had significant limitations and biases, showing the need to confirm that these methods are appropriate. Findings highlight the need to increase VRs and to ensure that energy saving and comfort measures do not compromise ventilation and IAQ.

Ultrasound-Guided Percutaneous Radiofrequency Lesioning When Treating Recalcitrant Plantar Fasciitis: Clinical Results.

PURPOSE: We evaluated the effects of ultrasound (US)-guided percutaneous radiofrequency thermal lesioning (RTL) and the impact of obesity when treating patients with recalcitrant plantar fasciitis. MATERIALS AND METHODS: 30 consecutive patients were enrolled. The visual analog scale (VAS), American Orthopedic Foot-Ankle Society (AOFAS) Ankle-Hindfoot Score, and plantar fascia thickness measured using US were recorded at baseline and at follow-up 1, 3, 6, and 12 months after surgery under local anesthesia. RESULTS: 12 patients in the obese (BMI ≥ 30 kg/m(2)) group and 18 patients in the non-obese group. There were significant postoperative decreases in VAS scores and in fascial thickness, and an increase in the AOFAS scores (all p < 0.001). The obese group showed delayed pain and functional improvement within the first 3 months after the index procedure (p < 0.01). Significant pain reduction and functional improvement were apparent earlier (after 1 month, p < 0.001) in the non-obese group than in the obese group (after 3 months, p < 0.05). Fascia thickness was positively correlated with the VAS score and negatively correlated with the AOFAS score (both p < 0.001). CONCLUSION: US should be regarded as a useful objective tool to guide RTL and to monitor the effectiveness of treatment. US-guided percutaneous RTL for recalcitrant PF is a minimally invasive treatment option that yields satisfactory results. Therefore, it should at least be considered before using more invasive procedures. Moreover, obesity leads to delayed improvement but does not affect overall outcome after 12 months. Plantar fascial thickness was correlated with VAS and AOFAS scores.

Preliminary Extracellular Vesicle Profiling in Drainage Fluid After Neck Dissection in OSCC.

Lymph node metastasis is related to poor prognosis in oral squamous cell carcinoma (OSCC), and few studies have explored the relevance of postoperative drainage fluid (PDF) in metastasis. Extracellular vesicles (EVs) are nanosized vesicles that can transfer oncogenic molecules to regulate tumorigenesis. However, the proteomic profile of postoperative drainage fluid-derived EVs (PDF-EVs) in OSCC has not been elucidated. Herein, we collected drainage fluid from OSCC patients after neck dissection to investigate the difference in PDF-EVs between patients with metastatic lymph nodes (the LN+ group) and nonmetastatic lymph nodes (the LN- group). The proteomic profile of PDF-EVs from the LN+ and LN- groups was compared using label-free liquid chromatography tandem-mass spectrometry-based protein quantification. The results revealed that PDF-EVs were mainly derived from epithelial cells and immune cells. A total of 2,134 proteins in the PDF-EVs were identified, and 313 were differentially expressed between the LN+ and LN- groups. Metabolic proteins, such as EHD2 and CAVIN1, were expressed at higher levels in the LN+ group than in the LN- group, and the levels of EHD2 and CAVIN1 in the postoperative drainage fluid were positively correlated with lymph node metastasis. Our study revealed previously undocumented postoperative drainage fluid-associated proteins in patients with metastatic OSCC, providing a starting point for understanding their role in metastatic and nonmetastatic OSCC.

Biomechanics during exercise with a novel stairclimber.

The current study aimed to investigate the stair-climbing biomechanics related to the lower extremities when subjects used the novel designed stair-climber, which could provide opportunity for both sagittal and frontal movements. 12 volunteers were required to step while either keeping the trunk static (STATIC) or allowing the trunk to shift with weight bearing (SHIFT). A motion analysis system and the 6-axis force and torque sensor embedded in the pedal were used to collect data. Foot contact forces and joint moments were calculated to represent loading characteristics. The joint angle and corresponding moments at the terminal point of the stance phase were computed to serve as the indicator of safety. Significant differences were found in peak foot contact forces, knee extensor moment, and hip abductor moment. At the end of the stance phase, various directions of moment between conditions were found in the knee and the ankle. The knee valgus angle, hip abductor moment, and knee extensor moment were significantly greater in SHIFT than in STATIC. The various stepping strategies caused differences in joint loading characteristics; therefore, these findings need to be given greater consideration in the design of training protocols.

Assessing four-dimensional radiotherapy planning and respiratory motion-induced dose difference based on biologically effective uniform dose.

Four-dimensional (4D) radiotherapy is considered as a feasible and ideal solution to accommodate intra-fractional respiratory motion during conformal radiation therapy. With explicit inclusion of the temporal changes in anatomy during the imaging, planning, and delivery of radiotherapy, 4D treatment planning in principle provides better dose conformity. However, the clinical benefits of developing 4D treatment plans in terms of tumor control rate and normal tissue complication probability as compared to other treatment plans based on CT images of a fixed respiratory phase remains mostly unproven. The aim of our study is to comprehensively evaluate 4D treatment planning for nine lung tumor cases with both physical and biological measures using biologically effective uniform dose (D =) together with complication-free tumor control probability, P+. Based on the examined lung cancer patients and PTV margin applied, we found similar but not identical curves of DVH, and slightly different mean doses in tumor (up to 1.5%) and normal tissue in all cases when comparing 4D, P0%, and P50% plans. When it comes to biological evaluations, we did not observe definitively PTV size dependence in P+ among these nine lung cancer patients with various sizes of PTV. Moreover, it is not necessary that 4D plans would have better target coverage or higher P+ as compared to a fixed phase IMRT plan. However, on the contrary to significant deviations in P+ (up to 14.7%) observed if delivering the IMRT plan made at end-inhalation incorrectly at end-exhalation phase, we estimated the overall P+, PB, and PI for 4D composite plans that have accounted for intra-fractional respiratory motion.

Jar-opening challenges. Part 2: estimating the force-generating capacity of thumb muscles in healthy young adults during jar-opening tasks.

This study discusses the force-generating capacity of thumb muscles during jar-opening tasks using two grip patterns: the power grip and the precision grip. This study develops a three-dimensional biomechanical model of the thumb to predict muscle forces in jar-opening activities based on external forces measured by a custom-designed jar device. Ten healthy subjects participated in the study. Each participant turned a jar lid of 66 mm diameter counterclockwise with maximal effort and preferred speed using both grip patterns. The average normal and tangential forces applied by the thumb to the jar lid show that the normal force is the primary contributive force for opening a jar. This normal force is approximately three times the tangential force. Muscular force-generating capacity measurements show that the major active muscles during a jar-opening activity for both grips include the flexor pollicis longus, flexor pollicis brevis, abductor pollicis brevis, adductor pollicis, and opponens pollicis. The total muscle force ratios for the precision grip and power grip with respect to externally applied forces are 5.6 and 4.7 respectively. These ratios indicate that the power grip pattern produces less muscle force per unit of external applied load. The technique proposed in this study provides a proper apparatus and model for measuring three-dimensional loads and estimating the force-generating capacity of each muscle and tendon of the thumb during jar-opening tasks.

Development of a three-degrees-of-freedom moveable platform for providing postural perturbations.

The purpose of this study was to design and develop a three-degrees-of-freedom moveable platform to provide postural perturbations for balance assessment and training. The platform consists of three motion mechanisms, which can provide forward-backward translation, upward-downward tilt, and clockwise-counterclockwise rotation. This platform can move in any of its degrees of freedom separately or simultaneously. The precision and accuracy of the platform movement were examined by calculating the standard deviations in repeated trials and comparing the real amplitude and velocity of the movement with the preset values. All the standard deviations in repeated trials were small in that the variation coefficients were less than 2 per cent, except that in the highest-velocity test, and all the mean differences were less than 1 mm for translational and 1 degree for tilt or rotational perturbations. The results demonstrated that the platform is a reliable and valid instrument for providing postural perturbations. The preliminary investigation of the kinematic postural responses to translational and tilt perturbation showed that this platform is a useful apparatus for balance research. Potential applications of this platform include investigation of the postural responses to yaw rotation or any combination of its degrees of freedom and studying the effects of perturbation-based balance-training programmes provided by this platform.

Jar-opening challenges. Part 1: an apparatus for assessing hand and finger torques and forces in a jar-opening activity.

A simulated jar apparatus was developed to record hand kinetics and torque contribution of a digit during jar-opening activities. The design of the apparatus, namely a jar body and a lid, is similar to a commercial jam jar that is regularly seen in daily living. One six-axis force-torque transducer and a torque cell were mounted inside the jar lid to detect the external force exerted from the digit and fixed on to the jar body to record the overall torque generated by the hand and wrist respectively. The applications of the apparatus were used to test the twisting torque of the hand and to measure the applied forces of the digit, which are both important factors in opening a jar. The contribution of each digit relative to the total twisting torque of the hand could be obtained via the apparatus. The intraclass correlation coefficient of the repeated measurements of the obtained forces and moments for different counterweights was approximately 0.96-1.00, which indicates that the reliability of the measured components of the apparatus is high. The high coefficient of determination (r2 > 0.99) indicates high accuracy of prediction of the measured values with respect to the expected loads. The validation outcomes support the design rationale and actual body part of the simulated jar. In addition, understanding the contribution of a single digit in opening a jar was also achieved via the apparatus and model.

Effects of age and gender on the movement workspace of the trapeziometacarpal joint.

While researchers have suggested that joint mobility would probably be affected by age and gender, research findings often present discrepancies. Little research has been performed on the factors which effect mobility of the trapeziometacarpal (TMC) joint. The purpose of this study was to address the effects of age and gender on the ranges of motion of the normal TMC joint. Eighty normal subjects divided into four age groups participated in this study. The TMC joint motions were recorded using an electromagnetic tracking system. In order to achieve a maximal range of TMC joint motion which was defined as the maximal workspace, each subject was asked to perform actively maximal circumduction, flexion-extension, and abduction-adduction of the TMC joint. Numerical and statistical methods were used to compute the TMC workspace and to detect significant differences. A workspace-to-length ratio was determined as an index to examine the effects of the age and gender on the joint mobility. The results demonstrated that age and gender had significant influences on the TMC workspace among the groups studied. The understanding of TMC joint mobility under different age and gender conditions is achieved through this study. The findings can be used to report clinical measures in the determination of the extent of impairment of osteoarthritis as well as the outcomes between pre- and post-surgical (or non-surgical) interventions.

Evaluation of emissions and exposures at workplaces using desktop 3-dimensional printer.

There is a paucity of data on additive manufacturing process emissions and personal exposures in real-world workplaces. Hence, we evaluated atmospheres in four workplaces utilizing desktop "3-dimensional" (3-d) printers [fused filament fabrication (FFF) and sheer] for production, prototyping, or research. Airborne particle diameter and number concentration and total volatile organic compound concentrations were measured using real-time instruments. Airborne particles and volatile organic compounds were collected using time-integrated sampling techniques for off-line analysis. Personal exposures for metals and volatile organic compounds were measured in the breathing zone of operators. All 3-d printers that were monitored released ultrafine and fine particles and organic vapors into workplace air. Particle number-based emission rates (#/min) ranged from 9.4 × 109 to 4.4 × 1011 (n = 9samples) for FFF3-d printers and from 1.9 to 3.8 × 109 (n = 2 samples) for a sheer 3-d printer. The large variability in emission rate values reflected variability from the printers as well as differences in printer design, operating conditions, and feedstock materials among printers. A custom-built ventilated enclosure evaluated at one facility was capable of reducing particle number and total organic chemical concentrations by 99.7% and 53.2%, respectively. Carbonyl compounds were detected in room air; however, none were specifically attributed to the 3-d printing process. Personal exposure to metals (aluminum, iron) and 12 different organic chemicals were all below applicable NIOSH Recommended Exposure Limit values, but results are not reflective of all possible exposure scenarios. More research is needed to understand 3-d printer emissions, exposures, and efficacy of engineering controls in occupational settings.

Effect of bilateral reaching on affected arm motor control in stroke--with and without loading on unaffected arm.

PURPOSE: To investigate the effect of bilateral reaching, with/without inertial loading on the unaffected arm, on hemiparetic arm motor control in stroke. METHODS: Twenty unilateral stroke patients were recruited. A three-dimensional optical motion capture system was used to measure the movement trajectory of the hemiparetic arm while performing three tasks: affected limb reaching forward; two-limb reaching forward; and two-limb reaching forward with inertia loading of 25% upper limb weight on the unaffected limb, respectively. Kinematical parameters were utilized to quantify the reaching performance of the affected arm. RESULTS: No matter whether loading was applied on the unaffected arm or not, the bilateral reaching task did not significantly facilitate smoother and faster movement. Furthermore, during bilateral reaching task with/without loading on the unaffected arm, stroke patients showed slower movement, lower maximal movement velocity, feedback control dominant and discontinuous movements in the affected arm than the same task with unilateral reaching. Subjects showed the greatest active upper extremity range of motion in proximal joints during the bilateral reaching task without unaffected arm loading. The amount of trunk movement also increased during bilateral reaching either with or without loading on the unaffected arm. Patients with moderate upper extremity motor impairment performed more discontinuous movements and less active elbow range of motion during bilateral reaching tasks; however, those with mild upper extremity motor impairment performed smoother movements and demonstrated greater active elbow range of motion during bilateral reaching tasks. CONCLUSIONS: Bilateral reaching tasks with/without loading on the unaffected arm could be considered as adding challenges during motor control training. Training with bilateral arm movements may be considered as a treatment strategy, and can be incorporated in stroke rehabilitation to facilitate greater arm active movement and improve motor control performance in the affected arm.

Movement of finger joints induced by synergistic wrist motion.

BACKGROUND: An experiment has recently been conducted to evaluate and compare the differences in tendon excursions between the flexor digitorum profundus and superficialis using three mobilization techniques. No previous studies deal with the total joint excursions with constant tendon length. The purpose of this study was to investigate the coordinated motion between the finger and wrist joints resulting from passive tension of the muscles while performing synergistic wrist motion. METHODS: The relative joint positions of the hand and wrist were measured using a three-dimensional motion analysis system with external retroreflective markers 2 mm in diameter placed on the dorsal surface of the hand. Fifty normal subjects, with a 1:1 gender ration, ranging in age from 20 to 40 years, and with no previous history of upper extremity injury, were recruited for the experiment. FINDINGS: The relationships of synergistic motion between the wrist and finger joints due to passive tension in the muscles were approximately linear. The ranges of wrist motion averaged 60 degrees extension and 60 degrees flexion. Moving the wrist from flexion into extension induced synergistic finger joint motion as follows: the distal interphalangeal joint angles changed from an average of 12 degrees of flexion to 31 degrees; proximal-interphalangeal joint angles changed from 19 degrees to 70 degrees; and metacarpal phalangeal joints changed from 27 degrees to 63 degrees of flexion. INTERPRETATION: The relationships of synergistic motion between the wrist and finger joints were systematically documented. Such a relationship could be considered in optimizing the design of dynamic splints used for rehabilitation in post-surgical tendon repair, as well as providing useful information about potential diagnoses of problems with the integrity of the flexor and extensor mechanisms.

Gait analysis after total hip arthroplasty with leg-length equalization in women with unilateral congenital complete dislocation of the hip--comparison with untreated patients.

Patients with Crowe Type-IV congenital dislocation of the hip (CDH) show significant clinical improvement after total hip arthroplasty (THA) because this surgery greatly reduces pain. Concomitant leg-length equalization in unilateral patients--a controversial procedure--theoretically should significantly improve these patients' ability to walk efficiently and comfortably. To understand the impact of leg-length equalization on these patients, we compared their gait parameters with those of untreated patients without pain but with leg-length discrepancy. Using a motion analysis system, three force platforms and computer calculation, the gait parameters during level walking of 22 women with unilateral Crowe Type-IV CDH were studied at an average of 58 months (27-98 months) following a successful cementless THA. The socket was placed in the best bone stock, which was close to the level of the true acetabulum. The leg-length discrepancy was equalized to within 2 cm in all patients. The Harris hip score averaged 94.8 (range, 88-100) at the time of the study. Nine women with untreated unilateral Crowe Type-IV CDH without major pain but with an average leg-length discrepancy of 4.7 cm (range, 2.5-6 cm) were also studied for comparison. The treated subjects (Group 1; THA and leg-length equalization) walked faster and had gait parameters with better bilateral symmetry than the untreated subjects (Group 2). We concluded that leg-length equalization in addition to THA in patients with unilateral Crowe Type-IV CDH significantly improves gait symmetry and efficiency.

Flexion-extension rhythm in the lumbosacral spine.

STUDY DESIGN AND OBJECTIVES: This study was conducted to depict the qualitative and quantitative changes of intervertebral rotation and translation from L1-L2 to L5-S1 during flexion, standing, and extension using dynamic lumbosacral radiographs. METHODS: A radiopaque ruler was placed on the back of each subject for the normalization of translational value. Eighty-nine volunteers were examined. RESULTS: From extension to flexion, all of the intervertebral rotations approached 0 degree from the lordotic position; the translations changed from slightly retro-listhetic to zero displacement. Using L3-L4 as a baseline for calculating the intervertebral differences in flexion, all of the rotational differences were less than 1.5 degrees, except at L5-S1, which remained 5 degrees. The mean translational difference was less than 0.6 mm, except at L5-S1, where it remained 1.5 mm. CONCLUSIONS: The amount of total flexibility was level-dependent and its frequency distribution is important. Qualitative rhythmic changes from extension to flexion and quantitative values of intervertebral difference in flexion help define the normal flexibility more accurately.

Gait analysis and energy consumption of below-knee amputees wearing three different prosthetic feet.

This study scientifically measures the dynamic gait characteristics and energy consumption of 16 male below-knee amputees, eight vascular and eight traumatic, while wearing solid ankle cushion heel (SACH), single axis and multiple axis prosthetic feet via six-camera motion analysis, metabolic measurement cart and heavy-duty treadmill. Subjective results are additionally determined via questionnaire after testing. Motion analysis showed statistically significant differences at P<0.05 between the SACH, single axis and multiple axis foot in the velocity, cadence, stride length and single limb stance. Significant differences were found in energy consumption between the traumatic and vascular groups, and significant changes in walking under different speeds and different inclines. Results provide quantitative and qualitative information about the dynamic performance of the various feet, which can be helpful in prescribing the optimal prosthetic foot for individual amputees.

Common abnormal kinetic patterns of the knee in gait in spastic diplegia of cerebral palsy.

We studied the kinetic characteristics of the knee in patients with spastic diplegia. Twenty three children with spastic diplegia were recruited and had their 46 limbs categorised into the following four groups: jump (n=7), crouch (n=8), recurvatum (n=14) and mild (n=17). In the crouch pattern, the patients usually had a larger and longer lasting internal knee extensor moments in stance suggesting that rectus femoris had a relatively high activation. In the recurvatum pattern, the internal knee flexor moment was large and long lasting in stance. The biceps femoris showed less activity on EMG although the knee flexor moment was large and we concluded that the soft tissue behind the knee joint provided this flexor moment. In the jump knee pattern there was abnormal power generation at the knee and ankle joints in initial stance, which did not contribute to normal progression but aided upward body motion. In the mild group the kinetic data was similar to that seen in normal children. Knowledge of kinetic patterns in these patients may help in their subsequent management.

Gait analysis after ankle arthrodesis.

The purpose of this study was to employ a computerized motion analysis system to identify the effect of ankle arthrodesis on the three-dimensional kinematic behavior of the rear and fore foot during level walking. A three-segment rigid body model was used to describe the motion of the foot and ankle. The results demonstrated that sagittal plane motion of the hindfoot was significantly decreased in the foot of patients having had ankle arthrodesis compared to normal subjects. The kinematic data indicated a generalized stiffness of the hindfoot on the involved foot in the sagittal plane. Sagittal plane movement in the forefoot and transverse plane movements in the hindfoot and forefoot increased in patients compared to controls.

Design and testing of a genetic algorithm neural network in the assessment of gait patterns.

It is important to be able to quantify changes in gait pattern accurately in order to understand the clinical implications of surgery or rehabilitation. Although supervised feed-forward backpropagation neural networks are very efficient in many pattern-recognition tasks, the genetic algorithm neural network (GANN), which can search in some appropriate space, has not been used previously for gait-pattern recognition. This study discusses how to use the GANN approach in gait-pattern recognition, and evaluates the complexity and training strategy of the particular classification problem. Both the GANN and a traditional artificial neural network (ANN) were used to classify the gait patterns of patients with ankle arthrodesis and normal subjects. The GANN model was able to classify subjects with recognition rates of up to 98.7%. In contrast, the ANN trained by using all possible predictor variables was only able to classify the subjects with recognition rates of 89.7%. It is suggested that the GANN model is more suitable to exploit the patient's gait pattern. The value of the neuron output can be used as an index of the difference from normal. By this means, all pathological gait patterns may be presented quantitatively.

Frequency spectral characteristics of standing balance in children and young adults.

Past research has shown that spectral frequency characteristics of a balance control system may be useful for early detection of minor changes in the system. Since there is a lack of information regarding the frequency spectrum of children's balance control system, this study was undertaken to investigate and compare the spectral frequency characteristics of standing balance control between children and young adults under altered sensory environments. Seventeen children (9 females, 8 males, mean age 7.8+/-0.9 years) and the same number of female and male young adults (mean age 21.1+/-1.3 years) were tested for standing balance under six sensory conditions. These conditions were created by crossing the three levels of the visual factor (open eye, closed eye, sway-referenced vision) with the two levels of the somatosensory factor (fixed foot support, compliant foot support). The median spectral frequencies of the shear forces in the anterior-posterior (A/P) and the medial-lateral (M/L) directions were used as dependent variables. The results showed that children had higher median spectral frequency of the A/P shear force than young adults and this difference was not affected by the somatosensory factor. The median spectral frequency in the M/L direction was not different between the groups. The higher rate of body mass vibration of children in the A/P direction implies that children may not have fully developed the ankle strategy for maintaining standing balance, but nonetheless have developed the same efficiency of using vision for their reference of standing balance.