Setting Expansion Characteristics of Three Phosphate-Bonded Investment Materials Used with High Fusing High-Noble Alloys, Cobalt-Chrome Alloys and Heat-Pressed Lithium Disilicate Ceramics

The purpose of this study was to investigate the setting expansion characteristics of three commercially available phosphate bonded investment materials for casting high fusing alloys and heat pressed lithium disilicate. The experimental groups in this study were P90 (Powercast 90% Special Liquid for Cobalt-Chrome alloys), P60 (Powercast 60% Special Liquid for High Noble Gold alloys), FF75 (FastFire 15 75% Special Liquid for Cobalt-Chrome alloys), FF50 (FastFire 15 50% Special Liquid for High Noble Gold alloys), and PVS (PressVest Speed 60% Special Liquid for Lithium Disilicate Veneers, Partial Crowns and Single Crowns). Twenty specimens per group were poured in a trough that conformed to ADA Specification No. 2 for the measurement of the linear setting expansion of gypsum bonded investments. Measurements of the setting expansion were taken at 2, 4, 6, 8, 12, and 24 hours after mixing. A one sample T-Test revealed that the setting expansion measured for all of the groups was statistically significant and that all of the groups exhibited statistically significant differences in setting expansion at the manufacturer’s recommended burn-out time from the setting expansion reported by the manufacturer (P≤0.01). P90, P60, FF50 and PVS showed less expansion than is required in order to fully compensate for solidification shrinkage during the casting procedures. FF75 was the only group that managed to fully compensate for the solidification shrinkage of the alloy it is intended for (P≤0.01). The delayed burn-out times evaluated in this study resulted in significantly different setting expansion for PVS. A statistically significant difference was detected after 4 hours. The dimension of the PVS specimens did not change significantly after 4 hours. No such difference in setting expansion at delayed burn-out times could be detected for P90, P60, FF75 & FF50 (P≤0.01). Within the limitations of this study, the setting expansion of the phosphate bonded investment materials tested could not fully compensate for the solidification shrinkage of the alloys except for FF75. Delayed burn-out times resulted in significantly different setting expansion for PVS. Significant differences at delayed burn-out times could not be detected for the rest of the groups

Differences between joint-space and musculoskeletal estimations of metabolic rate time profiles

Motion capture laboratories can measure multiple variables at high frame rates, but we can only measure the average metabolic rate of a stride using respiratory measurements. Biomechanical simulations with equations for calculating metabolic rate can estimate the time profile of metabolic rate within the stride cycle. A variety of methods and metabolic equations have been proposed, including metabolic time profile estimations based on joint parameters. It is unclear whether differences in estimations are due to differences in experimental data or due to methodological differences. This study aimed to compare two methods for estimating the time profile of metabolic rate, within a single dataset. Knowledge about the consistency of different methods could be useful for applications such as detecting which part of the gait cycle causes increased metabolic cost in patients. Here we compare estimations of metabolic rate time profiles using a musculoskeletal and a joint-space method. The musculoskeletal method was driven by kinematics and electromyography data and used muscle metabolic rate equations, whereas the joint-space method used metabolic rate equations based on joint parameters. Both estimations of changes in stride average metabolic rate correlated significantly with large changes in indirect calorimetry from walking on different grades showing that both methods accurately track changes. However, estimations of changes in stride average metabolic rate did not correlate significantly with more subtle changes in indirect calorimetry due to walking with different shoe inclinations, and both the musculoskeletal and joint-space time profile estimations did not correlate significantly with each other except in the most downward shoe inclination. Estimations of the relative cost of stance and swing matched well with previous simulations with similar methods and estimations from experimental perturbations. Rich experimental datasets could further advance time profile estimations. This knowledge could be useful to develop therapies and assistive devices that target the least metabolically economic part of the gait cycle

Altering gait variability with an ankle exoskeleton

Exoskeletons can influence human gait. A healthy gait is characterized by a certain amount of variability compared to a non-healthy gait that has more inherent variability; however which exoskeleton assistance parameters are necessary to avoid increasing gait variability or to potentially lower gait variability below that of unassisted walking are unknown. This study investigated the interaction effects of exoskeleton timing and power on gait variability. Ten healthy participants walked on a treadmill with bilateral ankle-foot exoskeletons under ten conditions with different timing (varied from 36% to 54% of the stride) and power (varied from 0.2 to 0.5 W∙kg-1) combinations. We used the largest Lyapunov exponent (LyE) and maximum Floquet multiplier (FM) to evaluate the stride-to-stride fluctuations of the kinematic time series. We found the lowest LyE at the ankle and a significant reduction versus powered-off with exoskeleton power (summed for both legs) of 0.45 W∙kg-1 and actuation timing at 48% of the stride cycle. At the knee, a significant positive effect of power and a negative interaction effect of power and timing were found for LyE. We found significant positive interaction effects of the square of timing and power for LyE at the knee and hip joints. In contrast, the FM at the ankle increased with increasing power and later timing. We found a significant negative effect of power and a positive interaction effect of power and timing for FM at the knee and no significant effects of any of the exoskeleton parameters for FM at the hip. The ability of the exoskeleton to reduce the LyE at the ankle joint offers new possibilities in terms of altering gait variability, which could have applications for using exoskeletons as rehabilitation devices. Further efforts could examine if it is possible to simultaneously reduce the LyE and FM at one or more lower limb joints

Altering gait variability with an ankle exoskeleton

Exoskeletons can influence human gait. A healthy gait is characterized by a certain amount of variability compared to a non-healthy gait that has more inherent variability; however which exoskeleton assistance parameters are necessary to avoid increasing gait variability or to potentially lower gait variability below that of unassisted walking are unknown. This study investigated the interaction effects of exoskeleton timing and power on gait variability. Ten healthy participants walked on a treadmill with bilateral ankle-foot exoskeletons under ten conditions with different timing (varied from 36% to 54% of the stride) and power (varied from 0.2 to 0.5 W∙kg-1) combinations. We used the largest Lyapunov exponent (LyE) and maximum Floquet multiplier (FM) to evaluate the stride-to-stride fluctuations of the kinematic time series. We found the lowest LyE at the ankle and a significant reduction versus powered-off with exoskeleton power (summed for both legs) of 0.45 W∙kg-1 and actuation timing at 48% of the stride cycle. At the knee, a significant positive effect of power and a negative interaction effect of power and timing were found for LyE. We found significant positive interaction effects of the square of timing and power for LyE at the knee and hip joints. In contrast, the FM at the ankle increased with increasing power and later timing. We found a significant negative effect of power and a positive interaction effect of power and timing for FM at the knee and no significant effects of any of the exoskeleton parameters for FM at the hip. The ability of the exoskeleton to reduce the LyE at the ankle joint offers new possibilities in terms of altering gait variability, which could have applications for using exoskeletons as rehabilitation devices. Further efforts could examine if it is possible to simultaneously reduce the LyE and FM at one or more lower limb joints

Modular footwear that partially offsets downhill or uphill grades minimizes the metabolic cost of human walking

Walking on different grades becomes challenging on energetic and muscular levels compared to level walking. While it is not possible to eliminate the cost of raising or lowering the centre of mass (COM), it could be possible to minimize the cost of distal joints with shoes that offset downhill or uphill grades. We investigated the effects of shoe outsole geometry in 10 participants walking at 1 m s−1 on downhill, level and uphill grades. Level shoes minimized metabolic rate during level walking (Psecond-order effect \u3c 0.001). However, shoes that entirely offset the (overall) treadmill grade did not minimize the metabolic rate of walking on grades: shoes with a +3° (upward) inclination minimized metabolic rate during downhill walking on a −6° grade, and shoes with a −3° (downward) inclination minimized metabolic rate during uphill walking on a +6° grade (P interaction effect = 0.023). Shoe inclination influenced (distal) ankle joint parameters, including soleus muscle activity, ankle moment and work rate, whereas treadmill grade influenced (whole-body) ground reaction force and COM work rate as well as (distal) ankle joint parameters including tibialis anterior and plantarflexor muscle activity, ankle moment and work rate. Similar modular footwear could be used to minimize joint loads or assist with walking on rolling terrain

Stride-time variability is related to sensorimotor cortical activation during forward and backward walking

Previous research has used functional near-infrared spectroscopy (fNIRS) to show that motor areas of the cortex are activated more while walking backward compared to walking forward. It is also known that head movement creates motion artifacts in fNIRS data. The aim of this study was to investigate cortical activation during forward and backward walking, while also measuring head movement. We hypothesized that greater activation in motor areas while walking backward would be concurrent with increased head movement. Participants performed forward and backward walking on a treadmill. Participants wore motion capture markers on their head to quantify head movement and pressure sensors on their feet to calculate stride-time. fNIRS was placed over motor areas of the cortex to measure cortical activation. Measurements were compared for forward and backward walking conditions. No significant differences in body movement or head movement were observed between forward and backward walking conditions, suggesting that conditional differences in movement did not influence fNIRS results. Stride-time was significantly shorter during backward walking than during forward walking, but not more variable. There were no differences in activation for motor areas of the cortex when outliers were removed. However, there was a positive correlation between stride-time variability and activation in the primary motor cortex. This positive correlation between motor cortex activation and stride-time variability suggests that forward walking variability may be represented in the primary motor cortex

Metabolically efficient walking assistance using optimized timed forces at the waist

The metabolic rate of walking can be reduced by applying a constant forward force at the center of mass. It has been shown that the metabolically optimal constant force magnitude minimizes propulsion ground reaction force at the expense of increased braking. This led to the hypothesis that selectively assisting propulsion could lead to greater benefits. We used a robotic waist tether to evaluate the effects of forward forces with different timings and magnitudes. Here, we show that it is possible to reduce the metabolic rate of healthy participants by 48% with a greater efficiency ratio of metabolic cost reduction per unit of net aiding work compared with other assistive robots. This result was obtained using a sinusoidal force profile with peak timing during the middle of the double support. The same timing could also reduce the metabolic rate in patients with peripheral artery disease. A model explains that the optimal force profile accelerates the center of mass into the inverted pendulum movement during single support. Contrary to the hypothesis, the optimal force timing did not entirely coincide with propulsion. Within the field of wearable robotics, there is a trend to use devices to mimic biological torque or force profiles. Such bioinspired actuation can have relevant benefits; however, our results demonstrate that this is not necessarily optimal for reducing metabolic rate

Design and development of a semi-rigid hip exoskeleton to reduce metabolic cost

Robotic exoskeletons can reduce metabolic cost in healthy individuals and restore mobility in patients with peripheral artery disease (PAD). PAD is a cardiovascular disease produced by atherosclerosis of the leg arteries. The primary symptom of PAD is claudication or pain in the legs during walking, which severely shortens the distance a patient can walk. Knowing that up to 40% of the metabolic cost of walking comes from the hip muscles, different groups have been developing rigid exoskeletons and soft exosuits that assist the hip. Assisting at the hip has the advantage that the exoskeleton mass is positioned close to the center of mass, which minimizes the energy cost of the added mass. Soft exosuits have the advantage that they allow greater freedom of movement. However, soft exosuits often cannot apply the same torque magnitudes as rigid exoskeletons, and they rely on friction with the skin to remain anchored. The purpose of this work was to develop a semi-rigid hip exoskeleton that can connect to and be powered by an existing actuation unit, to address the limitations of current existing soft exosuits. We evaluated the device performance by analyzing the match between desired and actual torque applied to the hip joint. Our exoskeleton\u27s semi-rigid design introduces advantages in comfort and efficiency of control in patients with PAD because it requires less friction and compression than soft exosuits. Our initial work demonstrates a good match between the desired and actual torque that the exoskeleton was able to generate for each leg

HOW CAN ACTUATION TIMING AND MAGNITUDE OF A BILATERAL SEMI-RIGID HIP EXOSKELETON OPTIMIZE METABOLIC COST

Semi-rigid exoskeletons could combine some advantages of rigid and soft approaches. The purpose of this study was to investigate the effects of timing and magnitude of assistance from a semi-rigid hip exoskeleton. For ten participants, we tested ten conditions that were combinations of 5 different end-timings, ranging from 21% to 49%, and 2 different moment magnitudes ranging from 0.06 to 0.12 Nm.kg-1. The participants walked in two reference conditions: a condition without actuation and a condition without the exoskeleton. A semi-rigid hip exoskeleton could alter metabolic rate. However, to produce a net assistive effect, it is necessary to design a lighter, more conforming device. In both actuation magnitude levels, the optimal end-timing was close to the maximum range, similar to findings from another study with human-in-the-loop optimization of a soft hip exosuit. This could indicate that the optimal timing with a semi-rigid device is not very different from a fully-soft prototype

Effects of Ankle Exoskeleton Power and Actuation Timing on Movement Variability and Metabolic Cost of Walking

Walking is one of the most primal movements and an essential part of everyday life. While humans use many strategies to reduce metabolic energy cost, walking still requires a considerable amount of metabolic energy. Lower extremity exoskeletons have become an established technology designed to reduce metabolic cost. Recently, it was reported that they might increase the variability of the locomotor system making the system more noisy and unstable, but optimal assistance properties for gait variability remain unclear. This could be a main concern for people with a mobility disorder. We investigated the effect of ankle exoskeleton power and actuation timing on gait variability and metabolic energy cost of walking. Data was collected for ten healthy participants wearing a powered ankle-foot exoskeleton during a 4-minute treadmill walking trial at 1.25 m∙s-1 in ten different assistance conditions. Largest Lyapunov exponent (LyE) was calculated to quantify the pattern of stride-to-stride fluctuations of the ankle angle kinematics. The metabolic rate and LyE were compared between conditions. Optimal assistance was achieved at 42% of the stride and average power of 0.4 W∙kg-1 for both the LyE and metabolic rate. This resulted to a 50% lower LyE and a 21% reduction in metabolic cost compared to walking with the exoskeleton deactivated. These results emphasize the importance of optimizing exoskeleton actuation properties to provide a more stable and metabolic efficient human locomotor system