Frequency analysis of ankle joint quasi-stiffness during quiet unperturbed standing in Chiari Malformation.

Chiari Malformation (Chiari) is a congenital condition occurring from an inferior herniation of the cerebellar tonsils into the foramen magnum. Given the role of the cerebellum in postural control, it is reasonable to expect joint motion to be affected in this patient population. In fact, joint stiffness is a common self-reported symptom of Chiari, however it has never been assessed in these individuals. This study aimed to examine if ankle joint quasi-stiffness is correlated with Chiari severity. The human body was considered as an inverted oscillating pendulum without damping. A Fast Fourier Transform was used to extract natural frequency from the center of pressure trajectories during upright standing. Ankle joint quasi-stiffness was then calculated using the relationship between natural frequency and moment of inertia. Twelve Chiari participants (Chiari), six with decompression surgery (Chiari-D) and six without (Chiari-ND), and eight control individuals (Control) participated. Participants completed three, 30-second quiet standing trials on a force plate, focused on a target three meters in front of them. Chiari, regardless of surgery, had significantly lower quasi-stiffness than controls (Chiari-D vs. Control p = 0.0011, Chiari-ND vs. Control, p < 0.001). The proposed method is advantageous as it incorporates the entire center of pressure signal, minimizes error from instantaneous muscular dynamics, and does not require motion capture.

Effects of ultrasound settings on temperature changes in NiTi implants.

BACKGROUND: Shape memory alloys (SMAs) are well-known for their unique ability to undergo a shape change in response to a thermal stimulus. A frequently-used SMA for biomedical devices is NiTi, although its superelastic features tend to be emphasized more than the ability to change shape. Minimally invasive NiTi implants which can reconfigure or adjust their shape across several temperature points could provide desirable surgical outcomes. For decades, therapeutic ultrasound has been used medically as a non-invasive method for tissue thermal therapy. Ultrasound's ability to quickly raise temperatures, and transcutaneously activate shape changes in NiTi implants is a novel approach for eliciting the martensitic thermoelastic transformation. METHODS: The purpose of this study was to investigate the features of therapeutic ultrasound that correspond with temperature changes in different NiTi specimens. For this purpose, ultrasound was applied to two NiTi specimens for two minutes each at varying low- and high-frequency and power settings using a Sonicator 740 and a Dynatron 150. FINDINGS: The baseline temperature for all 32 trials was room temperature (23.0 ± 1.7°C). This study successfully increased the specimen temperature with the application of Sonicator 740 and Dynatron 150 therapeutic ultrasound machines (2.2 ± 2.4°C and 1.5 ± 1.15°C, respectively). From the statistical analyses of the experimental data, it was clear that there is a significant difference between low- and high-power settings on mean temperature change using the Dynatron 150 (ANCOVA; p = 0.013). Interpretation Of clinical relevance, NiTi implants can quickly and easily increase in temperature when applying therapeutic ultrasound. Ultrasound power causes temperature changes and should be accounted for when designing orthopedic implants for applications where dimensional changes are desirable.

Comparison of side-cutting maneuvers versus low impact baseball swing on knee ligament loading in adolescent populations.

BACKGROUND: High impact sports are associated with an increased incidence rate for knee ligament injuries, specifically pertaining to the anterior cruciate ligament and medial collateral ligament. What is less clear is (i) the extent to which high impact activities preferentially load the anterior cruciate ligament versus the medial collateral ligament, and (ii) whether both ligaments experience similar stretch ratios during high loading scenarios. Therefore, the goal of this project was to assess how different loading conditions experienced through more at-risk sporting maneuvers influence the relative displacements of the anterior cruciate ligament and medial collateral ligament. The focus of the study was on adolescent patients - a group that has largely been overlooked when studying knee ligament biomechanics. METHODS: Through kinetic knee data obtained through motion capture experimentation, two different loading conditions (high vs low impact) were applied to 22 specimen-specific adolescent finite element knee models to investigate the biomechanical impact various sporting maneuvers place on the knee ligaments. FINDINGS: The high impact side cutting maneuver resulted in 102% and 47% increases in ligament displacement compared to the low impact baseball swing (p < 0.05) for both the anterior cruciate ligament and medial collateral ligament. INTERPRETATION: Quantifying biomechanical risks that sporting activities place on adolescent subjects provides physicians with insight into knee ligament vulnerability. More specifically, knowing the risks that various sports place on ligaments helps guide the selection of sports for at-risk patients (especially those who have undergone knee ligament surgery).

Diabetic Foot Considerations Related to Plantar Pressures and Shear.

Diabetic foot ulcers are a complex, multifaceted, and widespread complication of diabetes mellitus. Although there are a multitude of risk factors contributing to diabetic foot ulcer development, pressure and (more recently) shear stresses are two biomechanical metrics that are gaining popularity for monitoring risk factors predisposing skin breakdown. Other areas of diabetic foot ulcers under research include plantar temperature measuring, as well as monitoring wear-time compliance and machine learning/AI algorithms. Charcot arthropathy is another diabetes complication that has a relationship with diabetic foot ulcer development, which should be monitored for development alongside ulcer development. The ability to monitor and prevent diabetic foot ulcer development and Charcot neuroarthropathy will lead to increased patient outcomes and patient quality of life.

The effect of frictional coefficients and sock material on plantar surface shear stress measurement.

At present, there are no viable systems that can acquire in-shoe measurement of distributed shear forces. Foot-shoe interactions are such that skin shear is a notoriously difficult quantity to measure under the best of conditions. This is further complicated by the presence of forces normal to the skin surface that are large compared to the shear forces, which often results in crosstalk between pressure and shear signals. The present study used multibody dynamic simulations to investigate the combined effects of (i) coefficient of friction (COF) at skin-sock and sock-sensor interfaces, as well as (ii) sock stiffness on the accuracy of measured shear against the skin. These factors were systematically altered within a wide range (COF: 0.04, 0.34, 0.54, and 0.9; sock stiffness: 100, 250, 500, 1000, 1500 and 2000 N/m) to simulate a total of 96 scenarios. The correlation between the shear at the skin and at the sensor was used to compare each set of conditions. The results indicated that a high COF at the sock-sensor interface and a low sock stiffness would individually result in a significantly higher accuracy of shear measurements (p < 0.001). A low COF at the skin-sock interface was observed to reduce the occurred shear against the skin up to a factor of five, with very minimal effect on the accuracy of shear measurements (p = 0.98). These findings allow researchers to understand the potential effects of (i) sock stiffness, and (ii) coefficients of friction, on skin shear, and potentially correct for the effects of interface materials when trying to determine shear at the skin-sock interface.

Assessing the biomechanical properties of nitinol staples in normal, osteopenic and osteoporotic bone models: A finite element analysis.

OBJECTIVE: Bone staples are internal fixation devices that are frequently used in the foot, ankle, and hand to provide stabilization. Fixation stability is vital after fusion or fracture surgeries to ensure proper bone healing. Patients undergoing surgeries that require fixation to keep bones aligned and stable may present with diminishing bone mechanical properties, and this may compromise the ability of the fixation hardware to maintain a stable construct. The purpose of this study was to investigate the mechanical performance of shape memory and superelastic nitinol bone staples with different bridge geometries in normal, osteopenic, and osteoporotic bone models. Contact forces and maximum principal stress and strain in the bone were recorded. METHODS: Finite element simulations of a bone staple fixation procedure were performed to examine the initial and post-surgery contact force, as well as the maximum principal stress and strain of 15 mm bridge and 20 mm bridge staple-bone constructs. RESULTS: Shape memory nitinol staples exhibited higher contact forces compared to superelastic nitinol staples. Nitinol bone staples with 20 mm bridge lengths displayed higher contact forces and lower stresses in all bone types, as well as lower strains in osteoporotic bone models compared to nitinol staples with a 15 mm bridge length. CONCLUSION: Nitinol bone staple constructs with 20 mm bridge length staples provide higher contact forces and display lower stresses in the bone than 15 mm bridge staple-bone constructs, which may be beneficial in bone with diminishing mechanical properties. Both superelastic and shape memory effect nitinol staples provide adequate compression and stress relief. However, if osteopenia is present, shape memory effect nitinol staples with a 20 mm bridge length may provide more stress relief and compression, if the bone anatomy allows.

Examining feedback mechanisms of postural control in Chiari Malformation by average wavelet coefficient decomposition and the Hurst exponent.

BACKGROUND: Chiari Malformation (CM) is a congenital disorder occurring when the cerebellar tonsils descend into the foramen magnum, inhibiting cerebrospinal fluid (CSF) flow, causing headaches, dizziness, difficulty swallowing, muscle weakness, and loss of neuromuscular coordination. While there is no cure, surgical decompression of the hindbrain is used to alleviate symptoms. Loss of postural control is a main symptom reported by these patients; however, no study has examined postural stability in this cohort of patients. RESEARCH QUESTION: Do patients with CM exhibit impaired postural stability compared to healthy controls?. METHODS: Twelve female participants diagnosed with CM performed a postural stability test where six participants had undergone decompression (CM-D) surgery while six had not (CM-ND). Participants stood in Romberg fashion on an AMTI force plate according to an IRB-approved protocol. Postural stability measures were quantified by computing Hurst exponents. These values were determined from the Average Wavelet Coefficient method using a level 12 Symlet-2 wavelet to analyze anterior-posterior (AP) center-ofpressure (COP) trajectories in MATLAB. Identical procedures and analyses were performed on healthy control participants with no known neuromuscular disorders. RESULTS: CM participants displayed significantly impaired postural stability compared to healthy controls (p = 0.0002). CM-D participants displayed significantly impaired postural stability compared to CM-ND (p = 0.002). CM-D and CM-ND both displayed significantly impaired postural stability compared to controls (p < 0.0001 and p < 0.003, respectively). SIGNIFICANCE: Loss of postural stability is considered a main symptom of CM, however no study has previously quantified human postural control in this cohort of patients. Quantifying this relationship can provide further insight to neurologists studying the disorder and to therapists planning rehabilitation and pain relief methods.

Experimental thermal analysis of a novel prosthetic socket along with silicone and PCM liners.

Prosthetic liners and sockets insulate a residual limb, causing excessive heat, sweating, skin irritation and maceration. Circulation of a fluid through the socket wall has been shown to have positive cooling effects on the internal surface of the socket, i.e. skin temperature. Moreover, Phase Change Materials (PCMs) have been recognized as a practical method for cooling garments. These materials, such as water or many synthesized polymers, have a high latent heat and their application within a prosthetic liner allows for absorbing heat from the limb for retaining a constant temperature. In this study, a novel prosthetic socket has been designed and prototyped to investigate the interactive effects of fluid circulation and PCM materials on thermal comfort of prosthetic sockets. The results indicate a statistically significant difference (p-value < 0.001) in the duration a PCM liner can retain the appropriate skin temperature, compared to regular silicone liners. Likewise, the presence of air circulation within the socket wall was shown to have statistically significant influences (p-value = 0.018) on providing the efficient cooling effects compared to regular sockets. Hence, incorporating circulation cooling mechanisms along with PCM liners as proposed in this study holds a promising solution to enhance the thermal comfort of prosthetic socket systems.

Spatial frequency content of plantar pressure and shear profiles for diabetic and non-diabetic subjects.

How high does pressure and shear stress sensor resolution need to be in order to reliably measure the plantar pressure and shear profiles (PPSPs) under normal and diabetic feet? In this study, pressure and shear stress data were collected from 26 total diabetic and control subjects using new instrumentation that measures vertical and horizontal force vectors of the plantar contact surface during multiple instances in the gait cycle. The custom built shear-and-pressure-evaluating-camera-system (SPECS) performs simultaneous recordings of pressure and both components of the horizontal force vector (medio-lateral and antero-posterior) at distinctive regions under one׳s foot, at a spatial resolution for each sensor equal to 1.6mm by 1.6mm. A linear interpolation method was used to simulate the effect of increasing sensor size on PPSPs. Ten square-shaped sensors were included in the analysis, having edge lengths of: (1.6mm, 3.2mm, 4.8mm, 6.4mm, 8mm, 9.6mm, 11.2mm, 12.8mm, 14.4mm, and 16mm). A two-dimensional Discrete Fourier Transform was performed on each data set, for each of the ten sensor sizes. To quantify the difference between sensor sizes, a comparison was made using the maximum pressure and shear stress data over the entire plantar contact surface, equivalent to the peak of the spatial frequency spectrum. A reduction of 5% of any component of the stress vector (i.e., pressure, or medio-lateral shear stress, or anter-posterior shear stress) due to an increase in sensor size was deemed significant. The results showed that a sensor measuring 9.6mm by 9.6mm caused meaningful reductions in all three stress components (p<0.001), whereas sensors measuring 1.6mm by 1.6mm, up to 4.8mm by 4.8mm, can capture the full range of spatial frequencies in both pressure and shear stress data.

Expanded butterfly plots: A new method to analyze simultaneous pressure and shear on the plantar skin surface during gait.

The current method of visualizing pressure and shear data under a subject's foot during gait is the Pedotti, or "butterfly" diagram. This method of force platform data visualization was introduced in the 1970s to display the projection of the ground reaction force vector in the sagittal plane. The purpose of the current study was to examine individual sub-components of the vectors displayed in Pedotti diagrams, in order to better understand the relationship between one foot region and another. For this, new instrumentation was used that allows multiple Pedotti diagrams to be constructed at any instant during the gait cycle. The custom built shear-and-pressure-evaluating camera system (SPECS) allows for simultaneous recordings of pressure and both components of the horizontal force vector (medio-lateral and antero-posterior) at distinctive regions under one's foot during gait. Data analysis of such recordings affirms three conclusions: (i) pressure and shear values on individual sites on the plantar surface of the foot are not associated in a linear manner, (ii) force vectors in the heel and forefoot regions exhibit horizontal force components that oppose one another, and similarly, (iii) force vectors in the frontal plane transecting the forefoot region also exhibit medial-lateral shear components that counteract one another. This approach sheds light on individual vectors that collectively sum to each vector displayed in a Pedotti diagram. The results indicate that shearing between the foot and the ground is not simply a passive event. The structures of the arches and/or muscular activities are major contributors to the observed interfacial stresses.

Design of a novel prosthetic socket: assessment of the thermal performance.

Prosthetic liners and sockets insulate the residual limb, causing excessive sweating and concomitant skin maceration. When coupled with atypical loading conditions, further dermatologic problems can arise. This can significantly reduce the quality of life of an amputee patient. Improving the design of the prosthetic socket has been proposed as a means of reestablishing a normal thermal environment around the residual limb. In this study, a prosthetic socket was modified by incorporating a helical cooling channel within the socket wall using additive manufacturing techniques. Two sockets were modeled: a control socket, and a modified socket containing a 0.48 cm diameter cooling channel. Computer simulations and bench-top testing were used to assess the design's ability to create a greater temperature differential across the socket wall. A greater temperature drop across the socket wall suggested that the socket could provide cooling benefits to the residual limb by allowing for heat to be drawn away from the limb. The temperature difference across the socket wall was calculated for both sockets in each aspect of the study. Both socket type (p=0.002) and location on the socket (p=0.014) were statistically significant factors affecting the temperature difference between inner and outer socket walls. Compared with the control socket, the modified socket containing a helical cooling channel exhibited greater temperature differences across its wall of 11.1 °C and 6.4 °C in the computer simulations and bench-top testing, respectively. This finding suggested that socket modifications, such as the cooling channel presented, could provide a beneficial cooling effect to an amputee patient's residual limb.

Temperature as a predictive tool for plantar triaxial loading.

Diabetic foot ulcers are caused by moderate repetitive plantar stresses in the presence of peripheral neuropathy. In severe cases, the development of these foot ulcers can lead to lower extremity amputations. Plantar pressure measurements have been considered a capable predictor of ulceration sites in the past, but some investigations have pointed out inconsistencies when solely relying on this method. The other component of ground reaction forces/stresses, shear, has been understudied due to a lack of adequate equipment. Recent articles reported the potential clinical significance of shear in diabetic ulcer etiology. With the lack of adequate tools, plantar temperature has been used as an alternative method for determining plantar triaxial loading and/or shear. However, this method has not been previously validated. The purpose of this study was to analyze the potential association between exercise-induced plantar temperature increase and plantar stresses. Thirteen healthy individuals walked on a treadmill for 10 minutes at 3.2km/h. Pre and post-exercise temperature profiles were obtained with a thermal camera. Plantar triaxial stresses were quantified with a custom-built stress plate. A statistically significant correlation was observed between peak shear stress (PSS) and temperature increase (r=0.78), but not between peak resultant stress (PRS) and temperature increase (r=0.46). Plantar temperature increase could predict the location of PSS and PRS in 23% and 39% of the subjects, respectively. Only a moderate linear relationship was established between triaxial plantar stresses and walking-induced temperature increase. Future research will investigate the value of nonlinear models in predicting plantar loading through foot temperature.

Active functional stiffness of the knee joint during activities of daily living: a parameter for improved design of prosthetic limbs.

BACKGROUND: Exploring knee joint physiological functional stiffness is crucial for improving the design of prosthetic legs that aim to mimic normal gait. This study hypothesizes that knee joint stiffness varies among different activities of daily living, additionally while the knee performs natural movements; the magnitude of the stiffness indicates the degree of energy storage element sufficiency in terms of harvesting/returning energy. METHODS: This study examined sagittal plane knee moment vs. knee flexion angle curves from 12 able-bodied subjects during activities of daily living. Slopes of these curves were assessed to find the calculated stiffness during the peak energy return and harvest phases so that the activities, which can be performed when the prosthetic knee is supplemented by a spring, were identified. FINDINGS: For the energy return and harvest phases, the stiffness varied between 0.006 and 0.046 Nm/kg deg. and 0 and 0.052 Nm/kg deg. respectively. The optimum energy return phase stiffness was 0.024 (SD 0.013) Nm/kg deg. and energy harvest phase stiffness was 0.011 (SD 0.018) Nm/kg deg. INTERPRETATION: Knee joint stiffness varied significantly during activities of daily living, which indicated that a storage unit with a constant stiffness would not be sufficient in providing energy regenerative gait during all activities. However, by controlling the amount and timing of spring compression and release, an energy-regenerative prosthetic knee device could be developed during most of the activities. This study was directed to the development of a complete data set, which determined the torque-angle properties of the healthy knee joint.

Modeling and optimal control of an energy-storing prosthetic knee.

Advanced prosthetic knees for transfemoral amputees are currently based on controlled damper mechanisms. Such devices require little energy to operate, but can only produce negative or zero joint power, while normal knee joint function requires alternative phases of positive and negative work. The inability to generate positive work may limit the user's functional capabilities, may cause undesirable adaptive behavior, and may contribute to excessive metabolic energy cost for locomotion. In order to overcome these problems, we present a novel concept for an energy-storing prosthetic knee, consisting of a rotary hydraulic actuator, two valves, and a spring-loaded hydraulic accumulator. In this paper, performance of the proposed device will be assessed by computational modeling and by simulation of functional activities. A computational model of the hydraulic system was developed, with methods to obtain optimal valve control patterns for any given activity. The objective function for optimal control was based on tracking of joint angles, tracking of joint moments, and the energy cost of operating the valves. Optimal control solutions were obtained, based on data collected from three subjects during walking, running, and a sit-stand-sit cycle. Optimal control simulations showed that the proposed device allows near-normal knee function during all three activities, provided that the accumulator stiffness was tuned to each activity. When the energy storage mechanism was turned off in the simulations, the system functioned as a controlled damper device and optimal control results were similar to literature data on human performance with such devices. When the accumulator stiffness was tuned to walking, simulated performance for the other activities was sub-optimal but still better than with a controlled damper. We conclude that the energy-storing knee concept is valid for the three activities studied, that modeling and optimal control can assist the design process, and that further studies using human subjects are justified.

Exercise equipment used in microgravity: challenges and opportunities.

A variety of physiological changes are experienced by astronauts during both short- and long-duration space missions. These include space motion sickness, spatial disorientation, orthostatic hypotension, muscle atrophy, bone demineralization, increased cancer risk, and a compromised immune system. This review focuses on countermeasures used to moderate these changes, particularly exercise devices that have been used by National Aeronautics and Space Administration astronauts over the past six decades as countermeasures to muscle atrophy and bone loss. The use of these devices clearly has shown that a microgravity environment places unusual demands on both the equipment and the human users. While it is of paramount importance to overcome microgravity-induced musculoskeletal deconditioning, it also is imperative that the exercise system (i) is small and lightweight, (ii) does not require an external power source, (iii) produces 1g-like benefits to both bones and muscles, (iv) requires relatively short durations of exercise, and (v) does not affect the surrounding structure or environment negatively through noise and/or induced vibrations.

Simulation of lower limb axial arterial length change during locomotion.

The effect of external forces on axial arterial wall mechanics has conventionally been regarded as secondary to hemodynamic influences. However, arteries are similar to muscles in terms of the manner in which they traverse joints, and their three-dimensional geometrical requirements for joint motion. This study considers axial arterial shortening and elongation due to motion of the lower extremity during gait, ascending stairs, and sitting-to-standing motion. Arterial length change was simulated by means of a graphics based anatomic and kinematic model of the lower extremity. This model estimated the axial shortening to be as much as 23% for the femoropopliteal arterial region and as much as 21% for the iliac artery. A strong correlation was observed between femoropopliteal artery shortening and maximum knee flexion angle (r²=0.8) as well as iliac artery shortening and maximum hip angle flexion (r²=0.9). This implies a significant mechanical influence of locomotion on arterial behavior in addition to hemodynamics factors. Vascular tissue has high demands for axial compliance that should be considered in the pathology of atherosclerosis and the design of vascular implants.

Spatial relationships between shearing stresses and pressure on the plantar skin surface during gait.

Based on the hypothesis that diabetic foot lesions have a mechanical etiology, extensive efforts have sought to establish a relationship between ulcer occurrence and plantar pressure distribution. However, these factors are still not fully understood. The purpose of this study was to simultaneously record shear and pressure distributions in the heel and forefoot and to answer whether: (i) peak pressure and peak shear for anterior-posterior (AP) and medio-lateral (ML) occur at different locations, and if (ii) peak pressure is always centrally located between sites of maximum AP and ML shear stresses. A custom built system was used to collect shear and pressure data simultaneously on 11 subjects using the 2-step method. The peak pressure was found to be 362 kPa ± 106 in the heel and 527 kPa ± 123 in the forefoot. In addition, the average peak shear values were higher in the forefoot than in the heel. The greatest shear on the plantar surface of the forefoot occurred in the anterior direction (mean and std. dev.: 37.7 ± 7.6 kPa), whereas for the heel, peak shear the foot was in the posterior direction (21.2 ± 5 kPa). The results of this study suggest that the interactions of the shear forces caused greater "spreading" in the forefoot and greater tissue "dragging" in the heel. The results also showed that peak shear stresses do not occur at the same site or time as peak pressure. This may be an important factor in locating where skin breakdown occurs in patients at high-risk for ulceration.

Plantar shear stress distribution in patients with rheumatoid arthritis: relevance to foot pain.

BACKGROUND: Rheumatoid arthritis is an autoimmune disease that causes chronic, progressive joint inflammation; it commonly affects the joints of the feet. Biomechanical alterations and daily pain in the foot are the common outcomes of the disease. Earlier studies focusing on plantar pressure in such patients reported increased vertical loading along with peak pressure-pain associations. However, footwear designed according to the pressure profiles did not relieve symptoms effectively. We examined plantar shear and pressure distribution in patients with rheumatoid arthritis and compared the findings with those of controls, and we investigated a potential relationship between foot pain and local shear stresses. METHODS: A custom-built platform was used to collect plantar pressure and shear stress data from nine patients with rheumatoid arthritis and 14 control participants. Seven patients reported the presence of pain under their feet. Pressure-time and shear-time integral values were also calculated. RESULTS: Peak pressure, pressure-time integral, resultant shear-time integral, and mediolateral shear stress magnitudes were higher in the complication group (P < .05). An association between peak shear-time integral and maximum pain locations was observed. CONCLUSIONS: Increased mediolateral shear stresses under the rheumatoid foot might be attributable to gait instability in such patients. A correlation between the locations of maximum shear-time integral and pain indicate the clinical significance of plantar shear in patients with rheumatoid arthritis.

Design and validation of a general purpose robotic testing system for musculoskeletal applications.

Orthopaedic research on in vitro forces applied to bones, tendons, and ligaments during joint loading has been difficult to perform because of limitations with existing robotic simulators in applying full-physiological loading to the joint under investigation in real time. The objectives of the current work are as follows: (1) describe the design of a musculoskeletal simulator developed to support in vitro testing of cadaveric joint systems, (2) provide component and system-level validation results, and (3) demonstrate the simulator's usefulness for specific applications of the foot-ankle complex and knee. The musculoskeletal simulator allows researchers to simulate a variety of loading conditions on cadaver joints via motorized actuators that simulate muscle forces while simultaneously contacting the joint with an external load applied by a specialized robot. Multiple foot and knee studies have been completed at the Cleveland Clinic to demonstrate the simulator's capabilities. Using a variety of general-use components, experiments can be designed to test other musculoskeletal joints as well (e.g., hip, shoulder, facet joints of the spine). The accuracy of the tendon actuators to generate a target force profile during simulated walking was found to be highly variable and dependent on stance position. Repeatability (the ability of the system to generate the same tendon forces when the same experimental conditions are repeated) results showed that repeat forces were within the measurement accuracy of the system. It was determined that synchronization system accuracy was 6.7+/-2.0 ms and was based on timing measurements from the robot and tendon actuators. The positioning error of the robot ranged from 10 microm to 359 microm, depending on measurement condition (e.g., loaded or unloaded, quasistatic or dynamic motion, centralized movements or extremes of travel, maximum value, or root-mean-square, and x-, y- or z-axis motion). Algorithms and methods for controlling specimen interactions with the robot (with and without muscle forces) to duplicate physiological loading of the joints through iterative pseudo-fuzzy logic and real-time hybrid control are described. Results from the tests of the musculoskeletal simulator have demonstrated that the speed and accuracy of the components, the synchronization timing, the force and position control methods, and the system software can adequately replicate the biomechanics of human motion required to conduct meaningful cadaveric joint investigations.