FDA Regulation of Neurological and Physical Medicine Devices: Access to Safe and Effective Neurotechnologies for All Americans.

The United States Food and Drug Administration (FDA) ensures that patients in the U.S. have access to safe and effective medical devices. The Division of Neurological and Physical Medicine Devices reviews medical technologies that interface with the nervous system. This article addresses how to navigate the FDA's regulatory landscape to successfully bring medical devices to patients.

The coordinate system for force control.

The primary objective of this study was to establish the coordinate frame for force control by observing how parameters of force that are not explicitly specified by a motor task vary across the workspace. We asked subjects to apply a force of a specific magnitude with their hand. Subjects could complete the task by applying forces in any direction of their choice in the transverse plane. They were tested with the arm in seven different configurations. To estimate whether contact forces are represented in extrinsic or intrinsic coordinates, we applied the parallel transport method of differential geometry to the net joint torques applied during the task. This approach allowed us to compare the force variability observed at different arm configurations with the force variability that would be expected if the control system were applying an invariant pattern of joint torques at the tested configurations. The results indicate that for the majority of the subjects, the predominant pattern was consistent with an invariant representation in joint coordinates. However, two out of eleven subjects also demonstrated a preference for extrinsic representation. These findings suggest that the central nervous system can represent contact forces in both coordinate frames, with a prevalence toward intrinsic representations.

Stabilization strategies for unstable dynamics.

BACKGROUND: When humans are faced with an unstable task, two different stabilization mechanisms are possible: a high-stiffness strategy, based on the inherent elastic properties of muscles/tools/manipulated objects, or a low-stiffness strategy, based on an explicit positional feedback mechanism. Specific constraints related to the dynamics of the task and/or the neuromuscular system often force people to adopt one of these two strategies. METHODOLOGY/FINDINGS: This experiment was designed such that subjects could achieve stability using either strategy, with a marked difference in terms of effort and control requirements between the two strategies. The task was to balance a virtual mass in an unstable environment via two elastic linkages that connected the mass to each hand. The dynamics of the mass under the influence of the unstable force field and the forces applied through the linkages were simulated using a bimanual, planar robot. The two linkages were non-linear, with a stiffness that increased with the amount of stretch. The mass could be stabilized by stretching the linkages to achieve a stiffness that was greater than the instability coefficient of the unstable field (high-stiffness), or by balancing the mass with sequences of small force impulses (low-stiffness). The results showed that 62% of the subjects quickly adopted the high-stiffness strategy, with stiffness ellipses that were aligned along the direction of instability. The remaining subjects applied the low-stiffness strategy, with no clear preference for the orientation of the stiffness ellipse. CONCLUSIONS: The choice of a strategy was based on the bimanual coordination of the hands: high-stiffness subjects achieved stability quickly by separating the hands to stretch the linkages, while the low-stiffness subjects kept the hands close together and took longer to achieve stability but with lower effort. We suggest that the existence of multiple solutions leads to different types of skilled behavior in unstable environments.

Expert strategy switching in the control of a bimanual manipulandum with an unstable task.

The goal of this study is to better understand how the central nervous system switches between alternative stabilization strategies when presented with an unstable task. A haptic, bimanual manipulandum has been used to emulate an unstable task, which requires subjects to stabilize a virtual mass under the action of a saddle force field with two nonlinear springs, whose stiffness increases with the amount of stretch. Subjects learn to position the mass at various target points by adjusting the rest length, and thus the stiffness of the two springs. From a previous study we know that subjects can stabilize the mass by either 1) applying large forces to stretch the springs and increase the mechanical stiffness of the system beyond a critical level or by 2) applying small force impulses that intermittently adjust the position of the mass. In this study we report the performance of a subject who was trained extensively to use one strategy or the other in order to characterize the mechanism of target switching, from the high-stiffness to the low-stiffness regime and back.

The effect of trunk flexion on able-bodied gait.

This study examined the effect of sagittal trunk posture on the gait of able-bodied subjects. Understanding the effect of trunk posture on gait is of clinical interest since alterations in trunk posture often occur with age or in the presence of spinal pathologies, such as lumbar flatback. Gait analysis was conducted on 14 adults walking at self-selected slow, normal, and fast walking speeds while maintaining three trunk postures: upright, and with 25+/-7 degrees and 50+/-7 degrees of trunk flexion from the vertical. During trunk-flexed gait, subjects adopted a crouch posture characterized by sustained knee flexion during stance and an increase in ankle dorsiflexion and hip flexion angles. During stance, these kinematic adaptations produced a posterior shift in the positions of the trunk and pelvis, which helped to offset the anterior shift in the trunk mass that occurred with trunk flexion. In this way, kinematic adaptations may have been used to maintain balance by shifting the body's center of mass to a position similar to that of upright walking. These changes in lower limb joint kinematics created a phase lag in the position of the hip joint center relative to that of the ankle joint center in the sagittal plane. Alterations in the sagittal alignment of the hip and ankle joint positions were associated with a phase lag in the vertical position, velocity, and acceleration of the body's center of mass (BCOM) relative to upright walking. Since the vertical ground reaction force (GRF(v)) is proportional to the vertical acceleration of the BCOM, significant changes were also seen in the GRF(v) during trunk-flexed gait. In summary, kinematic adaptations necessary to maintain dynamic balance altered the trajectory and acceleration of the BCOM in the vertical direction, which was reflected in the GRF(v). The results of this study may help clinicians better understand the nature and impact of compensatory mechanisms in patients who exhibit trunk-flexed postures during gait.

The effect of trunk-flexed postures on balance and metabolic energy expenditure during standing.

STUDY DESIGN: This study analyzed force plate, kinematic, and metabolic energy data of 14 able-bodied subjects standing statically with upright and trunk-flexed postures. OBJECTIVE: To explore the effect of trunk-flexed postures on balance and metabolic energy expenditure during standing. SUMMARY OF BACKGROUND DATA: Abnormal trunk posture often occurs in the presence of spinal deformities, such as lumbar flatback. It is unclear whether alterations in trunk posture affect energy expenditure and the location of the body's center of mass in the transverse plane (BCOMtrans) during standing. METHODS: Kinematic, kinetic, and energy expenditure data were collected with upright trunk alignment and with 25 degrees +/- 7 degrees and 50 degrees +/- 7 degrees of trunk flexion from the vertical. The mean location of the BCOMtrans was estimated from the net center of pressure (COP), which is a weighted average of the COP beneath both feet. RESULTS: The fore-aft position of the net COP under the base of support was not significantly different between postures (P < 0.08). At each posture, the net COP was located 16% of the foot length anterior to the ankle joint centers. However, with increasing trunk flexion, there was a significant increase in oxygen consumption rate (P < 0.001 for all postures). CONCLUSION: Compensatory actions, such as ankle plantarflexion and hip flexion, allowed the mean position of the net COP to remain within a narrowly defined region irrespective of trunk posture. Changes in muscle activity associated with a trunk-flexed posture and the associated compensations likely contributed to the increased energy expenditure.