Satyajit Ambike

Purdue University | Purdue · Department of Health and Kinesiology

Previous reports show that the forces produced by the fingers of one hand drop exponentially over time in the absence of visual feedback on the forces. We study the force production by the index fingers of both hands with no visual feedback. Subjects produced a specified total force with a specific contribution from each finger by pressing on force...

A motor system configured to maximize the stability of its current state cannot dexterously transition between states. Yet, we routinely resolve the stability-dexterity conflict and rapidly change our current behavior without allowing it to become unstable prior to the desired transition. The phenomenon called anticipatory synergy adjustment (ASA)...

Maintaining a consistent relationship between each footfall and the body’s motion is a key mechanism to maintain balance while walking. However, environmental features, for example, puddles/obstacles, impose additional constraints on foot placement. This study investigated how healthy young individuals alter foot placements to simultaneously manage...

Humans closely coordinate the grip force exerted on a hand-held object with changes in the load arising from the object’s dynamics. Recent work suggests the grip force is responsive to the predictability of the load forces as well. The well-known grip-force–load-force coupling is intermittent when the load arising from volitional movements fluctuat...

A broad set of factors are associated with falling (e.g., age, sex, physical activity, vision, health), but their co-occurrence is understudied. Our objectives were to quantify the number and pattern of co-occurring fall-related factors. Data were obtained from the U.S. National Health and Nutrition Examination Survey (N = 1,957, 60-85 years). Twen...

During community ambulation, anticipatory adaptations in gait are key for navigating built, populated and natural environments. It has been argued that some instability in gait can be functionally beneficial in situations demanding high maneuverability, and while the mechanisms utilized to maintain locomotor balance are well understood, relatively...

In the referent control theory, grip force emerges by designating the referent aperture (Ra) as a threshold position inside the object. This study quantified Ra and investigated whether the synergistic control of digit referent coordinate (RC) and apparent stiffness (k) depend on the external mechanical constraints on the hand-held object. Subjects...

Interaction in mid-air can be fatiguing. A model-based method to quantify cumulative subjective fatigue for such interaction was recently introduced in HCI research. This model separates muscle units into three states: active (MA) fatigued (MF) or rested (MR) and defines transition rules between states. This method demonstrated promising accuracy i...

During community ambulation, anticipatory adaptations in gait are key for navigating built, populated and natural environments. It has been argued that some instability in gait can be functionally beneficial in situations demanding high maneuverability, and while the mechanisms utilized to maintain locomotor balance are well understood, relatively...

During steady-state gait, the instantaneous passive dynamics, quantified using the margin of stability in the anterior-posterior (AP) direction (MOSAP), are unstable. Although the instability indicates that substantial control will be required to reject a perturbation, the instability also allows for forward progression with low energy expenditure...

Tripping while walking is a main contributor to falls across the adult lifespan. Trip risk is proportional to variability in toe clearance. To determine the sources of this variability, the authors computed for 10 young adults the sensitivity of toe clearance to 10 bilateral lower limb joint angles during unobstructed and obstructed walking when th...

Community ambulation requires gait adaptations to navigate environmental obstacles. It is well known that while crossing obstacles, variables quantifying the gait pattern are controlled relative to the obstacle’s position. However, the stability of these gait variables is underexplored. We measured foot positions relative to an obstacle as young an...

Background Impaired movement stability is a common symptom of Parkinson’s disease (PD) that leads to falls and mishandled objects. Decline in synergistic stabilization of movement in PD patients has been observed in manual and postural tasks. However, locomotor synergies have not been quantified in PD patients. Research question The purpose of thi...

The authors examined how the stability of the current total isometric force ( FT ) produced by four fingers is influenced by previous and expected voluntary changes in FT . The authors employed the synergy index obtained from the across-trial uncontrolled manifold analysis to quantify the stability of FT . The authors compared two tasks with simila...

Stability is the ability of a system to maintain a desired static or dynamic motor pattern. Maneuverability, on the other hand, is the ability to transition between motor patterns, and it is antagonistic to stability. Animals frequently reduce the stability of an ongoing task to facilitate anticipated movement transitions. Such stability–maneuverab...

Falls that occur while negotiating steps are a leading cause of death in older adults. Although recent efforts have improved understanding of the mechanics and control of stepping behaviors, the double support phase during stepping is understudied. Therefore, we quantified the stability of the resultant forces and moments acting on the body during...

Joint angle variance in bipedal linked chain during curb negotiation

Anticipatory synergy adjustment (ASA) is a feed-forward control mechanism that describes a continuous decrease in the stability of the current motor state beginning about 150 ms prior to a state transition. Recently, we described an associated phenomenon in which the system stability was reduced solely in response to a cue that generates an expecta...

Clinical assessments indicate that manual dexterity decreases with age starting at age 60 years. These assessments involve measuring the time required to complete pick-and-place tasks conceived to approximate the fine motor functions that the fingers perform during activities of daily living. Here, we treat the hand as a redundant system comprised...

To cross an obstacle without tripping, the toe height must be greater than the obstacle height. Yet, Heijnen et al. (2012) report a decay (-1 mm/trial) in toe height during repeated obstacle crossing. We used the uncontrolled manifold (UCM) method to explore the variability in the trail toe height viewed as an output of the lower-limb joint angles.

Quantifying cumulative arm muscle fatigue is a critical factor in understanding, evaluating, and optimizing user experience during prolonged mid-air interaction. A reasonably accurate estimation of fatigue requires an estimate of an individual's strength. However, there is no easy-to-access method to measure individual strength to accommodate inter...

Cyclic isometric finger-force patterns established using visual feedback show systematic drifts when the feedback is removed. Force changes at multiple time scales and opposite directions have been reported. For further characterization of these drifts, healthy subjects produced isometric, cyclic finger force with and without visual feedback at var...

The soft human digit tip forms contact with grasped objects over a finite area and applies a moment about an axis normal to the area. These moments are important for ensuring stability during precision grasping. However, the contribution of these moments to grasp stability is rarely investigated in prehension studies. The more popular hard-contact...

We applied the theory of synergies to analyze the processes that lead to unintentional decline in isometric fingertip force when visual feedback of the produced force is removed. We tracked the changes in hypothetical control variables involved in single fingertip force production based on the equilibrium-point hypothesis, namely the fingertip refe...

The authors explored application of analytical inverse optimization (ANIO) method to the normal finger forces in unimanual and bimanual prehensile tasks with discrete and continuously changing constraints. The subjects held an instrumented handle vertically with one or two hands. The external torque and grip force changed across trials or within a...

We explored unintentional changes in forces during performance of constant and cyclic force-production tasks (F-tasks) after visual feedback removal. Based on earlier studies, we expected all force parameters to drop exponentially with time. We also explored possible role of working memory in the force drop phenomena. Healthy subjects performed con...

We use an approach rooted in the recent theory of synergies to analyze possible co-variation between two hypothetical control variables involved in finger force production based in the equilibrium-point hypothesis. These control variables are the referent coordinate (R) and apparent stiffness (C) of the finger. We tested a hypothesis that inter-tri...

We used the framework of the referent configuration hypothesis to explore the possibility that accurate performance can result from inter-trial co-variation among control variables that show relatively high variability. In particular, we assumed that force production by a finger in isometric conditions results from controlling the fingertip referen...

Grasping research in robotics has made remarkable progress in the last three decades and sophisticated computational tools are now available for planning robotic grasping in complex environments. However, studying the neural control of prehension in humans is more complex than studying robotic grasping. The elaborate musculoskeletal geometries and...

We investigated the unintentional drift in total force and in sharing of the force between fingers in two-finger accurate force production tasks performed without visual feedback by patients with Parkinson's disease (PD) and healthy controls. In particular, we were testing a hypothesis that adaptation to the documented loss of action stability coul...

This study used the framework of the referent configuration hypothesis and slow changes in the external conditions during vertical oscillation of a hand-held object to infer the characteristics of hypothetical control variables. The study had two main objectives: (1) to show that hypothetical control variables, namely, referent coordinates and appa...

We used the framework of the referent configuration (RC) hypothesis to reconstruct the hand and grip aperture referent trajectories during vertical oscillation of a hand-held object. Within the RC hypothesis, mechanical variables are consequences of shifts in referent values for salient body coordinates. For the studied task, this translates into t...

The extrinsic digit muscles naturally couple wrist action and grip force in prehensile tasks. We explored the effects of wrist position on the steady-state grip force and grip-force change during imposed changes in the grip aperture [apparent stiffness (AS)]. Subjects held an instrumented handle steady using a prismatic five-digit grip. The grip ap...

This study was motivated by the double action of extrinsic hand muscles that produce grip force and also contribute to wrist torque. We explored interactions between grip force and wrist torque in isometric force production tasks. In particular, we tested a hypothesis that an intentional change in one of the two kinetic variables would produce an u...

Identical muscles (FDP – flexion, EDC – extension) contribute to both grip force at the fingertips and wrist movements. We investigate the relation between grip force and wrist kinematics to examine: (1) whether grip force scales with grip strength, which was expected to vary with wrist angle; and (2) whether grip force can be predicted from accele...

This paper examines up to third-order geometric properties of wrist path and the first-order property of wrist trajectory (wrist speed) for spatial pointing movements. Previous studies report conflicting data regarding the time invariance of wrist-path shape, and most analyses are limited to the second-order geometric property (straightness, or str...

Extrinsic digit muscles contribute to both fingertip forces and wrist movements (FDP and FPL-flexion, EDC-extension). Hence, it is expected that finger forces depend on the wrist movement and position. We investigated the relation between grip force and wrist kinematics to examine whether and how the force (1) scales with wrist flexion-extension (F...

The leading joint hypothesis (LJH), developed for planar arm reaching, proposes that the interaction torques experienced by the proximal joint are low compared to the corresponding muscle torques. The human central nervous system could potentially ignore these interaction torques at the proximal (leading) joint with little effect on the wrist traje...

Trajectory tracking is accomplished by obtaining separate solutions to the geometric path-tracking problem and the temporal tracking problem. A methodology enabling the geometric tracking of a desired planar or spatial path to any order with a nonredundant manipulator is developed. In contrast to previous work, the equations are developed using one...

Path tracking using planar mechanisms with one degree of redundancy can be enhanced by matching (when possible) or approximating second-order path properties with first-order joint coordination. Second-order tracking can reduce the frequency of feedback for the desired accuracy, and since this paper provides analytical expressions for the joint spe...

Path tracking can be accomplished by separating the control of the desired trajectory geometry and the control of the path variable. Existing methods accomplish tracking of up to third-order geometric properties of planar paths and up to second-order properties of spatial paths using non-redundant manipulators, but only in special cases. This paper...

This paper addresses validation of a curvature-theory-based time-invariant inverse kinematic model and a related tracking algorithm for human motor control of reaching motions. Human subjects made unconstrained reaching motions in the horizontal plane to fixed targets at three self-selected speeds. Consistent shoulder/elbow joint speed ratios for m...

Planar motion can be synthesized with two-degree-of-freedom curvature theory by using the second-order Taylor series to coordinate planar path-tracking systems. Second-order control-variable coordination generates tracking error away from the reference point, and tracking becomes erratic when the driving variable experiences a dwell in its trajecto...

Design of three-point hitch systems used with agricultural tractors is quite evolved and is governed by an established standard. Freedom within the standard, though, can be exploited to tailor the individual hitch performance. A hitch can be treated as a four-bar linkage in the vertical longitudinal plane, yet it presents a complicated kinematic sy...

This paper presents an approach to obtain the first-order speed ratios for three-degree-of-freedom spatial mechanisms that allow three degrees of translational freedom. The approach eliminates the need to locate the canonical frame and find the instantaneous invariants of motion and provides a direct method of obtaining the first-order speed ratios...

This paper proposes that two-degree-of-freedom Curvature Theory pro- invariant kinematic model is fundamental to human motor coordination, Curvature Theory provides a concise, efficient mapping of a desired out- put trajectory geometry to the joint angles' instantaneous speed ratios. If the speed ratios for a motion are learned through experience,...

Abstract: The human central nervous system (CNS) builds models of the motor apparatus and the external world. Based on experimental evidence, it is hypothesized that the internal model built by the CNS separates the dynamic and kinematic aspects of motion. Further, the time- dependent and time-independent aspects of motion are also separated. It is...