Na Jin Seo

Investigation of grip force, normal force, contact area, hand size, and handle size for cylindrical handles.

Objective: To investigate relationships among grip forces, normal forces, contact area for cylindrical handles, handle diameter, hand size, and volar hand area. Background: Data describing those relationships are needed to predict thrust forces and torque capability. Method: Additional analyses were performed retrospectively on data collected in two previous studies in which participants performed maximum grip exertions on cylinders (diameter 38—83 mm) while grip force, normal force, and contact area were recorded. The length, width, and volar area of the hand were measured. Results: Average total normal force on cylinders was 2.3 times greater than grip force measured using a split cylinder (R 2 = 65%), regardless of the handle diameter examined. The ratio of handle diameter to hand length explained 62%, 57%, and 71% of the variances in grip force, normal force, and contact area, respectively. Estimated hand area (hand length × width) had a linear relationship with measured hand area (using photographs; R 2 = 91%), although it was 8% less than the measured area. Conclusion: This work describes the relationship between normal force and grip force independent of handle size (for handle diameters from 38 to 83 mm). Normal force and contact area can be explained by the interaction between handle size and hand size. Hand area can be estimated by hand length times width. Application: The quantitative relationships described in this paper can be used in the design of objects and hand tools to determine optimal handle sizes for maximizing grip force, total normal force, or contact area.

Effects of handle orientation, gloves, handle friction and elbow posture on maximum horizontal pull and push forces.

Biomechanical models were evaluated for effects of handle orientation, handle material, gloves and arm posture on maximal pull/push force. Eight healthy subjects performed maximum pull/push exertions on handles with two different orientations and two different surface materials, using bare hand and two types of glove as well as two arm postures. The empirical data supported the proposed biomechanical models: Pull/push forces for the bare hand on a rubber handle decreased 10% when the handle was parallel to the pull/push direction, compared with when perpendicular to it. For parallel handles, pull/push forces further decreased with decreasing hand–handle friction coefficient (simulated by different handle materials and gloves). Pull force exerted by the bare hand was 29% greater when the elbow was extended than when flexed. Pull force was greater than push force (with bare hand and flexed elbow). The biomechanical models suggest that friction between the hand and handle limits pull/push forces for parallel handles. Elbow strength may be responsible for decreased pull force for the flexed elbow posture and decreased force for pull compared with push in the postures examined. Statement of Relevance: Biomechanical models presented in this paper provide insights for causes of upper extremity strength limitations during pull/push tasks. Findings in this paper can be used directly in the design of workstation and objects to reduce fatigue and risk of musculoskeletal disorders.

Use of visual force feedback to improve digit force direction during pinch grip in persons with stroke: A pilot study

OBJECTIVE To investigate whether visual feedback of digit force directions for the index fingertip and thumb tip during repeated practice of grip force production can correct the digit force directions for persons with stroke during grip assessments. Following stroke, the paretic fingers generate digit forces with a higher than normal proportion of shear force to compression force during grip. This misdirected digit force may lead to finger-object slip and failure to stably grasp an object. DESIGN A case series. SETTING Laboratory. PARTICIPANTS Persons (N=11) with severe chronic hand impairment after stroke. INTERVENTIONS Four training sessions during which participants practiced directing the index finger and thumb forces in various target directions during pinch using visual feedback. MAIN OUTCOME MEASURE Digit force direction during pinch and clinical hand function scores were measured before and immediately after the training. RESULTS Study participants were able to redirect the digit force closer to the direction perpendicular to the object surface and increase their hand function scores after training. The mean ratio of the shear force to the normal force decreased from 58% to 41% (SD, 17%), the mean Box and Block Test score increased from 1.4 to 3.4 (SD, 2.0), and the mean Action Research Arm Test score increased from 10.8 to 12.1 (SD, 1.3) (P<.05 for all 3 measures). CONCLUSIONS Repeated practice of pinch with visual feedback of force direction improved grip force control in persons with stroke. Visual feedback of pinch forces may prove valuable as a rehabilitation paradigm for improving hand function.

Inward Torque and High-Friction Handles Can Reduce Required Muscle Efforts for Torque Generation

Objective: The effects of handle friction and torque direction on muscle activity and torque are empirically investigated using cylindrical handles. Background: A torque biomechanical model that considers contact force, friction, and torque direction was evaluated using different friction handles. Methods: Twelve adults exerted hand torque in opposite directions about the long axis of a cylinder covered with aluminum or rubber while grip force, torque, and finger flexor electromyography (EMG) were recorded. In addition, participants performed grip exertions without torque, in which they matched the EMG level obtained during previous maximum torque exertions, to allow us to determine how grip force was affected by the absence of torque. Results: (a) Maximum torque was 52% greater for the high-friction rubber handle than for the low-friction aluminum handle. (b) Total normal force increased 33% with inward torque (torque applied in the direction fingertips point) and decreased 14% with outward torque (torque in the direction the thumb points), compared with that with no torque. Consequently, maximum inward torque was 45% greater than maximum outward torque. (c) The effect of torque direction was greater for the high-friction rubber handle than for the low-friction aluminum handle. Conclusion: The results support the proposed model, which predicts a large effect of torque direction when high-friction handles are gripped. Application: Designing tasks with high friction and inward rotations can increase the torque capability of workers of a given strength, or reduce required muscle activities for given torque exertions, thus reducing the risk of fatigue and musculoskeletal disorders.

A comparison of two methods of measuring static coefficient of friction at low normal forces: A pilot study

This study compares two methods for estimating static friction coefficients for skin. In the first method, referred to as the ‘tilt method’, a hand supporting a flat object is tilted until the object slides. The friction coefficient is estimated as the tangent of the angle of the object at the slip. The second method estimates the friction coefficient as the pull force required to begin moving a flat object over the surface of the hand, divided by object weight. Both methods were used to estimate friction coefficients for 12 subjects and three materials (cardboard, aluminium, rubber) against a flat hand and against fingertips. No differences in static friction coefficients were found between the two methods, except for that of rubber, where friction coefficient was 11% greater for the tilt method. As with previous studies, the friction coefficients varied with contact force and contact area. Static friction coefficient data are needed for analysis and design of objects that are grasped or manipulated with the hand. The tilt method described in this study can easily be used by ergonomic practitioners to estimate static friction coefficients in the field in a timely manner.

Remote vibrotactile noise improves light touch sensation in stroke survivors’ fingertips via stochastic resonance

Background and purposeStroke rehabilitation does not often integrate both sensory and motor recovery. While subthreshold noise was shown to enhance sensory signal detection at the site of noise application, having a noise-generating device at the fingertip to enhance fingertip sensation and potentially enhance dexterity for stroke survivors is impractical, since the device would interfere with object manipulation. This study determined if remote application of subthreshold vibrotactile noise (away from the fingertips) improves fingertip tactile sensation with potential to enhance dexterity for stroke survivors.MethodsIndex finger and thumb pad sensation was measured for ten stroke survivors with fingertip sensory deficit using the Semmes-Weinstein Monofilament and Two-Point Discrimination Tests. Sensation scores were measured with noise applied at one of three intensities (40%, 60%, 80% of the sensory threshold) to one of four locations of the paretic upper extremity (dorsal hand proximal to the index finger knuckle, dorsal hand proximal to the thumb knuckle, dorsal wrist, volar wrist) in a random order, as well as without noise at beginning (Pre) and end (Post) of the testing session.ResultsVibrotactile noise of all intensities and locations instantaneously and significantly improved Monofilament scores of the index fingertip and thumb tip (p < .01). No significant effect of the noise was seen for the Two-Point Discrimination Test scores.ConclusionsRemote application of subthreshold (imperceptible) vibrotactile noise at the wrist and dorsal hand instantaneously improved stroke survivors’ light touch sensation, independent of noise location and intensity. Vibrotactile noise at the wrist and dorsal hand may have enhanced the fingertips’ light touch sensation via stochastic resonance and interneuronal connections. While long-term benefits of noise in stroke patients warrants further investigation, this result demonstrates potential that a wearable device applying vibrotactile noise at the wrist could enhance sensation and grip ability without interfering with object manipulation in everyday tasks.

Friction coefficients in a longitudinal direction between the finger pad and selected materials for different normal forces and curvatures.

This study investigated the effect of object curvature, normal force and material on skin friction coefficient. Twelve subjects slid their middle fingertip pad against a test object with small (11 mm), medium (18, 21 mm) or large (flat object) radii of curvature, while maintaining a normal force of 1, 10 or 20 N. Tested materials were aluminium and four rubber hoses. The average friction coefficient was 0.6 for aluminium and 0.9 for the rubber hoses. As normal force increased from 1 to 20 N, the average friction coefficient decreased 46%. Friction coefficient did not vary significantly with object curvature. The citation of friction coefficient data requires careful attention to normal force levels with which they are measured, but not so much to object curvature between 11 mm and infinity. This study provides skin friction coefficient data that are needed for design of objects that are manipulated with the hands. The investigation of the effect of object curvature on skin friction coefficient has important implications to ergonomics practices as many objects handled in everyday activities have curved surfaces.

The effect of handle friction and inward or outward torque on maximum axial push force.

Objective: To investigate the relationship among friction, applied torque, and axial push force on cylindrical handles. Background: We have earlier demonstrated that participants can exert greater contact force and torque in an “inward” movement of the hand about the long axis of a gripped cylinder (wrist flexion/forearm supination) than they can in an “outward” hand movement. Method: Twelve healthy participants exerted anteriorly directed maximum push forces along the long axis of aluminum and rubber handles while applying deliberate inward or outward torques, no torque (straight), and an unspecified (preferred) torque. Results: Axial push force was 12% greater for the rubber handle than for the aluminum handle. Participants exerted mean torques of 1.1, 0.3, 2.5, and —2.0 Nm and axial push forces of 94, 85, 75, and 65 N for the preferred, straight, inward, and outward trials, respectively. Left to decide for themselves, participants tended to apply inward torques, which were associated with increased axial push forces. Conclusion: Axial push force was limited by hand-handle coupling — not the whole bodys push strength. Participants appeared to intuitively know that the application of an inward torque would improve their maximum axial push force. Axial push forces were least when a deliberate torque was requested, probably because high levels of torque exertions interfered with the push. Application: A low-friction handle decreases maximum axial push force. It should be anticipated that people will apply inward torque during maximum axial push.

Dependence of safety margins in grip force on isometric push force levels in lateral pinch

This study examined the relationship between safety margin and force level during an isometric push task in a lateral pinch posture. Ten participants grasped an object with an aluminium- or rubber-finished grip surface using a lateral pinch posture and exerted 20%, 40%, 60%, 80% and 100% of maximum push force while voluntary grip force was recorded. Then minimum required grip force was measured for each push force level. Mean safety margin, the difference between voluntary and minimum required grip forces, was 25% maximum voluntary contraction (MVC) when averaged for all push levels. Safety margin significantly increased with increasing push force for both grip surfaces. Grip force used during maximum push exertion was only 74% lateral pinch grip MVC. Possible underlying mechanisms for increasing safety margin with increasing push force are discussed as well as the implication of this finding for ergonomic analysis. This study demonstrates that ergonomic analyses of push tasks that involve friction force should account for safety margin and reduced grip strength during the push. Failure to consider these can result in overestimation of peoples push capability.

Tactile feedback plays a critical role in maximum finger force production.

This study investigates the role of cutaneous feedback on maximum voluntary force (MVF), finger force deficit (FD) and finger independence (FI). FD was calculated as the difference between the sum of maximal individual finger forces during single-finger pressing tasks and the maximal force produced by those fingers during an all-finger pressing task. FI was calculated as the average non-task finger forces normalized by the task-finger forces and subtracted from 100 percent. Twenty young healthy right-handed males participated in the study. Cutaneous feedback was removed by administering ring block digital anesthesia on the 2nd, 3rd, 4th and 5th digits of the right hands. Subjects were asked to press force sensors with maximal effort using individual digits as well as all four digits together, with and without cutaneous feedback. Results from the study showed a 25% decrease in MVF for the individual fingers as well as all the four fingers pressing together after the removal of cutaneous feedback. Additionally, more than 100% increase in FD after the removal of cutaneous feedback was observed in the middle and ring fingers. No changes in FI values were observed between the two conditions. Results of this study suggest that the central nervous system utilizes cutaneous feedback and the feedback mechanism plays a critical role in maximal voluntary force production by the hand digits.