Biomechanics of Movement-Related Effort: Effects of Task

Experimental Brain Research, 2002

Proceedings of the Human Factors and Ergonomics Society Annual Meeting, 2014

Hand-based human-machine interfaces are complex tasks that involve repetitive or sustained movements and postures of the hands that can lead to overuse syndromes of the musculoskeletal system. Consequently, it is important to minimize the physical effort that occurs at these interfaces. The evaluation of physical effort can be performed either by subjective evaluation of the relative perceived effort (e.g., Borg scale) or by objective physiological measurements (e.g., electromyography -EMG). However, the relation between these two measures has not been sufficiently studied for localized low-effort activities. This study investigated the relation between EMG and Borg ratings, as well as the issue of gender differences during low-effort activity of forearm muscles. Nine females and nine males performed eight different hand gestures (localized low-effort activity), during which EMG signals were recorded from six forearm muscles and Borg ratings were obtained. On average, the female subjects rated the gestures as less effortful than the male subjects, and also demonstrated a higher positive correlation between the EMG and Borg ratings. Furthermore, the linear model that was fitted for predicting the Borg ratings based on gender and the combined activity of muscles provided an R-squared value of approximately 0.3.

Ergonomics, 2007

Motor Control, 2014

The way psychometric and neurophysiological measurements of fatigue are connected is not well understood. Thus, the time course of perceived effort changes due to fatigue, as well as the peripheral and central neurophysiological changes accompanying fatigue, were evaluated. Twelve healthy participants (35 ± 9 years old) undertook 10 min intermittent isometric fatiguing exercise of elbow flexors at 50% of maximum voluntary contraction (MVC). Perceived effort ratings, using the 0-10 numeric rating scale (NRS), were recorded at midrange of MVC. Single pulse TMS of the left motor cortex and electrical stimulation over the biceps muscle was used for the assessment of voluntary activation and peripheral fatigue. The fatiguing exercise caused a 44% reduction in the MVC (p < .001) accompanied by an 18% nonsignificant reduction of the biceps MEP amplitude. The resting twitch force decreased (p < .001) while the superimposed twitches increased (p < .001) causing a decrease (19%) of the voluntary activation (p < .001). The perceived effort ratings increased by 1 point at 30%, by 2 points at 50% MVC respectively on the NRS (p < .001) and were accompanied by an increase in mean biceps EMG. A substantial role of the perceived effort in the voluntary motor control system was revealed.

2011

Purpose: The purpose of this study was to investigate the biomechanical characteristics and physiological cost of three standardized pulling tasks. 12 female and 17 male subjects participated in the study. The subjects were instructed to pull a load at three different heights (hip, shoulder, eye -anatomical landmarks defined the end of a pull) at a rate of ten pulls per minute, for a period of ten minutes (one hundred total pulls). The pulls were sub-maximal and chosen to represent a task during a typical 8 hour workday.

Journal of Occupational Rehabilitation, 2004

The primary objective of this study was to assess the accuracy and precision with which analysts observe and estimate the force produced as subjects performed exertions on a work simulator. Eight analysts observed 32 subjects and estimated force as a percent of subjects' maximum voluntary contraction (% MVC). Analysts exhibited bias toward the mean, as high force exertions (>67% MVC) were underestimated (mean: 11.6% MVC) and low force exertions (<34% MVC) were overestimated (mean: 6.7% MVC). Average error for medium force exertions (34-67% MVC) was not statistically different from zero (2.1% MVC). For all force levels, precision of the estimate was very poor (standard deviation range: 16.2-20.7% MVC). Experience of the analyst in performing ergonomic analysis did not affect accuracy. A secondary objective of the study was to conduct a survey in which subjects identified activities of daily living they perceived as equivalent to controlled force levels. A total of 59 different activities ranging from minimal force required to near maximum were listed by at least 5% of the participants. This list may be used to assist health care practitioners and patients convey the force demands required of occupational tasks as well as for evaluating the diminished strength of the patient.

International Journal of Industrial …, 1994

Ten normal young adult females (mean age 26.8 years, mean weight 50.9 kg, mean height 165.9 cm) performed maximal and subjectively graded exertions of the stoop lift, hand grip, and finger pinch. The levels of graded exertion required were 80%, 60%, 40% and 20% of maximal voluntary contraction (MVC). The sequences of all conditions were fully randomized. Each of the randomized conditions was tried three times in succession. The entire experiment was carried out on four different days at the same time of the day on Monday, Wednesday, Friday of one week and Friday of the next week. The data obtained were subjected to descriptive and statistical analysis with t-test, analysis of variance, and correlation and regression. There were significant differences in the effort produced in three different activities (p < 0.01). The levels of exertion from 20% to 80% were all significantly different from each other (p < 0.01). However, there were no significant differences between the three trials of any given condition and the exertions produced on four different days. The 80% and 60% of exertions were overestimated and 20% was underestimated compared to the objective values based on MVC (p < 0.01). At 40% effort there was no significant difference between the objective level of exertion and subjectively gauged and produced effort for all three activities. The reliability of perception among the female subjects was similar for finger pinch, hand grip, and stoop lift activities.

Theoretical Biomechanics, 2011

Experimental Robotics, 2015

The human selection of specific postures among the infinity of possibilities is the result of a long and complex process of learning. Through learning, humans seem to come to discover the properties of their bodies and how best to put them to use when performing a task. Exploiting the body's kinematic characteristics, humans effectively use the body's mechanical advantage to improve the transmission of the muscles' tension into the forces the task requires. However, the efficiency of this transmission is also affected by the human muscle actuation physiology and dynamics. By also adjusting the body configurations to maximize this transmission of muscle tensions to resulting task forces, humans are in fact exploiting what can be termed the physiomechanical advantage of their musculoskeletal system. Here, we investigate the physiomechanical advantage of humans through several experimental validations. Based on the results of the analysis, we conclude that in learned tasks the optimization of the physiomechanical advantage corresponds to the overall minimization of the human muscular effort. The approach presented here can be applied for the motion control of human musculoskeletal models where the control is task-driven and the task consistent postures are driven by the muscular criteria.

Ergonomics, 2019

This study quantified changes in off-axis manual force production and upper extremity joint moments during sub-maximal one-handed push and pull tasks. Off-axis forces in the up/down and left/right directions were quantified in the presence or absence of constraints placed upon the direction of manual force application and/or arm posture. Resultant off-axis forces of 13.1% and 9.4% were produced for pulls and pushes, respectively. Off-axis forces during pulling were oriented downwards and to the right, and were associated with a decreased should flexion moment when posture was constrained. Off-axis forces in the up/down direction were minimized with increased on-axis force level. Off-axis forces during pushing tended to be oriented to the left and were associated with increased elbow flexion moment when off-axis forces were allowed. By not accounting for these off-axis forces, we may not be accurately reflecting actionable muscle-and joint-level loading characteristics derived from biomechanically-based proactive ergonomics assessment approaches.

Experimental Brain Research, 2008

Injury and Skeletal Biomechanics, 2012

It is thought that perception of effort during physical tasks is the conscious awareness of the central motor command sent to the active muscles. The aim of this study was to directly test this hypothesis by experimentally varying perception of effort and measuring movement-related cortical potential (MRCP). Sixteen healthy, recreationally active men made unilateral dynamic elbow flexions to lift a light (20% one repetition maximum, 1RM) and a heavier (35% 1RM) weight with a fatigued arm and a nonfatigued arm while rating of perceived effort (RPE), biceps brachii electromyogram (EMG), and MRCP were recorded. RPE, EMG amplitude, and MRCP amplitude at Cz during weight raising increased with weight and with muscle fatigue. There was a significant correlation between RPE and MRCP amplitude at the vertex during the weight raising epoch. This study provides direct neurophysiological evidence that perception of effort correlates with central motor command during movement execution.

Journal of Applied Biomechanics, 29:285-291

This study examined the influence of frequency and direction of force application on psychophysically acceptable forces for simulated work tasks. Fifteen male participants exerted psychophysically acceptable forces on a force transducer at 1, 3 or 5 repetitions per minute by performing both a downward press and a pull towards the body. These exertions were shown previously to be strength and balance limited, respectively. Workers chose acceptable forces at a lower percentage of their maximum voluntary force capacity during downward (strength limited) exertions than during pulling (balance limited) at all frequencies (4-11%, p = 0.035). Frequency modulated acceptable hand force; however only during downward exertions, where forces at five repetitions per minute were 13% less (p = 0.005) than those at one exertion per minute. This study provides insight into the relationship between biomechanically limiting factors and the selection of acceptable forces for unilateral manual tasks.

Keywords: grip strength maximal effort sincerity of effort submaximal effort wrist position a b s t r a c t Background: Several grip strength tests are commonly used for detecting sincerity of effort. However, there is still no widely accepted standardized sincerity of effort test. Therefore, this study aimed to examine whether grip strength test in three wrist positions could distinguish between maximal and submaximal efforts. Methods: Twenty healthy individuals (10 men and 10 women) with a mean age of 26.7 AE 3.92 years participated in this study. All participants completed two test conditions (maximal and submaximal efforts) in three wrist positions (neutral, flexion, and extension) using both hands. Each participant exerted 100% effort in the maximal effort condition and 50% effort in the submaximal effort condition. The participants performed three repetitions of the grip strength test for each session. Results: The results showed that there is a significant main effect of the type of effort (p < 0.001), wrist position (p < 0.001), and hand (p ¼ 0.028). There were also significant types of effort and wrist position interactions (p < 0.001) and effort and hand interactions (p < 0.028). The results also showed that grip strength was highest at the wrist in neutral position in both the maximal and the submaximal effort condition. Grip strength values of the three wrist positions in the maximal effort condition were noticeably greater than those in the submaximal effort condition. Conclusion: The findings of this study suggest that grip strength test in three wrist positions can differentiate a maximal effort from a submaximal effort. Thus, this test could potentially be used to detect sincerity of effort in clinical setting.