Multidirectional manual arm strength and its relationship with resultant shoulder moment and arm posture

The effect of handedness on upper extremity isometric strength symmetry

BACKGROUND

Understanding upper extremity strength symmetry can have important implications for ergonomics assessment and design. Few studies have examined isometric joint strength symmetry of left-handed individuals, or examined how handedness can influence strength. As such, the purpose of this study was to investigate the influence of handedness on dominant/non-dominant (D/ND) strength ratio in several moment directions about the shoulder, elbow, wrist, and hand. It was hypothesized that the isometric strength symmetry of left-handed individuals would be significantly different from that of right-handed individuals.

METHODS

The study recruited 28 participants to perform a series of maximal voluntary isometric contractions (MVCs) with both arms for various efforts about the shoulder, elbow, and wrist, as well as handgrip for a total of 68 MVCs. Strength symmetry ratios were computed, and a two-way mixed-model ANOVA evaluated the effects of handedness and MVC test on strength symmetry.

INTERPRETATION

Significant differences in D/ND ratios between right and left-handed individuals were found for 11 of the 17 explored exertion directions. Left-handed individuals tended to possess greater strength in their non-dominant limb, while right-handed individuals tended to have greater strength in their dominant limb. Left- and right-handed individuals often significantly differed in D/ND ratio, suggesting that separate normative values should be created to account for handedness when considering return-to-work or strength-based ergonomics design criterion.

A comparison between measured female linear arm strengths and estimates from the 3D Static Strength Prediction Program (3DSSPP)

This study performed a direct comparison between empirically measured female linear arm strengths and those estimated with the 3D Static Strength Prediction Program (3DSSPP). Linear arm strengths were collected from 15 female participants, at four hand locations and six primary directions (n = 360), and then estimated with 3DSSPP incorporating each participant's own segment lengths, body masses and joint strengths, and the measured arm postures from each trial to optimize the accuracy of 3DSSPP. In spite of this, the errors in 3DSSPP's estimated arm strength values were very high (RMS error = 56.0 N and 40.4%) and poorly correlated (r² = 29.2%) with measured strengths. These results seriously question the accuracy of 3DSSPP to estimate female linear arm strengths and percent capable values, for the range of conditions tested, likely due to the overly simplified assumptions made to estimate triaxial shoulder strength.

Determining best practices for manual pill crushing through evaluation of upper extremity muscle exposures

Nurses in long-term care homes often crush pills into a fine powder using a manual pill crushing device. This study provides novel quantitative data on muscle loading experienced during pill crushing. The influence of surface height, number of pills and device orientation were studied in twelve muscles of the upper extremity. Variations in the work surface height and number of pills crushed resulted in static shoulder and forearm muscle activations that exceeded recommended static limits. In most cases, working at approximately a 50th percentile female's hip height (87 cm) reduced the level of muscle activity, often to below the EMG-based exposure limit, compared to higher heights. A perpendicularly oriented device required substantially lower muscle activity in some shoulder muscles, with marginal differences occurring in muscles of the elbow and wrist. These data can inform practical design and work practice recommendations to reduce muscular activity while performing this important healthcare task.

The influence of hand location and handle orientation on female manual arm strength

Accurate estimations of manual arm strength (MAS) are crucial in the evaluation of occupational force demands relative to population capacity. Most current strength predictions assume force application with a vertically oriented handle, but it is unknown how uni-manual force capability changes as a function of handle orientation and hand location. This study evaluated the effect of handle orientation on MAS throughout the reach envelope. Fifteen female participants exerted maximum forces in six directions (i.e. superior, inferior, anterior, posterior, medial, lateral), at five different hand locations, and MAS was measured with the handle oriented at 0° (i.e. horizontal), 45°, 90° (i.e. vertical) and 135°. Handle orientation affected MAS in all but the anterior exertion direction, with significant interactions between hand location and grip orientation existing for the superior and inferior directions. These results suggest that handle orientation is important to consider in future predictive models of manual arm strength.

An evaluation of off-axis manual forces and upper extremity joint moments during unilateral pushing and pulling exertions

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. Practitioner Summary: Constrained arm postures and directions of manual force application influence the production of off-axis forces. As inaccurate estimation of true manual forces can markedly influence actionable outcomes of proactive ergonomic assessments, this study suggests that simplification of these estimates is insufficient and potentially misleading.

Spatial dependency of shoulder muscle demand during dynamic unimanual and bimanual pushing and pulling

Work involving extensive pushing and pulling is associated with higher frequency of shoulder complaints. While reports of shoulder muscle demand during submaximal isometric tasks are abundant, dynamic submaximal push-pull exertions are not well understood. We evaluated how muscle demand (weighted EMG average) of surface glenohumeral muscles varies with task type and target. Seventeen healthy young adults performed seated unimanual and bimanual pushes and pulls to 3 thoracohumeral elevations (20°, 90°, 170°) and 4 elevation planes (0°, 45°, 90°, 135°) with loading at 15% of isometric push-pull capacity. Pulling required less demand than pushing (p < 0.0001). Muscle demand varied more with elevation than elevation plane. The lowest target had highest demand for pulling (p < 0.01), and the most elevated target had highest demand for pushing (p < 0.0001). Working above the shoulder is known to increase demand during isometric tasks, however, these results suggest that for dynamic tasks working against gravity has a larger effect on demand than task target.

Fast Forward-Dynamics Tracking Simulation: Application to Upper Limb and Shoulder Modeling

OBJECTIVE

Musculoskeletal simulation can be used to estimate muscle forces in clinical movement studies. However, such simulations typically only target movement measurements and are not applicable to force exertion tasks which are commonly used in rehabilitation therapy. Simulations can also produce nonphysiological joint forces or be too slow for real-time clinical applications, such as rehabilitation with real-time feedback. The objective of this study is to propose and evaluate a new formulation of forward-dynamics assisted tracking simulation that incorporates measured reaction forces as targets or constraints without any additional computational cost.

METHODS

We illustrate our method with idealized proof-of-concept models and evaluate it with two upper limb cases: Tracking of hand reaction forces during an isometric force-generation task and constraining glenohumeral joint reaction forces for stability during arm elevation.

RESULTS

We show that the addition of reaction force optimization terms within our simulations generates plausible muscle force predictions for these tasks, which are strongly related to reaction forces in addition to movement. Execution times for all models tested were not different when run with or without the reaction force optimization term, ensuring that the simulations are fast enough for real-time clinical applications.

CONCLUSION

Our novel reaction force optimization term leads to more realistic shoulder reaction forces, without any additional computational costs.

SIGNIFICANCE

Our formulation is not only valuable for shoulder simulations, but could be used in various clinical situations (e.g., for different joints and rehabilitation therapy tasks) where the direction and/or magnitude of reaction forces are of interest.

Multi-directional one-handed strength assessments using AnyBody Modeling Systems

Digital human modeling tools support proactive ergonomics in optimizing work tasks and workplace layouts. Empirical-statistical model based tools are often used to estimate the force exertion capability of the operators. This work is intended to serve as an initial probing into the usability of a musculoskeletal model based software, AnyBody Modeling Systems (AMS), in evaluating the force exertion capability at different points in the workspace and for various exertion directions. As a first step, it focuses on the modeling approach and the accuracy of one-handed isometric strength estimates of AMS. An existing literature database was used to compare the predicted strength at 8 hand locations and in 26 exertion directions, while simulating the empirical postures. The results show a correlation coefficient of 0.7 between the simulated and the experimental strength. AMS emphasizes the biomechanical advantages in strength due to the alignment of force exertion direction with the shoulder. Additionally, some discrepancies have been identified and discussed.

The 'Arm Force Field' method to predict manual arm strength based on only hand location and force direction

This paper describes the development of a novel method (termed the 'Arm Force Field' or 'AFF') to predict manual arm strength (MAS) for a wide range of body orientations, hand locations and any force direction. This method used an artificial neural network (ANN) to predict the effects of hand location and force direction on MAS, and included a method to estimate the contribution of the arm's weight to the predicted strength. The AFF method predicted the MAS values very well (r² = 0.97, RMSD = 5.2 N, n = 456) and maintained good generalizability with external test data (r² = 0.842, RMSD = 13.1 N, n = 80). The AFF can be readily integrated within any DHM ergonomics software, and appears to be a more robust, reliable and valid method of estimating the strength capabilities of the arm, when compared to current approaches.