Comparison of comfort, discomfort, and continuum ratings of force levels and hand regions during gripping exertions

Abstract

The goal of this study was to compare three different evaluation systems of comfort, discomfort, and a continuum for the force levels and hand regions when gripping hand tools. Seventy-two participants were assigned to three groups, each group testing a different evaluation system. Each participant exerted ten levels of submaximal voluntary contractions on hand tools and used their assigned evaluation system to evaluate comfortable or uncomfortable feelings for the force levels and five different regions of the hand. The participants generally rated higher discomfort as required forces increased, but the discomfort and continuum evaluation systems were better than the comfort evaluation system because gripping exertion was related to the physical aspects of the hand tools and their users. Based on the results of the continuum evaluation system, the feeling changed from comfort to discomfort at 65% maximum voluntary contraction. The palm was the region of the hand with the most discomfort. Other design factors affecting comfort need to also be considered in order to better understand hand tool use.

Introduction

Human beings regularly express comfort and discomfort in relation to environments and products. An important issue in ergonomics is to identify influential factors in comfort and discomfort in order to enhance environments and products. Vink et al. (2006) introduced four case studies on working environments: office work, installation work, assembly work, and construction work. Direct participation of employees in modifying the configuration of office furniture such as chairs, tables, and VDT screens resulted in a reduction of neck and shoulder discomfort. About two hundred devices were developed to reduce musculoskeletal discomfort for manual material handling (Vink, 2005). Lowering part-picking heights in the assembly work of electric razors increased productivity due to reduced shoulder discomfort. Discomfort in the lower back of a bricklayer was decreased by raising the working height of bricks and mortar in construction work (Vink, 2005). The comfort and discomfort of many kinds of products such as seats and mattresses were also evaluated. Kyung and Nussbaum (2008) compared sedan and SUV seats using three types of subjective ratings. They revealed that sitting pressure related more strongly to comfort rather than discomfort. Buckle and Fernandes (1998) and Hänel et al. (1997) tested mattress comfort in terms of softness and concluded that mattress comfort was a multifactorial feeling.

De Looze et al. (2003) summarized the relationship between comfort and discomfort in sitting by literature reviews. They identified that pressure had a close association with subjective measures, and suggested a theoretical model which assumed that comfort and discomfort were independent entities, not two opposite extremes on a continuum. In their model, the physical factors of a human, product, or environment produced discomfort. But under the influence of emotional factors, discomfort could change to comfort. Zhang et al. (1996) presented the idea that comfort was associated with feelings of relaxation and well-being, whereas discomfort was closely related with pain, tiredness, soreness, and numbness.

Researchers have also paid a lot of special attention to hand tools. Researchers evaluated many different hand tools in terms of comfort and discomfort, such as garden tools (Chang et al., 1999), handsaws (Mirka et al., 2009), knives (Claudon, 2006), pliers (Chao et al., 2000, Groenesteijn et al., 2004, You et al., 2005), sanders (Spielholz et al., 2001), screwdrivers (Freund et al., 2000, Ulin et al., 1990), and wire-tying hooks (Li, 2002). Comfort is of importance in hand tool ergonomics and discomfort is also frequently assessed as a predictor of musculoskeletal injuries, which should always be reduced in any situation. In the use of hand tools, comfort is associated with positive feelings of reliability, safety, ease, and satisfaction, whereas discomfort is associated with negative feelings of pain, pressure, hardness, and irritation (Vink, 2005). Many researchers reported that comfort and discomfort using hand tools is influenced by many factors such as handle length (Mirka et al., 2009), size (Cochran and Riley, 1986), shape (Kong et al., 2007, Kong et al., 2008, Shih and Wang, 1996), material (Chang et al., 1999), and weight (Björing and Hägg, 2000).

Kuijt-Evers et al. (2005) reported that the descriptor of ‘adverse body effects’ could predict both comfort and discomfort, whereas other descriptors of ‘aesthetics,’ ‘functionality,’ and ‘physical interaction’ could also predict comfort using a questionnaire for the evaluation of hand tools. However, neither the relationship between objective and subjective measures of comfort and discomfort, nor the applicability of a continuum evaluation system to hand tool use, have been thoroughly studied. Some studies utilize both objective measures and subjective measures to recognize ergonomically well-designed hand tools (Strasser et al., 1996, Freund et al., 2000, Li, 2003, Kong and Freivalds, 2003, Groenesteijn et al., 2004, Kluth et al., 2004), but most studies common rely on subjective measures in comfort or discomfort (Kuijt-Evers et al., 2007). For instance, Freund et al. (2000) and Kuijt-Evers et al. (2005) focus on comfort, while Kilbom et al., 1993, Strasser et al., 1996, Chang et al., 1999, Freund et al., 2000, Kong and Freivalds, 2003, Groenesteijn et al., 2004, and Kuijt-Evers et al. (2005) concentrate on discomfort. In addition, there are only a few studies that use the concept of a continuum from comfort to discomfort, although it might be feasible in hand tool evaluation as it is in studies on seats conducted by Jensen and Bendix, 1992, Jianghong and Long, 1994, Wilder et al., 1994, and Vergara and Page (2000).

Gripping exertion is the first appreciable objective measure affecting comfort and discomfort in the use of hand tools. However, what level of maximum voluntary contractions (%MVC) of gripping exertion it would take to change from discomfort to comfort is not well known. Nor is it known which hand regions would mainly contribute to overall hand comfort and discomfort. Thus, one objective of this study is to examine which evaluation system evaluating by comfort level, by discomfort level, and on a continuum from comfort to discomfort – would be appropriate for evaluating gripping exertion. The other objective of this study is to identify the relationship between hand regions associated with gripping exertion levels of submaximal voluntary contractions and overall hand comfort or discomfort.