The effect of personality type on muscle coactivation during elbow flexion.

INTRODUCTION Work-related musculoskeletal injuries and illnesses are prevalent in many industries, and they result in both monetary and human costs (Putz-Anderson, 1988). Most of these injuries are believed to be caused either by an identifiable acute overexertion or by cumulative microtrauma to the tissues. Biomechanical models have been developed to describe the loading that can occur in joints and how that loading may lead to acute or cumulative trauma injury or illness. Recognizing that muscle forces play a major role in joint loading, recent models have become much more concerned with describing muscle coactivation. One effective modeling approach is to measure muscle activity using electromyography and then use these data to calculate agonist and antagonist muscle forces by applying basic principles of muscle physiology (Granata & Marras, 1993, 1995; Marras & Granata, 1995; Marras & Sommerich, 1991; McGill, 1991, 1992; McGill & Norman, 1986; Mirka & Marras, 1993). It is believed that as models become more proficient at describing muscle coactivation, the accuracy of the predictions of joint loading will improve and a more fundamental understanding of the etiology of injury will be gained. As advanced as these biomechanical models have become, it is clear that basic biomechanics cannot account for all of the variance associated with the injury/illness mechanism. Previous research has shown that personal factors also play a role in this process. These factors include age (Roto & Kivi, 1984), sex (Armstrong & Chaffin, 1979), physical size, physical strength, leisure activities, prior injuries (Putz-Anderson, 1988), contraceptive pill use (Sabour & Fadel, 1970), pregnancy (Massey, 1978), gynecological conditions (Cannon, Bemacki, & Walter, 1981), systemic diseases (Chammas et al., 1995; Massey, 1978), and psychosocial factors (Nadelson, 1992). Empirical studies of the relationship between psychosocial factors and musculoskeletal symptoms and injury suggest many potential risk factors, including low decision latitude at work (Bernard, Sauter, Fine, Petersen, & Hales, 1994; Faucett & Rempel, 1994), lack of social support (Bernard et al., 1994; Faucett & Rempel, 1994; Toomingas, Theorell, Michelsen, & Nordemar, 1997), high work pace (Houtman, Bongers, Smulders, & Kompier, 1994), low intellectual discretion (Houtman et al., 1994), high psychological demands (Toomingas et al., 1997), high job strain (Toomingas et al., 1997), and increased time or job pressures (Bernard et al., 1994). This list illustrates that the majority of the existing work in the area of psychosocial factors has focused on external factors. In contrast, Bongers, Winter, Kompier, and Hildebrandt (1993) proposed an illustrative model suggesting that individual differences such as personality type and coping styles (what we will refer to as internal psychosocial characteristics) may moderate (a) the relationship between external psychosocial factors at work and work-related stress, (b) the relationship between stress and muscle tone, and (c) the relationship between musculoskeletal symptoms and subsequent chronicity or disability status. Personality type is one such internal psychosocial characteristic that has not received much attention in the industrial ergonomics literature. Type A (coronary-prone) behavior is characterized by extremes of competitiveness, ambition, high performance standards, aggressiveness, hard-driving effort, time urgency, polyphasing, impatience, irritability, free-floating hostility, and hurried motor and speech patterns. Researchers have noted that Type A individuals experience "more frequent tense, hyperactive movements" (Sparacino, 1979, p. 46), "vigorous voice and psychomotor mannerisms" (Dembroski, Macdougall, & Lushene, 1979, p. 28), "restlessness, alertness, ... and hurried motor movement" (Jenkins, Zyzanski, & Rosenman, 1971, p. …