Spasmodic torticollis

The relationship between muscle tension and anxiety is widely known. (1-3) Earlier studies reported that the effects of stress and deliberate control of muscle tension indicated a relation between electromyographic (EMG) readings and personality. (4-6) Many authors think that the basic Eysenck's personality dimension of neuroticism depends largely on inherited characteristics and reflects physiological differences in the central nervous and related systems. (7) Moreover, neuroticism appears to be significantly positively related to muscle tension. (8,9)

In 1994, Schiff and Lamon found that left-side muscle contractions cause sadness and that right-side muscle contractions result in a more positive mood. They suggested that the contralateral cerebral hemisphere mediates between certain psychological states and unilateral muscle contractions. (10) In a clinical study on generalized anxiety disorders, tension in the left side of the gastrocnemius correlated with right but not left hemisphere activity. (1) The precise mechanism relating laterality or imbalance of muscle activities and psychological state was not clarified. The purpose of this study was to test our hypothesis that laterality and imbalance of muscle tension relate to personality dimensions.

METHODS

Study Subjects

Subjects were 23 healthy volunteers from Tokyo (6 men and 17 women). Subject age ranged from 26 to 65 years, and average age was 40. All subjects were right-handed and had no neuromuscular disease.

Measurement of Muscle Stiffness

We used a tissue stiffness meter (PEK-1, IMOTO), which directly measures muscle stiffness from the skin surface. The main pointer and subcylinders with different spring constants are pressed against the site of measurement and the distance that the main pointer moves is the stiffness index, measured in grams per square millimeters (g/[mm.sup.2]). This procedure has high reproducibility and validity for measuring the tension of body tissue in human subjects. (11) We measured right and left sides of 4 muscles--sternocleidomastoideus, trapezius, latissimus dorsi, and rectus abdominis--3 times and averaged the value for each muscle. We selected these muscles because they were often involved in stress-related disorders such as tension headache, (12) spasmodic torticollis, (13) and fibromyalgia. (14) The subjects reclined approximately 10 minutes for measuring muscle stiffness. The measurement was taken at the midpoint of the muscle.

Psychological Questionnaire

We asked all subjects to complete the Eysenck Personality Questionnaire (EPQ) before measuring muscle stiffness. The Japanese-adapted version of the EPQ includes the following 4 subscales: extraversion, neuroticism, psychoticism, and a lie scale. It has 25 items rated on a 5-point Likert-type scale. Extraversion represents sociability, liveliness, and urgency. Neuroticism represents emotional instability and anxiousness. The psychoticism scale represents tough-mindedness, aggressiveness, coldness, and egocentricity. Lie score reflects not only unsophisticated dissimulation, but also a stable personality factor associated with social naivety or conformity. (15) For the validity of the Japanese version of the EPQ, the Cronbach's alpha coefficients for the dimension of extraversion, neuroticism, psychoticism, and lie scale were .84, .85, .68, and .79, respectively. (16)

DATA ANALYSIS

We calculated laterality of muscle stiffness by subtracting the stiffness of the right side from the left side for each muscle. We divided subjects into 2 groups depending on whether muscle stiffness was greater on the subject's left or right side. We calculated imbalance of muscle stiffness as the absolute value of the difference of muscle stiffness between the right and the left sides for each muscle. We divided the subjects into 2 groups based on whether the subject's imbalance of muscle stiffness was higher or lower than the mean score of imbalance. All data are expressed as mean [+ or -] standard deviation (SD). We compared the data with student's t test. We used the SPSS version 10.0 (SPSS, Chicago, USA) for statistical analyses.

RESULTS

Table 1 presents the data for the muscle stiffness index of the left and right sides, laterality, and imbalance. Muscle stiffness of the bilateral latissimus dorsi was higher than that for other muscles. Muscle stiffness of the bilateral sternocleidomastoideus was lower than that for other muscles. There were considerable individual differences in laterality of muscle stiffness as indicated by individual difference because the standard deviations range from 3.3 to 5.7. Imbalance of muscle stiffness had standard deviations ranging from 2.0 to 3.4.

We have presented scores on the EPQ subscales in our study and reported normals in Table 2. Psychoticism was slightly higher than normal and neuroticism was slightly lower than normal. We found no significant difference in all subscales of the EPQ by sex and age (data not shown).

The relationships between laterality of muscle stiffness and personality dimensions are presented in Table 3. Subjects with left predominant muscle stiffness of the rectal abdominis had a significantly higher neuroticism score (18.6 [+ or -] 3.7) than subjects with right predominant muscle stiffness (14.7 [+ or -] 2.7, p = .02). No other muscle measures related to personality dimensions.

The relationships between imbalance of muscle stiffness and personality dimensions are presented in Table 4. Subjects with greater imbalance of muscle stiffness of the latissimus dorsi had a significantly higher neuroticism score (19.3 [+ or -] 4.0) than those with lower imbalance (16.4 [+ or -] 2.9, p = .03). Subjects with greater imbalance of muscle stiffness of the trapezius also had a significantly higher psychoticism score (14.4 [+ or -] 2.5) than those with lower imbalance (12.2 [+ or -] 2.5, p = .049). Other muscles did not relate to differences in personality dimensions.

DISCUSSION

The basic Eysenck's personality dimensions of neuroticism and psychoticism scales indicate a significantly positive relationship to muscle tension. (8,9) In our results, subjects with left predominant muscle stiffness of the rectal abdominis had a significantly higher neuroticism score than did subjects with right predominant muscle stiffness. Subjects with greater imbalance of muscle stiffness such as latissimus dorsi or trapezius had significantly higher neuroticism and psychoticism scores than did those with lower imbalance. We suggest that bilateral muscle stiffness, including laterality and imbalance of muscle tension, is related to personality dimensions.

A prior study showed that left-side muscle contractions related to sadness and right-side muscle contractions related to a more positive state that is not readily characterized, but appears to be a mixture of feelings of well-being and aggression. (10) The neuroticism scale might suggest negative feelings, as well as sadness, and it also suggests that contralateral cerebral hemisphere mediates between certain psychological states and unilateral muscle contraction. (10) Other studies showed that the neural connections between the lower facial muscles and the hemispheres were predominantly crossed. (17) Clinical and experimental evidence suggested that the left and right hemispheres were involved in positive and negative emotions, and concluded that the left corner of the facial muscles is activated through afferent feedback to the right hemisphere and its negative emotions. (18,19) In another study on generalized anxiety disorders, the left-side gastrocnemius correlated with right but not left hemisphere activity. (1) Therefore, asymmetrical brain activity may mediate between certain psychological states and laterality or imbalance of muscle activities.

The number of subjects in our study was relatively small and was made up of healthy volunteers. In the future, we need larger sample sizes and we need to compare healthy subjects with subjects that have psychosomatic disorders. Although the EMG has been widely and exclusively used to measure electrical activity in target muscles, we use a tissue stiffness meter to evaluate the muscle status so that the muscle tension itself was in the static state. It is necessary to investigate the relationship between each muscle stiffness and brain activities by electroencephalogram, positron emission tomography, and so on. It is important in future studies to confirm whether the laterality and imbalance of muscle stiffness could be changed by psychological variables.

To summarize our study, a part of the laterality of muscle stiffness is related to neuroticism and a part of the imbalance of muscle stiffness is related to neuroticism and psychoticism. These results suggest that asymmetrical muscle status may derive from contralateral cerebral hemisphere mediation between certain psychological states and unilateral muscle contraction.

NOTE

For comment and further information, please address correspondence to Naoki Nakaya, Department of Behavioral Medicine, Tohoku University Graduate School of Medicine, 2-1 Seiryomachi, Aoba-ku, Sendai 980-8575, Japan (nakaya-thk@ umin.ac.jp).

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Drs Nakaya, Kanazawa, and Fukudo are with the Department of Behavioral Medicine, Tohoku University Graduate School of Medicine, Sendai, Japan. Dr Kumano is with the Department of Stress Science and Psychosomatic Medicine, Graduate School of Medicine, University of Tokyo, Tokyo, Japan. Dr Minoda is with the Balance Therapy University, Fukuoka, Japan.

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