Twitch forces in human masseter during experimental muscle pain

Abstract

Motor control strategies in relation to pain are not completely understood. Decreased firing rates of single motor units (SMU), but with a constant bite force have recently been demonstrated in the presence of masseter pain suggesting the existence of compensatory mechanisms (Sohn et al., 2000). The aim of this study was to describe twitch force characteristics of masseter SMU using spike triggered averaging (STA) before and during experimental muscle pain. Ten healthy subjects (20–29 years) participated. Fine wire electrodes were inserted into the masseter to record SMU activity. Subjects performed an isometric contraction on a force transducer (Kistler, Switzerland) with their incisor teeth to keep the SMU steady firing. A template‐matching procedure was used to identify SMU and the corresponding force was determined with STA. One‐min test series were recorded before 1, 4, 7, 10, 15 and 20 min after injection of 0·2 mL 100 µg mL−1 capsaicin into the masseter. Pain was continuously scored on 10‐cm visual analogue scales (VAS). The amplitude of twitch force and contraction time was measured. Injection of capsaicin into the masseter caused a deep, painful sensation with a VAS peak of 5·5 ± 0·6 cm. Twitch amplitudes were significantly influenced by pain (anova: P < 0·001) with higher amplitudes during pain‐1‐min (56·1 ± 10·7 mN) and pain‐4‐min (53·5 ± 11·9 mN) compared with before pain (33·9 ± 11·6 mN; P < 0·05). The contraction time (51·6 ± 4·4 ms) was not significantly changed during pain (anova: P > 0·429). The present study suggests that increased twitch force during painful muscle contraction could compensate for decreased firing rates of SMU.