Motor unit discharge rate following twitch potentiation in human triceps brachii muscle C.S. Klein a , T.D. Ivanova b , C.L. Rice a,c , S.J. Garland b,d, * a School of Kinesiology, The University of Western Ontario, London, Ontario, N6G 1H1, Canada b School of Physical Therapy, The University of Western Ontario, London, Ontario N6G 1H1, Canada c Department of Anatomy and Cell Biology, The University of Western Ontario, London, Ontario, N6G 1H1, Canada d Department of Physiology, The University of Western Ontario, London, Ontario, N6G 1H1, Canada Received 10 September 2001; received in revised form 17 October 2001; accepted 17 October 2001 Abstract It has been proposed that during brief voluntary contractions, twitch potentiation may sustain force output despite a decline in motor unit discharge rate. This study examined the evoked twitch force and motor unit discharge rates during submaximal voluntary contractions of the triceps brachii muscle before and after a 5 s conditioning contraction (CC) at 75% of maximal voluntary force. After the CC, twitch force potentiated (,1.3–2-fold), and the discharge rate in 33 of 35 motor units declined significantly by 1–6 Hz. The increase in twitch force was significantly correlated with the decline in discharge rate (r ¼ 20:74). These findings suggest that the extent of the decrease in motor unit discharge rate following a CC is associated with the magnitude of twitch potentiation. q 2001 Elsevier Science Ireland Ltd. All rights reserved. Keywords: Motoneuron; Contractile properties; Rate coding; Recruitment The discharge rate of motor units has been reported to decline during submaximal constant-force contractions held for seconds or minutes [4,7,8]. It has been suggested that voluntary force may be maintained, despite the decline in discharge rate, by twitch potentiation [4]. However, no studies have tested this hypothesis, and little data are avail- able on whether twitch potentiation occurs following submaximal voluntary contractions [10]. Thus, the purpose of this study was to determine the relationship between changes in motor unit discharge rate and twitch force in the triceps brachii muscle following twitch potentiation induced by a submaximal conditioning contraction (CC). Six healthy men, 20–42 years (mean, 25 ^ 5 years), parti- cipated in the study and all procedures were approved by the University Ethics Committee. Subjects were seated with the non-dominant (left) forearm resting in front of them on a platform in the horizontal plane [6]. The shoulder and elbow were positioned securely in 80 and 908 of flexion, respec- tively. The forearm was semipronated and the wrist was secured in a U-shaped brace that was mounted on a force transducer to measure elbow extensor force. Motor units were recorded with subcutaneous fine wire electrodes positioned over the lateral head of the triceps brachii. The subcutaneous electromyogram (EMG) signals were amplified and filtered with a 10 Hz–10 kHz bandpass. Surface EMG was recorded with 8 mm electrodes placed in a bipolar configuration over the lateral head of triceps brachii and biceps brachii muscles. The surface EMG recordings were amplified and filtered at 10 Hz–2 kHz. Twitches were evoked with single pulses of 50 ms duration from a constant voltage electrical stimulator. The cathode was an 8 mm disc electrode, filled with conductive gel, and placed over the motor point of the lateral head of the triceps brachii. The anode was a lead plate (8.5 £ 5 cm), wrapped in gel-soaked gauze, and positioned lateral to the scapula over the teres minor muscle (to stimulate the radial nerve). A ground strap was placed around the right wrist. During the first of two sessions for each subject, the maximal voluntary force (MVC) was determined and the subject practiced the entire protocol, but without recording motor unit activity. The second session was held within a week of the first. The peak MVC force, determined in the first session, was used in the second session to avoid any potentiation that would be associated with performing a MVC. An initial maximal twitch was elicited first by apply- ing pulses of progressively greater voltage until the highest Neuroscience Letters 316 (2001) 153–156 0304-3940/01/$ - see front matter q 2001 Elsevier Science Ireland Ltd. All rights reserved. PII: S0304-3940(01)02389-8 www.elsevier.com/locate/neulet * Corresponding author. Tel.: 11-519-661-2111, ext. 88939; fax: 11-519-661-3866. E-mail address: jgarland@uwo.ca (S.J. Garland).