Lower excitability of the corticospinal tract to transcranial magnetic stimulation during lengthening contractions in human elbow flexors Hirofumi Sekiguchi * , Toshitaka Kimura, Kentaro Yamanaka, Kimitaka Nakazawa MotorDysfunctionDivision,ResearchInstitute,NationalRehabilitationCenterfortheDisabled,4-1,Namiki,Tokorozawa359-8555,Japan Received 24 May 2001; received in revised form 4 August 2001; accepted 7 August 2001 Abstract The purpose of this study was to characterize the neuromuscular control during shortening (SHO) and lengthening (LEN) contractions by investigating the input-output (I/O) property in the corticospinal tract. To this end, the relation between various stimulus intensities applied via transcranial magnetic stimulation and the size of motor evoked poten- tials was investigated in six healthy subjects during elbow flexion and extension. The measured I/O property demon- strates a sigmoidal shape, and is characterized by a plateau value, maximum slope and threshold. The results demonstrated that both the plateau value and maximum slope were significantly lower during LEN contraction than during SHO contraction (P , 0:05), whereas the threshold was found not to be significantly different. These results suggest that both the maximum excitation level and the gain of the corticospinal tract are reduced during LEN contrac- tions. q 2001 Elsevier Science Ireland Ltd. All rights reserved. Keywords: Lengthening contraction; Input-output property; Corticospinal tract; Transcranial magnetic stimulation It has been reported that the motor evoked potentials (MEP) elicited by transcranial magnetic stimulation (TMS) with a constant stimulus intensity (approximately 1.5 times the motor threshold at rest) were smaller during lengthening (LEN) contractions than those during shorten- ing (SHO) contractions in elbow flexors [1]. However, it is well known that the relation between MEPs and various stimulus intensities does not change linearly [3,4]. In fact, in this present study, we observed that this relation clearly showed a sigmoidal shape during both LEN and SHO contractions. Therefore, any conclusion derived from such constant stimulus intensity measurements may be limited, because the results might well be reversed depending on the stimulus intensity. Based on this reasoning, it is not enough to only compare the response evoked by a specific stimulus intensity for a detailed analysis of task dependency in the corticospinal tract. Devanne et al. [4] proposed that the input-output (I/O) property (MEPs versus stimulus inten- sity) in the corticospinal tract would be a useful measure in quantitative studies dealing with the involvement of the motor cortex in various motor tasks (see also [3]). This I/O property was characterized by the plateau value, maximum slope and threshold. They put forward that a clear task- dependent change of the involvement of the motor cortex would be represented by one or more of these parameters changing independently of the background electromyo- graphic activity (BGA EMG) levels [3]. Hence, the purpose of this study was to clarify whether the MEPs during LEN contractions are always smaller than those during SHO contractions independent of the stimulus intensities, and to characterize the neuromuscular control in each contraction type by using the parameters of the I/O property. Six healthy male subjects, aged 24–31 (mean ^ SD, 28.0 ^ 2.4) years, with no history of neurological disorders voluntarily participated in this study. They were right-hand dominant and their brachioradialis (BR) and biceps brachii (BB) were studied. All of the subjects gave informed consent prior to participating in this experiment and the local ethics committee approved the experimental proce- dures. Subjects were seated upright in a chair with their right shoulder positioned at 90 degrees of abduction. The experimental device allowed a load of various weights, operated by pulleys, to provide resistance to the elbow flex- ion movement in a horizontal plane. For SHO and LEN contractions, the subjects were required to rotate their fore- Neuroscience Letters 312 (2001) 83–86 0304-3940/01/$ - see front matter q 2001 Elsevier Science Ireland Ltd. All rights reserved. PII: S0304-3940(01)02197-8 www.elsevier.com/locate/neulet * Corresponding author. Tel.: 181-42-995-3100 ext. 2533; fax: 181-42-995-3132. E-mail address: hiro@rehab.go.jp (H. Sekiguchi).