Gradual increment/decrement of isometric force modulates soleus stretch reflex response in humans Toshitaka Kimura a, * , Daichi Nozaki b , Kimitaka Nakazawa b , Masami Akai b , Tatsuyuki Ohtsuki a a Department of Life Sciences, Graduate School of Arts and Sciences, The University of Tokyo, 3-8-1, Komaba, Meguro, Tokyo 153-8902, Japan b Motor Dysfunction Division, Research Institute, National Rehabilitation Center for the Disabled, 4-1, Namiki, Tokorozawa 359-8555, Japan Received 28 March 2003; received in revised form 7 May 2003; accepted 13 May 2003 Abstract We investigated how a gradual isometric force-increment and -decrement task modulates the behavior of a soleus stretch reflex. Six healthy subjects performed isometric plantar-flexion torque exertion tasks in which they adjusted the torque level to a target changing triangularly (3 s/cycle; the amplitude is 15% of maximal voluntary isometric contraction (MVC)) or to constant targets (3.75, 7.5, and 11.25% of MVC). The magnitude of the short latency stretch reflex evoked by dorsi-flexing mechanical perturbation was strongly modulated by the muscle contraction states; it was the largest and the smallest for the torque-increment and -decrement phases, respectively. On the other hand, within each muscle contraction, the magnitude was independent of the torque level. Similar results were observed for the medium latency stretch reflex. These results indicate that when the central nervous system increases or decreases the muscular force, it simultaneously regulates the sensitivity of the stretch reflex. q 2003 Elsevier Science Ireland Ltd. All rights reserved. Keywords: Muscle relaxation; Muscle contraction; Short latency stretch reflex; Medium latency stretch reflex; Soleus muscle; Human The muscle activity level needed to realize a given muscular force is not constant but dependent on its past state. For example, the firing rate of a single motor unit corresponding to a given force level is smaller during the force-decrement phase (relaxation phase) than during the force-increment phase (contraction phase) [3,11]. Such a lower firing rate of a motor unit during relaxation has been explained by the properties of muscle fiber itself, such as twitch potentiation [1,3]. However, little is known about how the central nervous system (CNS) takes such an asymmetry between contraction and relaxation phases into consideration to control the muscular force level. Recently, considerable attention has been paid to the modulation of a stretch reflex system by the type of movement, such as walking and sitting [7], or by the muscular contraction modes, i.e. isometric, lengthening and shortening contractions [8,10]. For example, it has been shown that the stretch reflex is smaller during lengthening than isometric muscle contraction in the elbow flexor [8] and planter-flexor muscles [10], which prevents undesired muscle shortening during a smooth lengthening contraction. From a functional viewpoint, the stretch reflex magnitude may also be modulated even during the muscle contraction or relaxation phase because its enhancement during relaxation might be unsuitable for a smooth decrement in muscular force, as in the case of a lengthening contraction. We aimed to examine this hypothesis by investigating how the gradual increment or decrement of isometric force influenced the soleus (SOL) stretch reflex response in humans. Experiments were performed on six healthy subjects (five male and one female) aged between 25 and 32 years. None had histories of neurological disorder, and all of them gave their informed consent prior to the study. The local ethical committee approved the experimental procedures. The subjects were comfortably seated in a chair with their trunk immobilized by a strap, and the ankle joint of their right leg was fixed on the footplate of a servo-controlled torque motor (Senoh, Japan). The hip and knee joint flexion was 908 and 308, respectively (08: full extension). The ankle joint was fixed at a 108 plantar-flexion position (08: neutral position) by a mechanical stopper. The subjects performed a triangular isometric plantar-flexion torque exertion task, in 0304-3940/03/$ - see front matter q 2003 Elsevier Science Ireland Ltd. All rights reserved. doi:10.1016/S0304-3940(03)00612-8 Neuroscience Letters 347 (2003) 25–28 www.elsevier.com/locate/neulet * Corresponding author. Tel.: þ 81-3-5454-6887; fax: þ81-3-5454-4317. E-mail address: cc07706@mail.ecc.u-tokyo.ac.jp (T. Kimura).