Stretch reflex excitability of the anti-gravity ankle extensor muscle in elderly humans N. Kawashima, 1 K. Nakazawa, 1 S.-I. Yamamoto, 2 D. Nozaki, 1 M. Akai 1 and H. Yano 1 1 Department of Rehabilitation for the Movement Functions, Research Institute of National Rehabilitation Center for Persons with Disabilities, Namiki, Tokorozawa city, Japan 2 Faculty of System engineering, Shibaura Institute of Technology, Fukasaku, Saitama city, Japan Received 2 April 2003, accepted 10 September 2003 Correspondence: N. Kawashima, Department of Motor Dysfunction, Research Institute NRCD, 4-1 Namiki, Tokorozawa, Saitama 359-8555, Japan. Abstract Aim: To examine whether the stretch reflex excitability of the soleus muscle changes with age, stretch reflexes at rest (REST) and during weak voluntary contractions (ACT) were elicited in 18 older and 14 younger subjects. Method: The amplitude of the stretch reflex responses and gain, defined as the gradient of the regression line for the relation between stretch reflex responses against the angular velocity of the applied perturbation, were evaluated in each short-latency (M1) and two long-latency components (M2 and M3). Results: It was found that in the older group, both the amplitude and gain of the M1 component did not change from the REST to the ACT conditions, whereas in the younger group both variables significantly increased from the REST to ACT conditions. The latency of the M1 component was signifi- cantly shorter under the REST condition (older vs. younger: 51.8 Æ 7.37 vs. 55.1 Æ 8.69 ms), while no group differences were found in those variables under the ACT condition, suggesting that the muscle-tendon complexes of SOL muscles of the older subjects were less elastic and had less slack, probably due to age-related histochemical alterations. Further, the Hoffman reflex (H-reflex), elicited during the REST condition in 10 older and 11 younger subjects showed no significant differences, suggesting that the soleus motoneuron response to the Ia input was comparable between the two subject groups. Conclusion: The histochemical alterations occurring with the ageing pro- cess might augment the short-latency stretch reflex in the SOL muscle without enhancement of motoneuronal excitability, and this effect might be masked when the muscle is voluntarily activated. Keywords ageing, gain modulation, stretch reflex, voluntary contraction. There is no doubt that age-related alterations occur in the human nervous system, which should in turn have certain influences on motor function. Surprisingly, however, the results reported thus far regarding age- related alterations in even the most simple motor function, the stretch reflex have been controversial. For example, it has been reported both that the stretch- reflex EMG response is attenuated (Corden & Lippold 1996, Lin & Sabbahi 1998) and remains unchanged (DeVries et al. 1985, Nardone et al. 1995) in the ageing process. In this study, we focused on the age-related changes in the stretch reflex of one of the main postural muscles, the soleus muscle, as this muscle plays a key role in stabilizing the ankle joint during upright posture or locomotive movements. Previously, Koceja et al. (1995) have demonstrated that elderly subjects lack the ability to reduce the Hoffman reflex (H-reflex) amplitude when Acta Physiol Scand 2004, 180, 99–105 Ó 2004 Scandinavian Physiological Society 99