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.
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).
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 significantly shorter under the REST condition (older vs. younger: 51.8 +/- 7.37 vs. 55.1 +/- 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.
The histochemical alterations occurring with the ageing process 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.
"In addition to cortical changes, also on the subcortical, spinal, and peripheral levels age-related alterations might occur and affect the transmission of efferent signals to the muscles (Kawashima et al., 2004; Kido et al., 2004; Wang et al., 1999). It has been proposed that pyramidal tract function is impaired (Léonard and Tremblay, 2007) and the number of descending projections in the pyramidal tract is decreased with age (Terao et al., 1994 "
"MEPs, H-reflexes and background EMG (bEMG) of the soleus of the right leg were recorded during two-legged hopping at distinct time intervals after ground contact: at the time of the short-latency response (SLR, 45 ms), the medium-latency response (MLR, 70) and the long-latency response (LLR, 120 ms). The latencies and durations of the abovementioned intervals were previously reported (Petersen et al. 1998, Grey et al. 2001, Kawashima et al. 2004, Leukel et al. 2008a,b, Taube et al. 2008). At the beginning of the experiment, subjects were asked to perform four bouts of two-legged hopping at 2.2 Hz. "
[Show abstract][Hide abstract] ABSTRACT: AimIt is accepted that leg stiffness (Kleg) increases when surface stiffness decreases, and vice versa. However, little is known how the central nervous system fulfills this task. To understand the effect of surface stiffness on the neural control of stretch-shortening cycle movements, this study aimed to compare modulation of spinal and corticospinal excitability at distinct phases after ground contact during two-legged hopping when changing from solid to elastic ground.Methods
Motor-evoked potentials (MEPs) induced by transcranial magnetic stimulation (TMS) and H-reflexes were elicited at the time of the short (SLR)-, medium (MLR)-, and long (LLR)-latency responses of the soleus muscle (SOL) during two-legged hopping on different stiffness surfaces, elastic and stiff.ResultsSOL H-reflexes during two-legged hopping on the elastic surface were lower at SLR and larger at LLR than on the stiff surface (p<0.05 for both comparisons). SOL MEP size was higher at the time of SLR during hopping on the elastic surface than on the stiff surface (p<0.05) although the background EMG was similar.Conclusion
It is argued that this phase-specific adaptation in spinal reflex excitability is functionally relevant to adjust leg stiffness to optimally exploit the properties of the elastic surface. Thus, the increased corticospinal excitability on the elastic surface may reflect a more supraspinal control of the ankle muscles in order to compensate the decrease in reflexive stiffness at the beginning of touch down and/or counteract the higher postural challenges associated with the elastic surface.This article is protected by copyright. All rights reserved.
"Age-related changes in spinal reflexes area Our results agree with most of previous works that recorded the stretch reflex responses at rest and during low contraction levels and showed non significant age-related difference for the SL component (Kawashima et al. 2004; Lin and Sabbahi 1998; Obata et al. 2010). The absence of a significant age-related decrement in H-reflex area contrasts with some studies (Angulo-Kinzler et al. 1998; Kido et al. 2004; Sabbahi and Sedgwick 1982) but not others (Kawashima et al. 2004; Scaglioni et al. 2003). As already mentioned, this discrepancy may be partly due to the variability of reflex responses, particularly when they are recorded at rest (Toft et al. 1991). "
[Show abstract][Hide abstract] ABSTRACT: During voluntary contractions, motor neurone activity is modulated by descending input and sensory feedback. Impaired excitatory afferent feedback with ageing may, therefore, alter motor control. This study investigated the age-related changes in afferent feedback through the recording of reflex responses during voluntary muscle activation. Short- and long-latency components of the stretch reflex and Hoffmann reflex (H-reflex) were recorded during voluntary contractions (10% of maximal voluntary contraction; MVC) of the ankle dorsiflexor muscles of young and elderly adults (≥70 years). Furthermore, the modulation of spinal reflex excitability was analyzed at different torque levels (10-50% MVC). The short-latency stretch reflex and the H-reflex areas were similar in the two age groups at 10% MVC whereas the area of the long-latency component of the stretch reflex augmented with ageing (P < 0.05). However, the area of the H-reflex increased linearly with the level of contraction up to 50% MVC in young adults, whereas it slightly increased to 30% MVC and plateaued thereafter in elderly adults. The absence of age-related changes in the short-latency stretch reflex and H-reflex areas suggests that the reflex circuitry and the sensitivity of the muscle spindles are not substantially affected by ageing. The modest increase in the H-reflex area with the contraction intensity in elderly adults, however, indicates that the modulation of afferent feedback is reduced with advancing age. This observation, associated with a greater long-latency stretch reflex, suggests that elderly adults rely more on central than peripheral mechanisms to regulate motor output of the dorsiflexor muscles.
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