Stretch reflex excitability of the anti-gravity ankle extensor muscle in elderly humans.
ABSTRACT 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.
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ABSTRACT: Both H (Hoffman) and T (tendon) reflexes have been used to study spinal cord excitability by activating the reflex pathway that links Ia afferents, spinal α-motoneurons (MNs) and muscle. Some experimental studies have shown that both reflexes are subject to reciprocal inhibition to a similar level. In the present study, we have used a complex model of the spinal cord and its innervated muscles to investigate the effects of reciprocal inhibition on both H and T reflexes. The recruitment curves of both electrically- and mechanically-evoked reflexes (H and T, respectively) were similar to experimental results in humans. The H-reflex had a latency of ~28 ms, whereas the T-reflex had a latency of ~37 ms. For an equivalent control condition, roughly the same number of MNs was recruited by either the electrical or mechanical stimulation. Reciprocal inhibition reduced the H-reflex amplitude by ~40% of its control value, whereas the amount of inhibition observed in T-reflex was ~53% of its control value. This difference was not statistically significant (p > 0.05), supporting the hypothesis that the postsynaptic inhibitory effect is similar in both H and T reflexes. The inhibition de-recruited approximately the same number of spinal MNs in both cases, with a more pronounced effect on high-threshold cells. Several other studies can be carried out with this neuromuscular model to better understand the spinal cord neurophysiology both in health and disease.XXIII Congresso Brasileiro de Engenharia Biomédica; 10/2012
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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.Acta Physiologica 07/2014; · 4.38 Impact Factor
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ABSTRACT: Falls are the primary cause of accidental death in older persons, producing increased morbidity, decreased independence, and billions in medical costs annually. Massage therapy (MT) may produce adaptations that decrease risk of falling. If MT can improve stability in older persons, it may provide a new intervention for this issue. Determine the acute effects of a 60-minute MT treatment on static and functional balance, neurological measures, heart rate, and blood pressure in healthy, older individuals. Laboratory A 2 by 4 (treatment by time) mixed factorial experimental design for the cardiovascular and postural control variables; independent variables were treatment with two levels (control, MT) and time with four levels (pretreatment baseline, immediate post-treatment, 20-minute post-treatment, 60-minute post-treatment). Neurological measures utilized a 2 by 2 mixed design, with testing conducted pre- and 60-minutes post-treatment. Thirty-five healthy, older volunteers (19 male and 16 female; ages 62.9 ± 4.6). A 60-minute full-body therapeutic massage. The control group rested quietly in the treatment room. Static (double-legged) and functional (single-legged) postural control with eyes-open and eyes-closed; Hoffmann-reflex measures; heart rate, and systolic and diastolic blood pressure. MT significantly decreased rectangular displacement area in both the eyes-open and eyes-closed, double-legged stance conditions (p < 0.05); displacement velocity in both eyes-open conditions (p < .05); and systolic and diastolic blood pressure (p < .05), while increasing heart rate (p < .05). MT also significantly lowered H(max)/M(max) ratios compared to controls (p = .002). Decreased H(max)/M(max) measures were correlated to improved stability. A single, 60-minute, full-body massage therapy treatment was shown to have a stabilizing effect on measures of static and dynamic balance and physiological factors related to stability in older adults. MT should be investigated as a potential intervention to decrease falls in older individuals.International Journal of Therapeutic Massage & Bodywork Research Education & Practice 09/2012; 5(3):16-27.