Modulation of reflex responses in activated ankle dorsiflexors differs in healthy young and elderly subjects

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 (C70 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 shortlatency 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.

Influence of age and posture on spinal and corticospinal excitability

“…During isometric contractions, the amplitude of the H reflex increases with the level of background EMG (Butler et al, 1993;Klass et al, 2011), such effect being known as the "automatic gain compensation" that reflects the increase in motor neurone excitability with the increase in background muscle activity (Matthews, 1986). In contrast to the automatic gain compensation, a lesser H max was recorded in elderly compared with young and middle-aged adults during submaximal isometric contraction, despite a greater soleus background EMG activity in this age group.…”

“…Indeed, elderly adults exhibit greater electromyographic (EMG) activity of leg muscles during upright standing compared with young adults (Baudry et al, , 2014bBillot et al, 2010). As the reflex amplitude is known to increase differently with the level in EMG activity in young and elderly adults (Klass et al, 2011), background EMG should be carefully monitored when comparing seated and standing posture. Whereas in some studies, details on how EMG was controlled between postures were not provided (Koceja et al, 1995;Tsuruike et al, 2003), AnguloKinzler et al (1998) asked their subjects to keep upright standing while matching different levels of background EMG for the soleus.…”

Age‐related influence of vision and proprioception on Ia presynaptic inhibition in soleus muscle during upright stance

“…Although these age‐related differences may originate from a loss of peripheral axons with ageing, a similar D1 inhibition between young and elderly adults during upright stance on a rigid surface was obtained for the same relative conditioning stimulus. Moreover, similar sizes for the short‐latency stretch reflex and H reflex recorded in the tibialis anterior for young and elderly adults indicates that the spinal reflex pathway is not changed substantially in healthy elderly subjects when seated (Klass et al 2011). In addition, elderly adults exhibited a greater decrease in peak motor unit discharge probability than young adults with a change in balance conditions.…”

“…Ageing is accompanied by a change in the modulation of the Ia afferent pathway during steady contractions of leg muscles (Earles et al 2001; Kido et al 2004; Klass et al 2011) and upright standing (Earles et al 2000). Observations indicate that elderly adults exhibit greater Ia presynaptic inhibition than young adults at rest (Morita et al 1995; Kido et al 2004), but modulate it less during isometric contractions (Earles et al 2001) and when going from seated to standing posture, as assessed by one method consisting of conditioning the H reflex (Koceja & Mynark, 2000).…”

Aging causes a reorganization of cortical and spinal control of posture

“…The observation of smaller early responses is consistent with the age-related decrease in H-reflex amplitude in the soleus (Koceja et al, 1995; Baudry and Duchateau, 2012). However, smaller H-reflexes were not observed in the tibialis muscle at rest or during small voluntary contractions (Klass et al, 2011). Therefore, the reduced postural response in the tibialis anterior may indicate a specific downmodulation in upright standing.…”