Intrinsic motor neuron excitability is increased after resistance training in older adults

Abstract

This study investigated the effects of high-intensity resistance training on estimates of the motor neuron persistent inward current (PIC) in older adults. Seventeen participants (68.5±2.8 years) completed a 2-week non-exercise control period followed by 6 weeks of resistance training. Surface electromyographic signals were collected using two 32-channel electrodes placed over soleus to investigate motor unit discharge rates. Paired-motor unit analysis was used to calculate delta frequency (ΔF) as an estimate of PIC amplitudes during (a) triangular-shaped contractions to 20% of maximum torque capacity, and (b) trapezoidal- and triangular-shaped contractions to 20% and 40% of maximum torque capacity, respectively, to understand their ability to modulate PICs as contraction intensity increases. Maximal strength and functional capacity tests were also assessed. For the 20% triangular-shaped contractions, ΔF (0.58-0.87 pps; p≤0.015) and peak discharge rates (0.78-0.99 pps; p≤0.005) increased after training, indicating increased PIC amplitude. PIC modulation also improved after training. During the control period, mean ΔF differences between 20% trapezoidal- and 40% triangular-shaped contractions were 0.09-0.18 pps (p=0.448 and 0.109, respectively), which increased to 0.44 pps (p<0.001) after training. Also, changes in ΔF showed moderate-to-very large correlations (r=0.39-0.82) with changes in peak discharge rates and broad measures of motor function. Our findings indicate that increased motor neuron excitability is a potential mechanism underpinning training-induced improvements in motor neuron discharge rate, strength, and motor function in older adults. This increased excitability is likely mediated by enhanced PIC amplitudes, which are larger at higher contraction intensities.

Ageing reduces persistent inward current contribution to motor neurone firing: Potential mechanisms and the role of exercise

“…Several recent studies quantified the level of Δ F neuromodulation capacity in older adults using a range of paradigms. In one study, we found that ∆ F values scaled linearly and positively with contraction intensity between 10% and 30% of maximum force in soleus motor units in younger adults (Orssatto, Mackay et al., 2021), but, in a follow‐up study, we found no such effect of contraction intensity (20–40% of maximum) in untrained older adults (Orssatto et al., 2023). This might explain the lower firing frequencies relative to younger adults at higher intensities of contraction (rate code compression) (Barry et al., 2007), which is also consistent with reduced PIC‐mediated amplification in motor neurone firing.…”

“…These neural adaptations regardless of age largely underpin the early increases in muscular strength in advance of substantial changes in muscle mass, and continuously contribute to subsequent increases in strength as the training progresses (Pearcey et al 2021). Recent animal (Krutki et al, 2017) and human (Orssatto et al, 2023) data suggest that increases in PIC magnitude observed after resistance training in older adults could be a mechanism underpinning the increases in motor unit firing frequencies, functional capacity and strength in this population. Krutki et al (2017) studied the effects of resistance training on the properties of lower-and higher-threshold motor neurones using an animal model.…”

“…Fs have been compared between older and young adults with contraction intensities up to 20% of maximal capacity (Guo et al, 2023;Hassan et al, 2021;Orssatto, Borg, Blazevich et al, 2021) and the effects of resistance training have been investigated with intensities up to 40% (Orssatto et al, 2023). This limitation is primarily a result of the method employed to estimate PICs.…”

“…2021). Recent animal (Krutki et al., 2017) and human (Orssatto et al., 2023) data suggest that increases in PIC magnitude observed after resistance training in older adults could be a mechanism underpinning the increases in motor unit firing frequencies, functional capacity and strength in this population. Krutki et al.…”

Can local vibration alter the contribution of persistent inward currents to human motoneuron firing?

“…Finally, it remains unclear whether the magnitude of changes in Δ F reported during ramp contractions in this study has meaningful implications in motor control. Nonetheless, overall changes in Δ F as small as 0.58 Hz have recently been associated with large changes in peak firing rates and with moderate to very large changes in broad measures of motor function (Orssatto et al., 2023).…”