External Resistance Is Imperative for Training-Induced Efferent Neural Drive Enhancement in Older Adults

Abstract

<jats:title>ABSTRACT</jats:title> <jats:p>Strength training performed with heavy loads and maximal intended velocity is documented to enhance efferent neural drive to maximally contracting musculature in older adults. However, it remains unclear whether the neural plasticity following training result from motor skill learning or if external resistance is a prerequisite. To investigate this, we assessed electrically evoked potentials (H-reflex and V-waves normalized to maximal M-wave) and voluntary activation (VA) in 36 older adults (73±4 years) randomized to 3 weeks of plantar flexion strength training, with (maximal strength training; MST) or without (unloaded ballistic training; UBT) heavy external loading (90% of one repetition maximum), or a control group. Both training groups aimed to execute the concentric phase of movement as fast and forcefully as possible. The MST group improved maximal voluntary contraction (MVC) and rate of force development (RFD) by 18±13% (p=0.001; Hedges g=0.66) and 35±17% (p&amp;lt;0.001; g=0.94), respectively, and this was different (MVC: p=0.013; RFD: p=0.001) from the UBT group which exhibited a 7±8% (p=0.033; g=0.32) increase in MVC and a tendency to increase RFD (p=0.119; g=0.22). Concomitant improvements in efferent neural drive (Vmax/Msup-ratio: 0.14±0.08 to 0.24±0.20; p=0.010) and a tendency towards increased VA (79±9% to 84±5%; p=0.098), were only apparent after MST. No changes were observed in Hmax/Mmax-ratio for the groups. In conclusion, external loading during exercise training appears to be a prerequisite for efferent neural drive enhancement in older adults. Thus, strength training with heavy loads should be recommended to counteract the typically observed age-related decline in motoneuron firing frequency and recruitment.</jats:p>

Maximal strength training-induced increase in efferent neural drive is not reflected in relative protein expression of SERCA

“…The improvements in maximal strength and RFD were accompanied by increased efferent neural drive ( V max / M sup -ratio). Interestingly, we have recently documented, albeit in an elderly population, that a potential learning effect of training is not manifested as increased V max / M sup -ratio (Unhjem et al 2021 ). This strengthens the assumption with which we can assume that the increases observed in V max / M sup -ratio are an effect of the strength training itself, although we cannot rule out that a learning effect contributed to the increases in 1RM and RFD.…”

Maximal intended velocity enhances strength training-induced neuromuscular stimulation in older adults

“…In the other condition the participants were instructed to perform the concentric action as fast as possible, emphasizing a maximal intentional velocity (MIV), as previously used in our laboratory (Wang et al 2017;Unhjem et al 2021). The purpose of applying MIV is to maximally stimulate the neural descending motor drive to contracting musculature and thus to maximally recruit muscle fibers (Tøien et al 2018b;Unhjem et al 2021;Behm and Sale 1993). Importantly, the actual movement velocity is relatively slow, especially at higher intensities, despite the maximal intended velocity.…”

The Application of Exercise Training for Diabetic Peripheral Neuropathy

“…Previous research has shown that DPN patients are slower to develop ankle and knee joint force, which has been attributed to diminished muscle activation [ 46 , 117 , 118 , 119 , 120 ]. This decrement in strength and power seen in patients with DPN can be further exacerbated in older adults who contend with age-related sarcopenia [ 14 , 121 ]. Engagement in regular RT has been demonstrated to be a primary intervention for increasing strength and power through neural adaptations across ages [ 122 , 123 , 124 ].…”

“…Engagement in regular RT has been demonstrated to be a primary intervention for increasing strength and power through neural adaptations across ages [ 122 , 123 , 124 ]. The use of heavy external loads, high velocity movements, and motor skill learning within RT aids in the increase of efferent neural drive through improved motor unit recruitment, motor neuron firing frequencies, motor unit synchronization, and the attenuation of antagonist co-activation [ 14 , 125 ]. With these neuromuscular adaptations in mind, various RT programs have been utilized to help reduce fall risk through enhanced balance and stability, increased core strength, work efficiency, and skeletal muscle fiber size [ 126 , 127 , 128 ].…”

“…For example, (1) microvascular dysfunction from type 2 diabetes-associated peripheral neuropathy is not detected with common macrovascular assessments (ankle brachial index); (2) muscle specific metabolic function is not adequately measured with whole body indices (homeostatic model assessment of insulin resistance score); and (3) muscle strength, a measure of motor neuropathy, is unable to decouple neural transmission from a muscle’s intrinsic contractility. Different forms of exercise training, including traditional and circuit-based resistance training, have been shown to increase the aforementioned mediators in older adults with and without diabetes, but research done specifically on DPN patients is lacking [ 11 , 12 , 13 , 14 , 15 , 16 , 17 ]. The purpose of this review is to (1) briefly describe the pathophysiology of DPN and (2) discuss the effects of exercise training interventions on sensorimotor, metabolic, and physical functions in people with DPN.…”