Physiological tremor is suppressed and force steadiness is enhanced with increased availability of serotonin regardless of muscle fatigue

Abstract

Although there is evidence that 5-HT acts as an excitatory neuromodulator to enhance maximal force generation, it is largely unknown how 5-HT activity influences the ability to sustain a constant force during steady-state contractions. A total of 22 healthy individuals participated in the study, where elbow flexion force was assessed during brief isometric contractions at 10% maximal voluntary contraction (MVC), 60% MVC, MVC, and during a sustained MVC. The selective serotonin reuptake inhibitor, paroxetine, suppressed physiological tremor and increased force steadiness when performing the isometric contractions. In particular, a main effect of drug was detected for peak power of force within the 8-12 Hz range (p = 0.004) and the coefficient of variation (CV) of force (p < 0.001). A second experiment was performed where intermittent isometric elbow flexions (20% MVC sustained for 2 min) were repeatedly performed so that serotonergic effects on physiological tremor and force steadiness could be assessed during the development of fatigue. Main effects of drug were once again detected for peak power of force in the 8-12 Hz range (p = 0.002) and CV of force (p = 0.003), where paroxetine suppressed physiological tremor and increased force steadiness when the elbow flexors were fatigued. The findings of this study suggest that enhanced availability of 5-HT in humans has a profound influence of maintaining constant force during steady state contractions. The action of 5-HT appears to suppress fluctuations in force regardless of the fatigue state of the muscle.

The Neuromuscular Fatigue-Induced Loss of Muscle Force Control

“…This serotonergic projection has the potential to have a profound impact on motor neuron excitability, increasing the efficacy of synaptic inputs by three-to five-fold [92]. It has recently been demonstrated that ingestion of paroxetine, a selective serotonin reuptake inhibitor which increases the availability of serotonin, decreased muscle force CV (i.e., increased steadiness) during contractions in unfatigued muscle, and during a two-minute fatiguing contraction at 20% of MVC [93]. Given the coherence between muscle force CV and the cumulative motor unit spike train, differences in CV between conditions have been suggested to reflect differences in common synaptic input [79].…”

Effect of New Zealand Blackcurrant Extract on Force Steadiness of the Quadriceps Femoris Muscle during Sustained Submaximal Isometric Contraction

“…As tryptophan is a precursor to serotonin and kynurenine, its availability may influence the CNS. In addition, Henderson et al [ 36 ] demonstrated that enhanced serotonin availability decreases physical tremor amplitude and improves steadiness during a sustained isometric contraction of the elbow flexors with the presence of fatigue. Interestingly, Lomiwes et al [ 8 ] demonstrated that during moderate intensity walking, blackcurrant juice was able to lower the perception of exertion, with a strong trend ( p = 0.06) for an increase in perceived mood, alongside an inhibition of the neurotransmitter platelet monoamine oxidase-B.…”

Muscle contractile properties directly influence shared synaptic inputs to spinal motor neurons

“…Recent investigations have demonstrated that oscillations in force output within the alpha band can be modulated by a range of factors. These factors include changes in nociceptive input (Yavuz et al, 2015), enhancement of serotonin availability by pharmacological means (Henderson et al, 2022), and alterations in the demands of a visuomotor task (Laine et al, 2014). Of particular interest are changes in muscle length which have been shown to exert a direct influence on alpha oscillations in force output, with shorter muscle lengths being associated with an increase in physiological tremor (Jalaleddini et al, 2017).…”