The effects of α-lactalbumin supplementation and handgrip contraction on soleus motoneuron excitability

Introduction We tested two strategies that hypothetically increase serotonin availability (alpha-lactalbumin consumption and a remote submaximal handgrip contraction) on estimates of persistent inward currents (PICs) amplitude of soleus muscle in healthy participants. Methods With a randomised, double-blind, and cross-over design, 13 healthy participants performed triangular-shaped ramp contractions with their plantar flexors (20% of maximal torque), followed by a 30-s handgrip sustained contraction (40% of maximal force) and consecutive repeated triangular-shaped contractions. This was performed before and after the consumption of either 40 g of alpha-lactalbumin, an isonitrogenous beverage (Zein) or an isocaloric beverage (Corn-starch). Soleus motor units discharge rates were analysed from high-density surface electromyography signals. PICs were estimated by calculating the delta frequency (Delta F) of motor unit train spikes using the paired motor unit technique. Results Delta F (0.19 pps; p = 0.001; d = 0.30) and peak discharge rate (0.20 pps; p < 0.001; d = 0.37) increased after the handgrip contraction, irrespective of the consumed supplement. No effects of alpha-lactalbumin were observed. Conclusions Our results indicate that 40 g of alpha-lactalbumin was unable to modify intrinsic motoneuron excitability. However, performing a submaximal handgrip contraction before the plantar flexion triangular contraction was capable of increasing Delta F and discharge rates on soleus motor units. These findings highlight the diffused effects of serotonergic input, its effects on motoneuron discharge behaviour, and suggest a cross-effector effect within human motoneurons.

Automated summary

This study tested the effects of two strategies that hypothetically increase serotonin availability on persistent inward currents (Pics) amplitude of soleus muscle in healthy participants. The results showed that performing a submaximal handgrip contraction increased the discharge rates and Delta F of soleus motor units, while alpha-lactalbumin supplementation had no effect on motoneuron excitability.