The Effect of Creatine Loading on Neuromuscular Function

Abstract

Although the existing body of creatine (Cr) literature is robust and supports its role as a metabolic ergogenic aid, a limited number of studies have examined the effects of Cr supplementation on neuromuscular function. PURPOSE: The purpose of the present study was to examine the effects of fatigue on voluntary peak force (PF), percent voluntary activation (%VA), peak twitch force (PTF), peak twitch rate of force development (RFD), and electromyographic (EMG) amplitude of the soleus (SOL) and medial gastrocneimus (MG) muscles prior to and following short term creatine loading. METHODS: Using a double-blind, placebo-controlled, randomized design, 29 moderately trained participants were assigned to a creatine (Cr; n = 16, age: 21.9 ± 2.6 yrs, stature: 174.6 ± 10.9 cm, body mass: 73.0 ± 10.9 kg) or placebo (Pl; n = 13, age: 22.2 ± 3.5 yrs, stature: 175.7 ± 9.5 cm; body mass: 71.9 ± 14.2 kg) group. Subjects supplemented their diet four times daily for five days with 5g of Cr + 15g dextrose (Cr) or 20g dextrose (Pl). At baseline (week 1) and post-testing (week 2) all subjects performed an isometric maximal voluntary contraction (MVC) and the twitch interpolation technique prior to and immediately following a four minute fatigue protocol of the plantar flexor muscles on a custom-built load cell apparatus attached to a calibrated isokinetic dynamometer. The fatiguing protocol included four sets of 10 intermittent isometric contractions (7-sec on / 3-sec off) at 70% of MVC. Additionally, a familiarization session was completed prior to baseline measurements. Separate three-way mixed factorial ANOVAs [acute (pre vs. post) x chronic (week 1 vs. week 2) x treatment (Cr vs. Pl)] were used to analyze the PF, %VA, PTF and RFD. A four-way mixed factorial ANOVA [muscle (SOL vs. MG) x acute (pre vs. post) x chronic (week 1 vs. week 2) x treatment (Cr vs. Pl)] was used to analyze EMG values. RESULTS: The acute bout of fatiguing contractions resulted in a significant main effect for acute resulting in a loss in voluntary strength (–15.7%, P < 0.001), %VA (−7.3%, P = 0.003) and an increase in RFD (4.7%, P = 0.037) for both groups. There was a significant chronic × treatment interaction for MVC. The chronic effects of Cr supplementation from week 1 to week 2 demonstrated a maintenance in voluntary strength (MVC) for the Cr group (−0.5%, P = 0.710) while the PL group showed a significant decline in strength (−5.8%, P = 0.035). There was also a significant chronic × treatment interaction for the evoked twitch properties, resulting in an increase in PTF (5.7%, P = 0.019) and a non-significant increase in RFD (4.6%, P = 0.071) for the Cr group only. In addition, there was a main effect for treatment for %VA where the Cr group was 7.3% more active than the PL group across all time points (P = 0.023). Furthermore, there was a main effect for chronic where EMG amplitude was higher (20.0%, P = 0.023) for both groups at week 2 compared to week 1. CONCLUSIONS: These findings indicated that chronic Cr supplementation preserved maximal voluntary strength and enhanced evoked twitch properties. However, there were no acute differences between groups before and after the fatiguing protocol. PRACTICAL APPLICATIONS: Cr supplementation enhanced maximal contractile properties, but had a limited influence during a fatiguing bout of exercise in the present study. The ability to maintain maximal force between training sessions may lead to practical increases in strength.