Caffeine-induced increase in voluntary activation and strength of the quadriceps muscle during isometric, concentric and eccentric contractions

Abstract

This study investigated effects of caffeine ingestion (8 mg/kg) on maximum voluntary torque (MVT) and voluntary activation of the quadriceps during isometric, concentric and eccentric contractions. Fourteen subjects ingested caffeine and placebo in a randomized, controlled, counterbalanced, double-blind crossover design. Neuromuscular tests were performed before and 1 h after oral caffeine and placebo intake. MVTs were measured and the interpolated twitch technique was applied during isometric, concentric and eccentric contractions to assess voluntary activation. Furthermore, normalized root mean square of the EMG signal was calculated and evoked spinal reflex responses (H-reflex evoked at rest and during weak isometric voluntary contraction) as well as twitch torques were analyzed. Caffeine increased MVT by 26.4 N m (95%CI: 9.3-43.5 N m, P = 0.004), 22.5 N m (95%CI: 3.1-42.0 N m, P = 0.025) and 22.5 N m (95%CI: 2.2-42.7 N m, P = 0.032) for isometric, concentric and eccentric contractions. Strength enhancements were associated with increases in voluntary activation. Explosive voluntary strength and voluntary activation at the onset of contraction were significantly increased following caffeine ingestion. Changes in spinal reflex responses and at the muscle level were not observed. Data suggest that caffeine ingestion induced an acute increase in voluntary activation that was responsible for the increased strength regardless of the contraction mode.

Figure 1

Effect of caffeine (CAF) on (A) maximum voluntary torque (iMVT), (B) voluntary activation and (C) normalized muscle activity of the quadriceps at MVT (Q RMS-EMG_MVT/M_max) during isometric, concentric and eccentric MVCs. ∗ denotes a significant difference between trials (ANCOVA with baseline-adjustment, ∗ P ≤ 0.05; ∗∗ P ≤ 0.01) and † denotes a statistical tendency towards a significant difference between trials (ANCOVA with baseline-adjustment, P ≤ 0.09). PLA: placebo trial

Figure 2

Effect of caffeine (CAF) on (A) rate of torque development (RTD) and (B) normalized muscle activity (Q RMS-EMG_RTD/M_max). ∗ denotes a significant difference between trials (ANCOVA with baseline-adjustment, ∗ P ≤ 0.05). PLA: placebo trial

Figure 3

An overview of the procedures carried out during neuromuscular testing of the knee extensors and the extracted parameters. The thin arrow indicates electrical stimulation at submaximal intensity and the thick arrow indicates electrical stimulation at supramaximal intensity. H_max: maximal H-reflex at rest, M_max: maximal M-wave at rest, H_sup: maximal H-reflex at 10% of MVC strength during an isometric voluntary contraction, M_sup: maximal M-wave at 10% of MVC strength during an isometric voluntary contraction, MVT: maximum voluntary torque, RTD: rate of torque development, RMS-EMG: root mean square of the EMG signal, ISO: isometric, CON: concentric, ECC: eccentric.