Neuromuscular responses to experimental muscle pain induced by exercise and saline injection

Abstract

The present thesis aimed to investigate neuromuscular responses to experimental pain induced by exercise and saline solution injection. For this, three studies were designed. Twenty physically active volunteers participated in the studies, 9 participants in study 1 (saline-induced pain), and 11 participants in studies 2 and 3 (exercise-induced pain). In study 1, participants performed two maximal isometric contractions (MVC) of the knee extensors, six explosive contractions and six contractions induced by electrical stimulation (twitch and octets) before and after (15 min of rest) an intramuscular injection of hypertonic saline (pain inducer) or isotonic (placebo) in two laboratory visits separated by seven days. In study 2, two MVC and five concentric-eccentric contractions of the knee extensors were performed. Assessments were repeated immediately after (0h), 24 hours (24h) and 48 hours (48h) after the intense exercise protocol. Electromyography was measured using decomposition electromyography during the protocol, and Delayed-onset muscle soreness (DOMS) was assessed before, 24h and 48h after the protocol. Skin temperature was recorded at all stages. In study 3, an protocol was carried out with 3 sets of 30 maximal coupled concentric-eccentric contractions of the knee extensor muscles. Electromyography was measured using decomposition electromyography during the protocol in the vastus lateralis (VL) and rectus femoris (RF) muscles. The results found were: (study 1) reduction in peak torque in the MVC in both conditions and in the rate of torque development (RTD) in the placebo. There was an increase in the RTD for single pulse and octets in the pain condition and in peak torque for the single pulse in both conditions, and in the total RTD for octets; (study 2) immediately after the protocol, a positive and moderate correlation was observed between the loss of force (dynamic) and the motor unit action potential amplitude (MUAP) of the VL. Two positive and moderate correlations with temperature, between RF firing rate and minimum temperature, and between VL recruitment threshold and mean temperature. 24h after, there was a positive and moderate correlation between peak torque (dynamic) and the MUAP of the VL; (study 3) reduction in torque for concentric and eccentric contractions in all series. The VL and RF muscles showed similar responses, with reductions in recruitment and decruitment thresholds. In conclusion, our studies have shown that saline- and exercise-induced pain result in different neuromuscular responses, including reduced torque and changes in muscle recruitment thresholds. Correlations between motor unit action potential amplitude and loss of strength suggest specific neuromuscular adaptations. The use of decomposition electromyography has provided detailed insights into muscle activity in response to pain.