Hypertonic Saline-Evoked Muscle Pain in the Quadriceps Reduces Neuromuscular Performance and Alters Corticospinal Excitability.

Muscle pain can alter corticospinal function, but the specific excitatory/inhibitory effects on the quadriceps across different levels of corticospinal neuron recruitment remain unclear. Furthermore, maximal force production is reduced with muscle pain, but how the rate of force development, a key component of neuromuscular function remains less-known. To investigate this, healthy participants completed an isometric maximal voluntary contraction (MVC) followed by submaximal, intermittent contractions after receiving a hypertonic saline injection into the vastus lateralis to cause quadriceps pain (HYP) or isotonic saline, a non-painful control (ISO). Peripheral nerve stimulation was delivered during and after MVCs to determine neuromuscular function. Transcranial magnetic stimulation (TMS) was delivered at 120% and 150% of active motor threshold during submaximal contractions to determine corticospinal excitability/inhibition, along with paired-pulse TMS to determine short-interval intracortical inhibition (SICI). Results revealed a moderate effect size (ES) reduction in MVC force (ES = -0.68, = 0.020), early-phase rate of force development (ES = -0.57, = 0.029), and voluntary activation (ES = -0.66, = 0.008) in HYP compared to ISO. Corticospinal excitability increased in HYP compared to ISO (ES = 0.60, = 0.023), whereas corticospinal inhibition decreased in HYP at higher stimulation intensities only (ES = 0.63, = 0.017). Conversely, SICI increased in HYP compared to ISO (ES = 0.58, = 0.035). Our findings indicate that muscle pain induced by a hypertonic saline injection reduced quadriceps neuromuscular function due to centrally mediated mechanisms, potentially involving both excitatory and inhibitory effects on the corticospinal tract.