###### Abstract  
The main objective of this study was to assess the ergogenic effect of N-acetylcysteine (NAC) on myotendinous isometric force production in physically active people after being subjected to a fatigue protocol. Twenty-three physically active people were randomly divided into the following two groups: NAC (n = 12; age = 26.8 \(\pm\) 4.5 years, height = 173.1 \(\pm\) 7.2 cm, and weight = 75.5 \(\pm\) 7.5 kg), who received 2400 mg oral NAC, and control (n = 11; age = 23.4 \(\pm\) 5.8 years, height = 175.9 \(\pm\) 4.5 cm, and weight = 72.3 \(\pm\) 9.9 kg), who received a placebo, for eight days. The isometric force production was assessed pre- and post-NAC supplementation during a maximal voluntary contraction test (MVC) and also during a fatigue protocol composed of seven sets of ten maximal isometric contraction repetitions of 5, with 5 s of rest between repetitions and 20 s between sets. No differences were observed between the groups in the force production values at any moment, and no side effects were found after NAC supplementation. After supplementation, a significant decrease in force was observed in both groups, but this significant loss of force started one set later in the NAC group compared to the control group (4 th set vs. 5 th set), which could be an ergogenic effect of the treatment. Therefore, oral daily supplementation with 2400 mg of NAC for eight days, could delay the decrease in force production during an isometric exercise protocol and without adverse side effects. N-acetylcysteine; muscle fatigue; muscle; strength; ergogenic effects; side effects Article  
[PERSON], [PERSON]. Changes in factors such as loss of strength, changes in the rate of muscle activation, and even stiffness resulting from strenuous exercise have been assessed mainly by artificial stimulation, i.e., mechanical and evoked muscle responses associated with electromyography or examinations of skeletal muscle and/or motor nerve structures at rest [6]. As well as contractile mechanisms, the decrease in voluntary and/or electrically induced force with fatigue also depends on the elongation capacity of elastic components, both in series and in parallel [7]. Fatigue may increase the compliance of the muscle-tendon complex due to repetitive cycles of long-duration contractions [8; 9; 10] that lengthen the force transmission time to the bone due to the inability to store and release elastic energy [9; 11]. In addition, intense exercise is associated with the production of reactive oxygen species (ROS), the accumulation of which in muscle cells impairs muscle contraction and reduces the force production [12]. In intermittent sports or where the accumulation of competition occurs in a short space of time, this can be one of the factors that limits performance, so many recovery strategies have focused on fatigue control [13; 14]. N-acetylcysteine (NAC) has been commonly used as an ergogenic aid for respiratory-related health problems, although there are already a number of research studies on its ergogenic effect in sport [15]. The main hypothesized mechanisms of action of NAC include improvement in potassium homeostasis, preservation of skeletal muscle Na+/K+ pump activity and inhibition of calcium ATPase oxidation in the sarcoplasmic reticulum [16]; so in sports where there is a high accumulation of exercise-induced ROS, their effect and performance decrement could be attenuated with an acute dose of NAC before or after exercise [17; 18]. A recent review on the use of NAC in sport indicates that its antioxidant effect may be beneficial during the recovery process by scavenging ROS from muscle contraction and by providing cysteine for glutathione synthesis [4]. The studies analyzed in the review used a dose ranging from 1.2 to 20 g per day, for a loading period of 5 to 9 days. Following this dose recommendation, [PERSON] and [PERSON] [5] found that oral NAC extended the time to fatigue at 80% of peak power, but other studies did not find a performance improvement under fatigue conditions [19; 20].