Introduction: The quadriceps femoris is one of the strongest muscle groups of the lower limbs and plays a key role in stabilizing the knee joint, absorbing forces, and generating power during various movements. During maximal voluntary isometric contraction (MVIC), the primary outcome is the peak torque a muscle can generate. The interpolated twitch technique is a method used to assess the level of voluntary muscle activation (VA) by applying electrical stimulation to the muscle or its peripheral nerve during MVIC. Olympic weightlifting and powerlifting are sports that require high levels of strength and whole-body muscle activation, particularly of the quadriceps femoris. Purpose: The aim was to determine the effect of fatigue on the level of maximal voluntary activation of the quadriceps femoris in strength-trained athletes and untrained individuals. Methods: The study included 17 strength-trained athletes and 16 untrained individuals. MVIC and VA were measured using the interpolated twitch technique. The experimental protocol consisted of four parts: 1. familiarization and warm-up, 2. baseline measurement of MVIC and VA, 3. fatigue protocol – 30 maximal concentric and eccentric contractions at an angular velocity of 180 °/s, 4. post-fatigue measurement of MVIC torque and LA. Results: Trained individuals had a significantly higher body mass (p = 0.002) and body mass index (p < 0.001). Between-group comparisons showed that the difference in maximal voluntary isometric contraction torque between the groups was statistically significant at each measurement time point (p < 0.001). The relative decline in maximal voluntary isometric contraction torque was greatest immediately after the fatigue protocol, with the trained group showing a decrease of -0.38 (0.07) Nm and the untrained group -0.32 (0.08) Nm. Between-group comparisons at individual post-fatigue measurement time points revealed significant differences between the groups at the second (p = 0.045) and third (p = 0.005) measurements of voluntary activation level. Discussion and conclusion: Contrary to our initial expectations, strength-trained athletes did not exhibit significantly higher quadriceps neuromuscular activation levels, suggesting that the difference in torque production was primarily due to differences in muscle mass.
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