Neuromuscular fatigue development during maximal concentric and isometric knee extensions

Abstract

This study aimed to investigate mechanisms of neuromuscular fatigue during maximal concentric and isometric leg extensions inducing similar torque decrements. Nine physically active men performed two separate fatiguing sessions maintained until similar torque decreases were obtained. The first session, only conducted under isokinetic concentric conditions (CON), consisted of three series of 30 maximal voluntary concentric knee extensions (60 degrees/s). The second session, exclusively isometric (ISO), mimicked the torque decreases registered during the CON session while performing three long-lasting ISO contractions. Maximal voluntary torque, activation level (twitch interpolation technique), electromyographic activity (root mean square and median frequency) of the vastus lateralis muscle, and electrically evoked doublet-twitch mechanical properties were measured before and at the end of each of the three series. After the three series, similar torque decrements were obtained for both fatiguing procedures. The total fatiguing contraction durations were not different among procedures. With equivalent voluntary torque decrements, the doublet-twitch amplitude reduction was significantly greater (P<0.01) during the two first series of the CON procedure compared with ISO. No difference was observed for the third series. Although no difference was recorded with fatigue for median frequency changes between CON and ISO, activation levels and root mean square values demonstrated greater reductions (P<0.05) for all three series during the ISO procedure compared with CON. Performing CON or ISO fatiguing exercises demonstrated different fatigue origins. With CON exercises, peripheral fatigue developed first, followed by central fatigue, whereas with ISO exercises the fatigue pattern was inverted.

Assessment of voluntary muscle activation using magnetic stimulation

“…Hence, we conclude that the MS activation levels were reproducibly estimated using MS. Importantly, the SEs found here, albeit with a limited population of young healthy adults, show a level of variation which is much lower than the changes seen in musculoskeletal disorders such as anterior knee pain (17%) (Suter et al 1998) and osteoarthritis (7%) (Pap et al 2004), in ageing (12%) (Onambele et al 2006), with fatigue (10-25%) (Babault et al 2006) and following exercise interventions such as resistance training (3-4%) (Harridge et al 1999;Scaglioni et al 2002). This indicates that MS is sensitive enough to detect physiologically meaningful changes in activation.…”

Adjustments Differ Among Low-Threshold Motor Units During Intermittent, Isometric Contractions

“…According to this scheme, only those physiological processes that are stressed by the physical activity will be challenged enough so that they are eventually impaired and contribute to muscle fatigue. For example, the decline in voluntary activation of muscle differs during maximal sustained, intermittent, and shortening contractions and thus contributes differently to the observed muscle fatigue during these tasks (Babault et al 2006;Taylor et al 2006). Similarly, the type of load supported by a limb during a submaximal isometric contraction influences the rate at which the motor unit pool is recruited and the duration that the contraction can be sustained (Enoka and Duchateau 2008;Maluf and Enoka 2005).…”

Neuromuscular Fatigue and Physiological Responses After Five Dynamic Squat Exercise Protocols

“…It has 3 been stated that maximal-dynamic strength and power of the lower limbs are strongly correlated to many 4 sport-specific variables of athletic performance, such as vertical jump height, 10-m shuttle run time, and 5 sprint times over several distances (10 -30m), respectively (35,42). However, for well-designed strength 6 training sessions it is required to have a comprehensive knowledge about the short-term fatigue effects of 7 different strength training methods.…”