The estimation of critical power (CP) is a result of the hyperbolic relationship between specific power output levels and the corresponding time that the power output can be sustained. Originally, Monod and Scherrer (1965) described CP based on the results of several (three to five) bouts of repetitive lifting exercises performed using different isolated muscle groups, and they noted that exhaustion did not occur when the dynamic work intensity was inferior or equal to the CP. Theoretically, CP represents the highest power output that can be sustained without exhaustion. Subsequent investigation of this sustainability has shown that exhaustion occurs after about 30 minutes of exercise at CP (Brickley et al., 2002). Moritani and colleagues (1981) extended the CP concept to cycling exercise and found CP to be highly correlated with the ventilatory anaerobic threshold. However, this method still requires a subject to perform exhaustive exercise at different constant work rates on separate days.
Vanhatalo and colleagues (2007) developed a three-minute all-out cycling test (3MT) which yielded a stable power output at the end of the test, thus generalizing the CP concept to all-out exercise. The rationale for the measurement of CP in a single bout of all-out cycling stems from its mathematical definition as the asymptote of the hyperbolic power-time relationship and further outlines a finite and rate-independent capacity for work above the CP (Vanhatalo et al., 2007). The validity and reliability of the 3MT has been demonstrated by comparing the CP assessed from the traditional multi-trial constant load to exhaustion cycling test and the newly developed 3MT, while sensitivity has been shown through similar alterations due to an interval training intervention (Vanhatalo et al., 2008). The 3MT is most often used to evaluate performance through estimation of CP, however, several studies have examined the physiological response to the testing protocol, primarily focused on the metabolic aspects (Bergstrom et al., 2013; McClave et al., 2011; Sperlich et al., 2014) and electromyographic (EMG) amplitude parameters (Bergstrom et al., 2013; Vanhatalo et al. 2011). The 3MT provides an advantageous alternative to the conventional protocol of multiple exhaustive exercise tests to determine CP, and may offer a unique method to examine mechanisms of fatigue.
Muscle fatigue represents a multifaceted phenomenon with physical and chemical changes in muscle as well as alterations in nervous system efficiency, which are related to different causes, mechanisms and symptoms (Cifrek et al., 2009). The monitoring of local muscle fatigue can be conducted by measuring myoelectric activity via EMG which may represent biochemical and physiological changes during exercise (De Luca, 1984). Traditionally, EMG is used in detecting fatigue through incremental or constant exercise protocols, during which slow twitch and fast twitch fibers are progressively recruited as exercise proceeds (Guffey et al., 2012; Malek et al., 2006; Travis et al., 2011). During maximal all-out exercise, characterized by the attainment of peak power at the start of the 3MT, the majority of available motor units should be recruited along with an alteration of firing rate and a reversal of the progressive muscle activation pattern utilized during incremental or constant exercise may occur (Sargeant et al., 1981; McCartney et al., 1983). Subsequently, fast twitch fibers would be expected to generate a greater proportion of the total external power output (Beelen and Sargeant 1993). Thereafter, these fibers would become progressively fatigued at a rapid rate, potentially leading to a greater reliance on slow twitch fibers, resulting in decreased power output due to the high fatigue resistance but low contraction speed characteristics of slow twitch fibers (McCartney et al., 1983; Sargeant et al., 1981). Due to this unique muscle recruitment pattern and alteration of firing rate, changes in the EMG signal during the 3MT should be evident. However, the changes in muscle fiber conduction velocity, assessed via surface EMG frequency, associated with this testing protocol have yet to be evaluated.
Mean frequency (MNF) and median frequency (MDF) are two useful frequency-domain parameters of EMG analysis which are frequently used to detect fatigue in the target muscles (De Luca, 1984). According to the definition, both MNF and MDF can represent the shift of the frequency spectrum of the EMG signal. Thus, the general behavior of MNF and MDF should be analogous, however, there are conflicting results regarding which frequency parameter is more suitable for evaluation of fatigue and the results may vary among different muscles or exercise protocols (Stulen and De Luca, 1981). Additionally, some studies have reported differences in variance between MNF and MDF, the MNF had a lower standard deviation (Balestra et al., 1988; Knaflitz et al., 1990). EMG frequency, specifically MNF, has been used to detect fatigue thresholds during graded exercise testing (Camic et al., 2010). To the best of our knowledge, no one has ever compared MNF and MDF in corresponding to the changes in neuromuscular function during the 3MT.
Thus, the purpose of this study was to evaluate the time course of EMG frequency (MNF and MDF) changes during a 3MT cycling session and to examine which frequency parameter (MNF or MDF) is more suitable for evaluation of changes in neuromuscular function throughout the 3MT. The hypotheses were as follows: (1) Both MNF and MDF would decrease over the course of the 3MT, based on the all-out nature of the exercise test; and (2) MNF would be more suitable than MDF to evaluate neuromuscular fatigue during the 3MT due to lower variability.
Eighteen male participants (mean [+ or -] SD; age: 23.5 [+ or -] 3.1 yrs; height: 1.79 [+ or -] 0.06 m; mass: 85.1 [+ or -] 9.5 kg) volunteered to participate in this study. The study was approved by New England Institutional Review Board. Testing procedures were fully explained before obtaining written informed consent from each participant. All participants were habitually active (participating in a regular physical activity program or accumulating 150 min per week or more of moderate intensity exercise). In an attempt to eliminate the potential for reduced power output, the participants were asked to refrain from any strenuous physical activity for the previous 72 hours. In addition, participants were instructed to arrive at each testing session 2 hours fasted and in a euhydrated state. Each participant completed a confidential medical and activity questionnaire in order to identify any exclusion criteria, including the inability to perform physical exercise and any chronic illness that required continuous medical care.
All study participants completed two testing sessions on nonconsecutive days separated by a minimum of 48 hours. During the first testing session (T1), participants performed a standardized warm-up consisting of cycling and lower body exercise, the latter included 10 bodyweights squats and 10 alternating lunges. Immediately following the warm up...