In a series of electromyographic (EMG) fatigue studies, Moritani et al. 12 demonstrated an increase in EMG activity during incremental cycling exercise. It has been suggested that the rise in electrical activity is a result of progressive recruitment of additional motor units (MU) and/or an increase in the firing frequency of MUs that have already been recruited 3. An exercise-induced decrease in intramuscular pH, due to increases in hydrogen ions [H⁺], may interfere with the excitation-contraction coupling process of skeletal muscle, which, in turn, may lead a to decrease in power output and fatigue 4. Thus, if power output is to be maintained, either additional MUs must be recruited or the firing rates of the already active MUs must increase. In either case, a rise in EMG amplitude with power output takes place due to increases in muscle activation.

The physiological mechanism responsible for the increase in EMG amplitude over time during a fatiguing task is unknown. Three potential mechanisms, however, include the accumulation of metabolic by-products (lactate, hydrogen ions (H⁺), inorganic phosphate (Pi), and ammonia), the depletion of stored energy substrates (ATP, phosphocreatine (PCr), and glycogen) and/or impaired muscle cation (potassium (K⁺), sodium (Na⁺) and calcium (Ca²⁺) regulation 567. Several investigations have used surface electromyography to characterize the fatigue-induced increase in EMG amplitude, as well as to identify the power output associated with the onset of neuromuscular fatigue during cycle ergometry 2891011. Furthermore, surface EMG has been shown to be an acceptable method for non-invasive assessment of muscle fatigue of active muscles 1213.

Matsumoto et al. 9 and Moritani et al. 2 have proposed an incremental cycle ergometer test utilizing fatigue curves to identify the maximal power output at which an individual can maintain without evidence of fatigue, called the electromyographic fatigue threshold (EMG_FT). The EMG_FT test is an adaptation to deVries original monopoloar physical working capacity at the fatigue threshold (PWC_FT) test 14, using a bipolar supramaximal protocol, which involves determining the rate of rise in electrical activity from the vastus lateralis during four, 60 second work bouts on a cycle ergometer, with varying power outputs. The four power outputs are then plotted as a function of four EMG slope coefficients, with the y-intercept defined as the electromyographic fatigue threshold (EMG_FT).

Matsumoto et al9 described the EMG_FT as the highest intensity sustainable on a cycle ergometer without signs of neuromuscular fatigue. In addition, Moritani et al. 2 suggested a strong physiological link between myoelectrical changes at fatigue and anaerobic threshold. Furthermore, the EMG_FT method has been reported as a valid and reliable technique for examining the transition from aerobic to anaerobic metabolism during exercise 81213.

Harris et al. 15 were the first to demonstrate a 20% increase in muscle creatine (Cr) after ingestion of 20 g/day of Cr (four doses of 5 g) (Cr) for 5 days, with a 30% increase as phosphocreatine (PCr). The effect Cr supplementation on anaerobic activities, utilizing anaerobic energy reserves (adenosine triphosphate (ATP) PCr) has received a large amount of attention. It has been suggested that skeletal muscle PCr may serve as a temporal energy buffer as well as a modulator of glycolysis, and that, if increased via Cr supplementation, may influence neuromuscular fatigue 1116. Although Cr has been reported to be most effective in anaerobic activities, some evidence suggests that Cr may serve as an effective temporal energy buffer 1718 and may enhance aerobic activities111920.

Although women have been underrepresented in the Cr literature, recent studies suggest that there are no gender differences in the amount of increase of total creatine (TCr) following loading 1521 nor are there differences in high intensity exercise performance 22. In addition, Stout et al. 11 demonstrated improvements in neuromuscular fatigue after 5 days of Cr loading (4 × 5 g·d^-1) in trained female athletes, with a significant (p < 0.05) increase in PWC_FT. To date, however, no studies have examined the effects of Cr loading on EMG_FT in women. Therefore, the purpose of this study was to examine the effects of 5 days of Cr loading on EMG_FT in college-aged women.

Table 2 shows the mean and standard error values for EMG_FT and BW. A two-way ANOVA resulted in a significant interaction (p = 0.024) for group and time. There was a significant (p = 0.031) difference in absolute change from pre- to post-treatment in EMG_FT between the Cr and the PL treatment groups (Fig 2). In addition, dependent-samples t-test indicated a 14.5% ± 3.5% increase in EMG_FT from pre- to post-supplementation for the Cr treatment (p = 0.009), but no significant change for the PL treatment (-2.2 ± 5.8%; p = 0.732) (Table 2). Cr supplementation resulted in a significant 1.0 kg ± 0.34 kg increase (p = 0.049) in BW (Fig 2b) from pre- to post-supplementation, but no change in the PL treatment group (-0.2 kg ± 0.2 kg).

Additionally, independent samples t-tests demonstrated no significant differences between either pre-supplementation variables, p > 0.05 for EMGFT and BW respectively.

Several studies have reported 4 to7 days of Cr loading is effective for enhancing anaerobic performance during repeated sprint cycling 24252627, resistance training 2829, and anaerobic treadmill running 30. In addition, a few studies have demonstrated Cr loading to be an effective ergogenic aid for supramaximal workloads (>120% VO_{2 max}) on a cycle ergometer 27313233. The increase in exercise performance is believed to be attributed to an increase in intramuscular stores of total PCr 1534. A few investigators 111633 have suggested that increasing skeletal muscle PCr from Cr loading may improve supramaximal cycle ergometry performance by decreasing the reliance on anaerobic glycolysis for energy. In theory, decreasing the contribution of anaerobic glycolysis to meet energy demands during supramaximal workloads may reduce intramuscular lactate and H⁺ accumulation and, in turn, delay the onset of fatigue. In addition, Cr loading may also delay fatigue by increasing the rate of PCr resynthesis between intense exercise bouts, enhance buffering capacity, and provide greater shuttling of mitochondrial ATP into the cytoplasm 151835.

McClaren et al. 5 suggested that a decrease in muscle pH, as a result of the accumulation of intramuscular H⁺ or an increase in intracellular and extracellular ammonia, may be responsible for fatigue-induced increases in motor unit recruitment and the corresponding increase in EMG amplitude. In agreement, Taylor et al. 36 also found that, for incremental cycle ergometry, the accumulation of plasma lactate and ammonia was associated with an increase in EMG amplitude. Therefore, there is evidence to suggest that a significant reliance on anaerobic glycolysis leads to an increase in EMG amplitude from the working muscles, as a result of changes in muscle and blood lactate levels and the corresponding decrease in skeletal muscle pH 236.

Moritani et al. 2 speculate that the EMG_FT may be closely associated with a steady state of lactate metabolism and H+ accumulation in the active muscle. The level of exercise just below the point at which the bicarbonate buffering system can no longer maintain a homeostatic environment, due to an increase in H⁺, has been described as anaerobic threshold (AT) 37. In support, deVries 14 and Matsumoto et al. 9 demonstrated a close relationship between AT and the EMG_FT test. Furthermore, Matsumoto et al. 9 reported a significant relationship (r = 0.823; p < 0.01) between AT and EMG_FT with a similar subject sample of college-age women. In addition, Wasserman et al. 38 and Sirotic et al. 39 have demonstrated a strong relationship between AT and the ability of the cardiovascular system to supply oxygen during exercise, coupled with an improvement in performance. The results of our study indicated a 14.5% increase in EMG_FT after 5 days of Cr loading, which are very similar to Nelson et al. 20 who reported a 15% increase in AT in men and women from 7 days of Cr loading using a graded exercise test (GXT) on a cycle ergometer. In addition, Stout et al. 11 using a similar cohort of college-age women, reported a 13% increase in neuromuscular fatigue threshold during a GXT. While our results were similar to Nelson et al. 20 and Stout et al. 11 at increasing estimated fatigue thresholds, the present study utilized a supramaximal discontinuous protocol, compared to a submaximal incremental test, to estimate anaerobic and neuromuscular fatigue thresholds. To our knowledge this is the first study to report the effect of Cr loading on the supramaximal EMG_FT assessment, in women.

The EMG_FT theoretically represents the highest power output that can be sustained, during cycle ergometry, for an extended period of time without signs of neuromuscular fatigue 9. The primary findings of this study suggest that Cr loading may significantly increase the EMG_FT in women, and thereby increase the power output that can be maintained on a cycle ergometer for an extended period of time without exhaustion. Additionally, in contrast to previous studies using women 344041, the results of the present study for BW (Table 1) showed a significant increase from pre- to post-Cr loading. Although there was a significant increase in BW, the change (<0.8 kg) may have represented day-to-day fluctuations.

In conclusion, Cr loading in college-age women resulted in a significant increase in EMG_FT (14.5%) when compared to the placebo group (-2.5%). The increase in EMG_FT may have been due to the increased intramuscular PCr levels, attenuating the contribution of anaerobic glycolysis to ATP resynthesis 1116334243. Future studies that directly measure changes in skeletal muscle PCr and AT measures (EMG_FT, lactate, ventilatory and ammonia thresholds) may be warranted to confirm these results.

The author(s) declare that they have no competing interests.

All authors contributed equally to this work. All authors have read and approved the final manuscript.