Effects of creatine loading on electromyographic fatigue threshold during cycle ergometry in college-aged women
Abbie E Smith
1
Ashley A Walter
1
Trent J Herda
0
Eric D Ryan
0
Jordan R Moon
1
Joel T Cramer
0
Jeffrey R Stout
1
0
Biophysics Laboratory, Department of Health and Exercise Science, University of Oklahoma
,
Norman, OK 73019
,
USA
1
Metabolic and Body Composition Laboratory, Department of Health and Exercise Science, University of Oklahoma
,
Norman, OK 73019
,
USA
The purpose of this study was to examine the effects of 5 days of Creatine (Cr) loading on the electromyographic fatigue threshold (EMGFT) in college-aged women. Fifteen healthy college-aged women (mean SD = 22.3 1.7 yrs) volunteered to participate in this double-blind, placebocontrolled study and were randomly placed into either placebo (PL - 10 g of flavored dextrose powder; n = 8) or creatine (Cr - 5 g di-creatine citrate plus 10 g of flavored dextrose powder; n = 7; Creatine Edge, FSI Nutrition) loading groups. Each group ingested one packet 4 times per day (total of 20 g/day) for 5 days. Prior to and following supplementation, each subject performed a discontinuous incremental cycle ergometer test to determine their EMGFT value, using bipolar surface electrodes placed on the longitudinal axis of the right vastus lateralis. Subjects completed a total of four, 60 second work bouts (ranging from 100-350 W). The EMG amplitude was averaged over 10 second intervals and plotted over the 60 second work bout. The resulting slopes from each successive work bouts were used to calculate EMGFT. A two-way ANOVA (group [Cr vs. PL] time [pre vs. post]) resulted in a significant (p = 0.031) interaction. Furthermore, a dependent samples t-test showed a 14.5% 3.5% increase in EMGFT from pre- to post-supplementation with Cr (p = 0.009), but no change for the PL treatment (-2.2 5.8%; p = 0.732). In addition, a significant increase (1.0 0.34 kg; p = 0.049) in weight (kg) was observed in the Cr group but no change for PL (-0.2 kg 0.2 kg). These findings suggest that 5 days of Cr loading in women may be an effective strategy for delaying the onset of neuromuscular fatigue during cycle ergometry.
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Background
In a series of electromyographic (EMG) fatigue studies,
Moritani et al. [1,2] 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 [5-7]. 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 [2,8-11].
Furthermore, surface EMG has been shown to be an
acceptable method for non-invasive assessment of muscle
fatigue of active muscles [12,13].
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 (EMGFT). The EMGFT
test is an adaptation to deVries original monopoloar
physical working capacity at the fatigue threshold (PWCFT) 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 (EMGFT).
Matsumoto et al[9] described the EMGFT 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 EMGFT method has been reported as a valid and
reliable technique for examining the transition fro (...truncated)