Creatine monohydrate (Cr) has become one of the most widely used ergogenic aids among athletes. The ergogenic effect of acute Cr intake can result in an improved performance especially during high-intensity intermittent exercise. Moreover, oral Cr supplementation has been reported to increase maximal force and power output during short maximal exercise bouts (26). A large body of scientific literature has been generated during the last decade to elucidate how Cr can elicit its ergogenic effect, but still, the physiological mechanisms implicated are not completely understood. Most studies have focused on the Cr kinase reaction (7,21-24,31) and reported that Cr ingestion substantially increases muscle's total Cr concentration, which elicits a faster phosphocreatine (PCr) resynthesis during recovery (9) and reduces plasma ammonia accumulation in muscle (10).
In contrast to the large body of evidence on the metabolic effect of Cr ingestion, only a limited number of studies investigated the neuromuscular modification induced by this nutritional intervention. van Leemputte et al. (27) found a decrease in muscle relaxation time (RT) during intermittent isometric elbow flexions without any changes in force production. The mechanism proposed is that an increased adenosine triphosphate (ATP) availability elicited by an enhanced PCr content would improve Ca2+ kinetics in the sarcoplasmic reticulum improving muscle contractile properties. Also, it has been shown in rats that, after Cr supplementation, twitch and tetanic half RT (HRT) are decreased (28). On the other hand, Jakobi et al. (14) did not find an influence of Cr intake on isometric elbow flexion force, electrically elicited twitch force, and recovery from fatigue. Moreover, in the above-mentioned studies on humans, the effect of Cr was investigated only during isometric contractions.
The limited number of studies and the inconsistency of results highlight the need for a deeper evaluation of the effects of Cr supplementation on neuromuscular function in humans.
The information extracted from the surface EMG reflects central and peripheral properties of the neuromuscular system. In particular, muscle fiber conduction velocity (CV) is a basic parameter estimated from EMG and is related, among many other factors, to ions concentration and pH (32). The propagation velocity of action potentials could be influenced by the alterations of the intracellular environment, which follows the Cr uptake into muscle. It is reasonable to suppose that if any alteration of the contractile apparatus occurs, the simultaneous recording of force and EMG signals could reveal some of the changes. Very few studies, however, have measured EMG activity after Cr supplementation (14,15,27), but none of these studies have estimated muscle fiber CV to assess changes in the neuromuscular function.
For the above-mentioned reasons, the purpose of the present study was to investigate the effect of oral Cr supplementation on neuromuscular activation of upper limb muscles during electrically induced contractions (single twitch), maximal isokinetic contractions (torque-velocity relationship), and fatiguing dynamic contractions.
Sixteen moderately active men (25.2 ± 5.1 yr; 78.0 ± 8.7 kg) gave their informed written consent before participation in the study. The sample size was chosen following statistical power calculations for maximal strength and HRT (α level of P < 0.05 and 90% of power) by using data reported by Hespel et al. (11).
All volunteers were asked to maintain their normal level of physical activity and normal diet and to refrain from caffeine consumption throughout the study. None of the participants reported any record of renal, metabolic, cardiovascular, or neuromuscular disease. Exclusion criteria included having received Cr supplementation within the previous 12 months. The local ethics committee approved the protocol of the study.
A minimum of three experimental sessions were conducted during a period of 2 wk. During the first visit, volunteers familiarized with the experimental procedures and performed all the tests. Moreover, their body weight was measured, and their body composition was estimated by using the sum of seven skinfold measurements according to the method of Jackson and Pollock (13). One week later, subjects returned to the laboratory for the preliminary session (PRE). Anthropometric data were assessed, and they performed the experimental tests as described later. Thereafter, half of the participants were randomly assigned to a Cr supplementation group (CRE) and half to a placebo group (PLA) using a double-blind design. CRE supplemented their diet four times a day for 5 d with 5 g of Cr (DSN Fine Chemicals, Österreich, Austria) + 15 g of maltodextrin, whereas PLA assumed 20 g of maltodextrin. Cr and placebo powders appeared and tasted identically. After 2 d (1 wk later from the PRE session), participants performed a POST session repeating all the tests and anthropometric measurements of the PRE session.
Elbow flexion torque of the dominant limb was measured with an isokinetic dynamometer (Kin-Com, Chattanooga, TN). Participants were seated comfortably in the dynamometric chair and were stabilized by chest and waist straps. The position of the upper arm was parallel to the trunk, and the forearm was halfway between pronation and supination. The wrist was secured in a padded cuff attached to the load cell. The center of rotation of the lever arm was aligned to the distal lateral epicondyle of the humerus.
The surface EMG (sEMG) signals were recorded with a linear array of four electrodes (silver bars 5 mm long, 1 mm thick, 10 mm apart; LISiN, Torino, Italy) from the biceps brachii (BB). After gentle skin abrasion and cleaning with ethyl alcohol, electrodes were attached on the skin over the BB along a line connecting the acromion to the cubital fossa. The optimal position and orientation of the electrodes were determined to be conveniently distant from the innervation zone and the tendon as previously described. A ground electrode was placed around the wrist of the contralateral limb. Three sEMG were detected in a single-differential mode. Two double differentials were computed offline and were used for further analysis. Signals were amplified (×1000), band-pass-filtered (10-450 Hz; LISiN EMG 16), and sampled at 2048 Hz with 12-bit resolution (amplitude range ± 10 V; DAQ card AI-16XE-50; National Instruments, Austin, TX), recorded, and stored on a personal computer.
During the test trial, participants were requested to perform the following tasks: 1) maximal twitch; 2) isometric maximal voluntary contractions (MVC); 3) maximal isokinetic contractions; and 4) isokinetic fatiguing contractions.
After a period of standardized warm-up at submaximal intensity, the experimental trial started with the assessment of the motor point (MP) on the muscle belly (LISiN STIM-PRO). A stimulation pen was used, and the point that elicited the maximal response with the minimum stimulation amplitude was the MP. A small round electrode was placed on the MP (cathode), and a large rectangular electrode was placed on the distal tendon (anode) (4). Trains of 10 single impulses of 496 μs in duration with a biphasic rectangular wave and constant envelope were delivered. Increments of 10-mA amplitude from 50 mA to a maximum of 100 mA were carried out to assess the maximum mechanical response (maximal twitch).
The joint angle was fixed at 90° (0°, full extension). The MVC task consisted of rapidly increasing the force exerted by elbow flexors to a maximum. A visual feedback was provided to the subjects by setting a target line on the computer screen at a value 20% higher than the best MVC. All subjects were verbally encouraged to exceed the target force, producing a maximal contraction and to maintain it for at least 2-3 s before relaxing (2). A minimum of three maximal attempts were performed separated by 5 min to recover from fatigue. Participants were asked to perform further attempts if the MVC of their last trial exceeded the previous trials by at least 10%. However, in no instances did MVC attempts exceed the number of five per subject.
Isokinetic concentric contractions.
After the MVC task, participants performed a set of three maximal concentric elbow flexions at 15°·s−1, 30°·s−1, 60°·s−1, 90°·s−1, 120°·s−1, 180°·s−1, and 240°·s−1. The range of motion (ROM) was 90° starting from 40° to 130°. The order of the trials was randomized to minimize the effect of skill acquisition. Each contraction was followed by a rest period lasting from 5 min (after 240°·s−1) to 30 min (after 15°·s−1) to prevent cumulative fatigue (17).
Isokinetic fatiguing contractions.
Participants were asked to perform 5 sets of 30 maximal isokinetic flexions at 180°·s−1 with 1-min rest between the sets. The ROM was the same of the previous exercise.
All data collected during the experiments were analyzed offline (LabVIEW 8.0 software; National Instruments). For the twitch task, peak torque (PT) was calculated as the average of maximal torque values obtained during the train of 10 pulses. The highest average across the different stimulation intensities was chosen as maximal PT. Moreover, time to peak (TTP) from the onset of force trace and HRT (i.e., time to halve the PT) were calculated. The MVC that showed the highest value for force was chosen for the analysis, and the MVC torque value was calculated as the mean torque of a 1-s window centered at that peak value. For each set of the isokinetic task, the repetition that showed the highest value of force was used for the analysis. Isokinetic maximal torque values were expressed as percentage of the MVC value obtained during the PRE session.
EMG signals were recorded simultaneously to mechanical data. CV was estimated from the two double differentials with the cross-correlation technique. The cross-correlation function technique was used to estimate the time delay between the two signals (i.e., the amount of time shift that must be applied to one signal to minimize the mean square error with the other). This time shift is the same, which maximizes the cross-correlation between the signals (20). Estimates of CV were accepted only when cross-correlation values were higher than 0.8.
Trials chosen for CV estimation were selected on the basis of maximal force. During isokinetic contractions, maximal CV was estimated during 250-ms windows, and this windowing was applied over the 90°-120° ROM being the ROM portion where it was more likely to reach the maximal value of torque (Fig. 1). During maximal twitch, CV window was selected manually to isolate the M-wave elicited during the twitch and to avoid stimulus artifacts.
FIGURE 1-Angle at wh...Image Tools
During dynamic fatiguing contractions, power, mean peak force, total work, and fatigue indexes for force and CV were also calculated. Fatigue index for force represented the decay of peak force during the five bouts. Fatigue index for CV was calculated on single bouts as the percentage of difference between the initial values of CV (mean of the first three contractions) and final values (mean of the last three contractions).
A repeated measures ANOVA [between factors: CRE vs PLA; within factors: pre vs post, angular velocities (0°·s−1, 15°·s−1, 30°·s−1, 60°·s−1, 90°·s−1, 120°·s−1, 180°·s−1, and 240°·s−1)] was used to compare the dependent variables (CV, force). A t-test with Bonferroni correction was implemented when appropriate. Data are expressed as mean ± SD in the text and tables and as mean ± SE in figures. Statistical significance was accepted if the P value was <0.05. Regression lines for individual data sets of torque versus angular velocity were computed using the least-squares method.
Body weight increased by 1.1 kg in the CRE group, whereas it was unchanged in the PLA group (Table 1). Such an increment in body weight after Cr loading is in line with what typically observed, and that has been shown to be associated to an enhancement of muscle Cr content (18,29). The mechanical parameters recorded in the familiarization and the presupplementation trials did not differ by more than 5%. This consistency in performance provides the evidence to exclude the possibility that the differences between the PRE and POST trial were due to learning of the motor task.
The values of PT, TTP, and HRT calculated during the single twitch are reported in Figure 2. The values are expressed as percentage of the value recorded in the PRE trial. Cr supplementation resulted in an increased PT (P < 0.05), whereas this was unchanged in the PLA group (112.1 ± 14.9% and 79.3 ± 12.5% the value recorded during the PRE trial in the CRE and the PLA groups, respectively). Cr supplementation also significantly reduced TTP (P < 0.05; 41.7 ± 37.9% of the PRE value), whereas TTP was unchanged in the group that ingested placebo. Differently, no statically significant difference was observed for HRT between the two groups (82.3 ± 29.9% for CRE and 92.5 ± 13.1% for the PLA group). The changes observed in the mechanical parameters were not reflected in the CV values estimated during the maximal twitch, as these were unchanged in the two groups after the supplementation period (Fig. 3).
MVC and Maximal Isokinetic Contractions
Figure 4 depicts the torque-velocity relationship for the elbow flexors obtained in the PRE and POST trials in CRE and PLA group. Exponential regression lines were also fitted. After supplementation, maximal torque was significantly increased at 180°·s−1 (+9.1%) and 240°·s−1 (+9.0%) in the subjects supplemented with Cr, whereas the difference at the other velocities did not reach statistical significance. Differently, maximal torque was unchanged in PLA group at all velocities considered. Mean fiber CV before and after supplementation was significantly enhanced (15% on average) in the CRE group at all angular velocities (P < 0.05), whereas it remained unchanged in the PLA group (Fig. 5). Raw data for torque and CV are reported in Table 2.
Fatiguing Isokinetic Contractions
Table 3 shows the values of mean force, total power, total work, and fatigue index during the five sets of 30 isokinetic contractions at 180°·s−1. No significant differences were found after supplementation for all the mechanical parameters considered in both the PLA and the CRE groups. Similarly, Cr supplementation did not affect significantly the CV fatigue index, as shown in Figure 6.
The main findings of the present study are that 1) oral Cr supplementation increased PT and reduced time needed to reach the maximum tension during electrically induced twitches, 2) Cr supplementation significantly affects the torque-velocity relationship by enhancing especially muscle torque exerted at the highest angular velocities, and 3) Cr loading increases mean muscle fiber CV estimated during maximal dynamic contractions performed at different angular velocities.
Several studies have documented the ergogenic effect of oral Cr supplementation on performance during continuous or intermittent high-intensity exercise (26). The improved exercise performance has been attributed to an increased capacity for ATP resynthesis resulting from an augmented availability of PCr in muscle, a higher capacity of maintaining ATP stores through a smaller reduction of adenosine nucleotides, and a higher capacity of H+ buffering (26). The electrically induced muscle twitch and its duration provide significant information on muscle contractile properties (12), which, in turn, represent important determinants of muscle strength and endurance. Most of the studies investigating the effect of Cr loading on single twitch and tetanic tension in humans did not report significant changes of peak force or duration. In particular, Jakobi et al. (14,15) could not show significant changes in PT, TTP, and HRT during single twitch induced on the elbow flexor muscles of young and older individuals. Moreover, no difference in PT and TTP was observed by van Leemputte et al. (27) during voluntary maximal isometric contraction and by Hespel et al. (11) during electrically induced maximal contractions, whereas both studies documented a reduction of muscle RT. Furthermore, van Leemputte et al. (27) reported that the longer the initial RT, the better the response to Cr intake, which resulted in a more pronounced shortening of RT. Our data indicate only a tendency toward a reduction of the HRT, which did not reach statistical significance. However, the large variability of the experimental protocols used makes the comparison between the various studies difficult. Moreover, a significant increase of PT was observed in the present study together with a reduction of TTP as an effect of Cr supplementation. The present data, therefore, point toward an effect of Cr loading on muscle contractile properties.
Although the measurements performed in the present investigation have mainly a functional significance, it is tempting to speculate about the mechanism that may have lead to the changes induced by Cr supplementation. In this regard, the improved contractile capacity registered could be partially explained by osmotic changes resulting from the increase of intracellular Cr content, which is accompanied by a concomitant increase of intracellular water content to maintain cellular osmolarity (19). Indications in this directions are suggested by evidence obtained in vitro on rat single muscle fibers, which demonstrate that the reduction in ionic force consequent to the increase in intracellular water content results in an increase in maximal tension induced by Ca2+ activation and in an augmented Ca2+ sensitivity (same level of force produced with a lower Ca2+ concentration) (19). Furthermore, it has been proposed (11,27) that Cr loading may induce a facilitation of Ca2+ reuptake in the sarcoplasmic reticulum by virtue of the action on the Ca2+-adenosine triphosphatase (ATPase) pump (8). Such a condition would predispose for a faster detachment of the actomyosin bridges, which would benefit the capacity of producing force rapidly. These phenomena could possibly explain the mechanical changes observed during the single twitch after Cr supplementation in the present study. In particular, a faster cycling of the actomyosin bridges could explain the reduction of TTP, whereas the increase in Ca2+ sensitivity may have favored the enhancement of PT after supplementation. However, whether these mechanisms are those actually responsible for the muscle twitch changes observed after Cr ingestion should be proven in future studies.
On the other hand, our data show that mean fiber CV is not influenced by Cr loading during maximal twitch. Several factors could explain this finding. First of all, changes in the propagation velocity of the action potentials are dependent upon many factors, not only on changes in the supposed Ca2+-ATPase pump activity. It is reasonable to suppose that any alteration on fibers contractile properties, if present, could be reflected on the estimated CV only when a substantial number of fibers are recruited (i.e. during voluntary contraction). During single twitch, even if maximal, the number of fibers stimulated from the MP is relatively smaller than fibers that can be recruited during an MVC, as manifested also by the different mechanical output.
Jakobi et al. (14,15) have adopted the isometric model which represents a controlled condition to assess both voluntary and stimulated contractile properties. Isometric exercise was also used by van Leemputte et al. (27) to assess fatigue recovery during rapid intermittent maximal contractions. Nevertheless, most of the physical activities are characterized by dynamics tasks. Moreover, it has been shown (10) that the increase in Cr pool induced by Cr intake improves markedly the performance capacity during repeated bouts of isokinetic exercise. To the best of our knowledge, the present study is the first investigating the effect of muscle Cr loading on the torque-velocity relationship of elbow flexor muscles in young moderately active individuals. Our findings show an upward shift of the torque-velocity curve. Torque enhancement is more evident at the highest angular velocities (right portion of the torque-velocity curve). Maximal isometric torque, conversely, did not change substantially after supplementation, which is in line with what reported in previous investigations (11,27).
CV, estimated from sEMG recorded during the same maximal isokinetic contractions, was significantly higher after supplementation at all angular velocity considered. In analogy to what hypothesized for the electrically induced contractions, the effect of an improved Ca2+ kinetics on the conduction of the action potential along the muscle fibers in voluntary conditions cannot be excluded. In addition, given that both the increased Ca2+ sensitivity and the improvement in the function of the ATPase calcium pump are more evident in the muscular regions where ATP utilization is higher (19), it is reasonable to suppose that such mechanisms are more evident in the Type II muscle fibers (19), as an interaction between Type II muscle fibers and Cr has been proposed (9). Therefore, when it is required to express maximal level of force and in a short time, the advantage arising from an improved CV of the action potential along the Type II muscle fibers could result determinant, explaining in this way the shift of the right portion of the torque-velocity curve toward higher values. Moreover, a higher CV could be explained by changes in neuromuscular recruitment strategies, with CV being higher when a large number of motor units (particularly Type II) are recruited. In our study, we documented for the first time a higher CV after Cr supplementation. Although the experimental protocol we used did not directly assess motor unit recruitment strategies, we may speculate that Cr supplementation could also affect central sites of the nervous system. Some studies (1,3) suggest that Cr could have a neuroprotective effect that attenuates motor dysfunctions of certain brain pathologies such as amyotrophic lateral sclerosis (1,16) and Duchenne muscular dystrophy (25). Mechanisms of action by which Cr exerts these effect on the brain and nervous system are still unclear, but in the brain, for example, Cr has been shown to be associated with synaptic membranes (6) and to facilitate glutamate uptake into vesicles (30), thus being directly involved in the energetics of neurotransmitter uptake (5). Motor unit recruitment capacity, in turn, may be influenced, and future studies should be designed to investigate this possibility.
Unpredictably, the present data do not show significant differences in total power, total work, and in the mechanical indexes of fatigue during the dynamic fatiguing contractions. It can be hypothesized that the mechanical and neural components of muscle performance are not necessarily governed by the same factors, and the neuromuscular function would benefit more from a higher efficiency of the contractile apparatus than to a higher capacity for ATP resynthesis. Support to this interpretation is provided by the tendency for a slower decay of the CV during the last two sets of the fatiguing exercise protocol in the subjects who were supplemented with Cr, although this tendency did not reach statistical significance.
In conclusion, the present study show an improvement in neuromuscular function of the elbow flexor muscle after oral Cr supplementation, which is evident as an enhancement of muscle contractile properties during electrically evoked and voluntary contractions performed at high angular velocities. Moreover, the present data show for the first time that muscle Cr loading induces an increase in the velocity of propagation of the action potential along the muscle fibers during contraction conducted at different angular velocities.
This work was supported by a grant from the University of Rome "Foro Italico" (grant no. G-44.04).
The authors wish to thank Leonardo Gizzi for software design and helpful discussion.
The results of the present study do not constitute endorsement by ACSM.
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