NAP: Training (2007)

Citation:

Rockwell MS, Rankin JW, Dixon H. Effects of muscle glycogen on performance of repeated sprints and mechanisms of fatigue. Int J Sport Nutr Exerc Metab. 2003; 13 (1): 1-14.

PubMed ID: 12660402
 
Study Design:
Randomized crossover trial
Class:
A - Click here for explanation of classification scheme.
Quality Rating:
Neutral NEUTRAL: See Quality Criteria Checklist below.
Research Purpose:
  • To determine whether initial muscle glycogen influences performance of repeated 60-second sprint exercise
  • In addition, the hypothesis was tested that lower initial muscle glycogen accelerates the decline in sarcoplasmic reticulum function and creatine phosphate concentration during exercise, thus providing an explanation for the effect of diet on performance.
Inclusion Criteria:
Competitive male cyclists between ages 18 and 30 years, currently cycling four to six times per week for at least two hours each ride.
Exclusion Criteria:
Subjects were screened for contraindications to strenuous exercise, diet manipulation and muscle biopsies.
Description of Study Protocol:
  • Recruitment: Recruitment methods not specified
  • Design: Randomized crossover trial
  • Blinding used: Lab tests; subjects were unaware of their performance.
  • Intervention: Subjects performed two cycling trials following consumption of either a high-carbohydrate or low-carbohydrate diet.

Statistical Analysis

  • Data are presented as averages with standard error of the mean
  • A one-tailed paired T-test was performed to detect differences between trials in performance measurements
  • All other data were analyzed by repeated-measures ANOVA to test for effect of group, time and group x time interaction with Tukey as the post-hoc test
  • Since statistical analysis showed that performance differences were not influenced by the order in which trials were performed, order was not included in subsequent analyses
  • Pearson's correlation analysis was performed to determine associations between dependent measures.
  • Significance defined as P<0.05.
Data Collection Summary:

Timing of Measurements

  • Pre-testing and familiarization trials on Day One
  • Baseline trial on Day Eight
  • Glycogen-depleting ride on Day 13 with Diet Manipulation on Day 14 and then Performance Trial on Day 15
  • Second Glycogen-depleting ride on Day 20 with Diet Manipulation on Day 21 and Performance Trial on Day 22
  • Muscle biopsies and blood samples collected at baseline, following a 15% (15% fatigue) and 30% decline in sprint performance (30% fatigue), when exercise was terminated.

Dependent Variables

  • Muscle biopsies analyzed for glycogen, creatine phosphate, free creatine, ATP, sarcoplasmic reticulum calcium uptake and release 
  • Blood samples analyzed for glucose and lactate
  • Body weight
  • Performance trials separated by seven days, after a 12-hour fast, with only water provided during sprints. 

Independent Variables

  • Diets consisted of either high-CHO (80% to 85% CHO, 10% to 15% protein, 5% fat) or low-CHO (5% to 10% CHO, 15% to 20% protein, 65% to 75% fat)
  • Total energy provided was mathed to 24-hour dietary record at baseline
  • Foods were provided to subjects
  • Diets were consumed 36 hours after glycogen depletion.
Description of Actual Data Sample:
  • Initial N: Nine competitive male subjects
  • Attrition (final N): Eight males; one dropped out after first trial
  • Age: Mean, 23.9±1.6 
  • Ethnicity: Not mentioned 
  • Other relevant demographics: Mean body fat percentage, 8.5±1.2%
  • Location: Virginia.
Summary of Results:

  High-CHO; Baseline High-CHO; 15% Fatigue High-CHO; 30% Fatigue Low-CHO; Baseline Low-CHO; 15% Fatigue Low-CHO; 30% Fatigue
Muscle Glycogen (mmol/kg dw) 346±19 214±20 204±16 222±19 139±15 118±13

Muscle CP (mmol/kg dw)

86±5

59±4 58±6 86±3

61±5

53±6

Muscle Free Creatine (mmol/kg/dw) 47±4 72±4 74±6 54±3 80±5 87±6
Muscle ATP (mmol/kg dw) 29±1 27±1 23±1 26±2 24±1 23±1
Serum Glucose (mmol/L) 5.64±0.15 9.75±0.82 8.98±1.19 5.08±0.26 7.98±0.91 7.20±0.62

Serum Lactate (mmol/L)

1.89±0.25

15.73±0.62 13.13±1.65 1.45±0.28

16.30±0.49

11.88±1.82

Other Findings

  • All subjects maintained their body weight during the study
  • All subjects were compliant (returning less than 200kcal of food), except one subject returning 1,200kcal of food
  • There was superior performance in the high-CHO trial (14.3±2.5 sprints, 1,311.9±279.8kJ total work), as compared to the low-CHO trial (10.4±0.9 sprints, 960.6±118.0kJ total work). Total exercise time was 37% longer in the high-CHO trial (57.5±10.0 minutes) than the low-CHO trial (42.0±3.6 minutes, P=0.04).
  • There was no difference in peak power between trials or the average amount of work performed per sprint.
  • Approximately 93% more work was completed between 15% and 30% fatigue in the high-CHO condition (753.5±231.5 kJ) than the low-CHO condition (389.1±69.2kJ, P<0.05)
  • Baseline muscle glycogen levels (346±19 for high-CHO and 222±19mmol per kg dry weight for low-CHO) and total exercise time to 30% fatigue were higher following high-CHO than low-CHO (57.5±10.0 vs. 42.0±3.6 minutes, P<0.05)
  • Similar significant decreases (P<0.05) over the entire exercise bout were seen in muscle glycogen (43%), creatine phosphate (35%) and sarcoplasmic reticulum calcium uptake in isolated homogenized muscle (56%) for both trials (P>0.05 between trials)
  • The percentage decline in sarcoplasmic reticulum calcium release was less for high-CHO than low-CHO (36% and 53%, respectively) but this was not statistically different.
Author Conclusion:
  • Performance of our repeated sprint protocol was enhanced 37% by a diet that resulted in elevated muscle glycogen levels throughout exercise relative to low-CHO
  • The effect of diet was evident during the latter stage of exercise, from 15% to 30% fatigue, when exercise time was 87% longer after subjects had consumed high-CHO
  • Initial muscle glycogen levels did not influence the rate of glycogen utilization, decrease in muscle creatine phosphate or impairment of sarcoplasmic reticulum function
  • Thus, other unmeasured factors must explain differential fatigue development when carbohydrate content of the diet is manipulated
  • The possibility that glycogen was selectively depleted during this intense exercise bout from specific muscle fibers or cellular compartments, including sarcoplasmic reticulum, deserves further research.
Funding Source:
Government: NIH
Industry:
Gatorade Sports Science Institute
Food Company:
University/Hospital: Virginia Tech
Not-for-profit
0
Foundation associated with industry:
In-Kind support reported by Industry: Yes
Reviewer Comments:
  • Recruitment methods not specified
  • Dropout resulted in unbalanced design
  • Foods provided to subjects, but not all subjects were necessarily compliant.
Quality Criteria Checklist: Primary Research
Relevance Questions
  1. Would implementing the studied intervention or procedure (if found successful) result in improved outcomes for the patients/clients/population group? (Not Applicable for some epidemiological studies) Yes
  2. Did the authors study an outcome (dependent variable) or topic that the patients/clients/population group would care about? Yes
  3. Is the focus of the intervention or procedure (independent variable) or topic of study a common issue of concern to dieteticspractice? Yes
  4. Is the intervention or procedure feasible? (NA for some epidemiological studies) Yes
 
Validity Questions
1. Was the research question clearly stated? Yes
  1.1. Was (were) the specific intervention(s) or procedure(s) [independent variable(s)] identified? Yes
  1.2. Was (were) the outcome(s) [dependent variable(s)] clearly indicated? Yes
  1.3. Were the target population and setting specified? Yes
2. Was the selection of study subjects/patients free from bias? ???
  2.1. Were inclusion/exclusion criteria specified (e.g., risk, point in disease progression, diagnostic or prognosis criteria), and with sufficient detail and without omitting criteria critical to the study? Yes
  2.2. Were criteria applied equally to all study groups? Yes
  2.3. Were health, demographics, and other characteristics of subjects described? Yes
  2.4. Were the subjects/patients a representative sample of the relevant population? ???
3. Were study groups comparable? ???
  3.1. Was the method of assigning subjects/patients to groups described and unbiased? (Method of randomization identified if RCT) ???
  3.2. Were distribution of disease status, prognostic factors, and other factors (e.g., demographics) similar across study groups at baseline? Yes
  3.3. Were concurrent controls or comparisons used? (Concurrent preferred over historical control or comparison groups.) Yes
  3.4. If cohort study or cross-sectional study, were groups comparable on important confounding factors and/or were preexisting differences accounted for by using appropriate adjustments in statistical analysis? N/A
  3.5. If case control study, were potential confounding factors comparable for cases and controls? (If case series or trial with subjects serving as own control, this criterion is not applicable.) N/A
  3.6. If diagnostic test, was there an independent blind comparison with an appropriate reference standard (e.g., "gold standard")? N/A
4. Was method of handling withdrawals described? ???
  4.1. Were follow-up methods described and the same for all groups? Yes
  4.2. Was the number, characteristics of withdrawals (i.e., dropouts, lost to follow up, attrition rate) and/or response rate (cross-sectional studies) described for each group? (Follow up goal for a strong study is 80%.) ???
  4.3. Were all enrolled subjects/patients (in the original sample) accounted for? Yes
  4.4. Were reasons for withdrawals similar across groups? ???
  4.5. If diagnostic test, was decision to perform reference test not dependent on results of test under study? N/A
5. Was blinding used to prevent introduction of bias? Yes
  5.1. In intervention study, were subjects, clinicians/practitioners, and investigators blinded to treatment group, as appropriate? Yes
  5.2. Were data collectors blinded for outcomes assessment? (If outcome is measured using an objective test, such as a lab value, this criterion is assumed to be met.) Yes
  5.3. In cohort study or cross-sectional study, were measurements of outcomes and risk factors blinded? N/A
  5.4. In case control study, was case definition explicit and case ascertainment not influenced by exposure status? N/A
  5.5. In diagnostic study, were test results blinded to patient history and other test results? N/A
6. Were intervention/therapeutic regimens/exposure factor or procedure and any comparison(s) described in detail? Were interveningfactors described? Yes
  6.1. In RCT or other intervention trial, were protocols described for all regimens studied? Yes
  6.2. In observational study, were interventions, study settings, and clinicians/provider described? N/A
  6.3. Was the intensity and duration of the intervention or exposure factor sufficient to produce a meaningful effect? Yes
  6.4. Was the amount of exposure and, if relevant, subject/patient compliance measured? Yes
  6.5. Were co-interventions (e.g., ancillary treatments, other therapies) described? Yes
  6.6. Were extra or unplanned treatments described? Yes
  6.7. Was the information for 6.4, 6.5, and 6.6 assessed the same way for all groups? Yes
  6.8. In diagnostic study, were details of test administration and replication sufficient? N/A
7. Were outcomes clearly defined and the measurements valid and reliable? Yes
  7.1. Were primary and secondary endpoints described and relevant to the question? Yes
  7.2. Were nutrition measures appropriate to question and outcomes of concern? Yes
  7.3. Was the period of follow-up long enough for important outcome(s) to occur? Yes
  7.4. Were the observations and measurements based on standard, valid, and reliable data collection instruments/tests/procedures? Yes
  7.5. Was the measurement of effect at an appropriate level of precision? Yes
  7.6. Were other factors accounted for (measured) that could affect outcomes? Yes
  7.7. Were the measurements conducted consistently across groups? Yes
8. Was the statistical analysis appropriate for the study design and type of outcome indicators? Yes
  8.1. Were statistical analyses adequately described and the results reported appropriately? Yes
  8.2. Were correct statistical tests used and assumptions of test not violated? Yes
  8.3. Were statistics reported with levels of significance and/or confidence intervals? Yes
  8.4. Was "intent to treat" analysis of outcomes done (and as appropriate, was there an analysis of outcomes for those maximally exposed or a dose-response analysis)? N/A
  8.5. Were adequate adjustments made for effects of confounding factors that might have affected the outcomes (e.g., multivariate analyses)? Yes
  8.6. Was clinical significance as well as statistical significance reported? Yes
  8.7. If negative findings, was a power calculation reported to address type 2 error? N/A
9. Are conclusions supported by results with biases and limitations taken into consideration? Yes
  9.1. Is there a discussion of findings? Yes
  9.2. Are biases and study limitations identified and discussed? Yes
10. Is bias due to study's funding or sponsorship unlikely? Yes
  10.1. Were sources of funding and investigators' affiliations described? Yes
  10.2. Was the study free from apparent conflict of interest? Yes