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NAP: Recovery (2014)


Etheridge T, Philp A, Watt PW. A single protein meal increases recovery of muscle function following an acute eccentric exercise bout. Appl Physiol Nutr Metab. 2008; 33: 483-488.

PubMed ID: 18461101
Study Design:
Randomized Crossover Trial
A - Click here for explanation of classification scheme.
Quality Rating:
Neutral NEUTRAL: See Quality Criteria Checklist below.
Research Purpose:

To examine the effects of acute protein ingestion on the recovery of muscle function and markers of muscle damage in the 72 hours post-eccentric exercise.

Inclusion Criteria:
  • Male
  • Recreationally active.
Exclusion Criteria:
Not discussed.
Description of Study Protocol:

The study's protocol followed a randomized, cross-over design, in which subjects were assigned to either the protein (PRO) group or control (CON) group. Subjects visited the laboratory on two occasions, separated by a minimum of 28 days. They performed three maximal five-second knee extensions and sprints on a cycle ergometer, followed by a downhill running protocol on a treadmill. Immediately after exercise, subjects in the PRO group ingested a protein solution, while subjects in the CON group ingested a placebo solution.

Blinding Used

The study was conducted in a double-blinded manner.


Immediately after exercise, subjects ingested one of two solutions:

  • Protein (PRO): A 100g milk protein concentrate (40g EAAs with no CHO)
  • Placebo (CON) solution: Flavored water.

Statistical Analysis

  • A repeated-measures ANOVA was used to analyze differences in blood markers, performance variables and assessment of DOMS between conditions
  • A Tukey's post-hoc test was performed when a significant time or condition interaction was obtained to identify the point of difference
  • Pearson's product-moment correlation test was performed to assess relationships between plasma CK and PC concentration and subjects' perceptions of muscle soreness. 


Data Collection Summary:

Timing of Measurements

Plasma creatine kinase (CK), protein carbonyl (PC) content, peak isometric maximum voluntary contraction (MVC) of the quadriceps and peak power output (PPO) were measured prior to exercise. Along with assessment of delayed-onset muscle soreness (DOMS), these measurements were recorded again 24, 48 and 72 hours after exercise.

Dependent Variables

  • CK (marker of muscle damage): Analyzed from blood samples
  • PC (marker of oxidative stress): Analyzed from blood samples
  • MVC: Determined as the highest value recorded from three maximal five-second knee extensions; measured using a goniometer
  • PPO: Determined as the highest value recorded from three maximal five-second sprints on a cycle ergometer
  • DOMS: Muscle soreness values were obtained using a visual analog scale ranging from no muscle soreness (one) to very, very sore (10). A strain gauge algometer was used to assess deep muscle pain and trigger point tenderness of the quadriceps muscle group.

Independent Variables

Post-exercise beverage: (PRO) vs. (CON).

Control Variables

  • Standardized exercise protocol
  • Throughout the study, all subjects maintained their habitual diet and abstained from alcohol, anti-inflammatory drugs, supplementary vitamins and nutritional supplements.


Description of Actual Data Sample:
  • Initial N: Nine males
  • Attrition (final N): Nine
  • Age: 21±1 years.


  • Mass: 76±3kg
  • Height: 1.80±0.01m.


Eastbourne, United Kingdom.

Summary of Results:

Key Findings

  • A significant increase in pain sensation and perceived soreness was experienced by all subjects (P<0.05). There were no differences observed in rating of muscle soreness at any time point between the PRO and CON conditions.
  • Serum CK was significantly elevated after the downhill run, peaking at 24 hours in both the PRO and CON conditions (P<0.01), declining to within pre-exercise values at 48 and 72 hours (P>0.05). Serum PC concentrations were significantly raised post-exercise (P<0.05), peaking after 24 hours in the PRO trial and after 72 hours in the CON trial. No differences were observed in CK and PC between the PRO and CON trials at all time points.
  • A significant difference was observed between the PRO and CON conditions for both MVC and PPO (P<0.05). This difference was found at 48 hours for both measures of muscle function.


Author Conclusion:

Ingesting a single dose of protein immediately after exercise accelerates recovery of static force and dynamic power production during the DOMS period.

Funding Source:
University/Hospital: Welkin Research Laboratories, University of Brighton
Reviewer Comments:
  • Small population
  • Inclusion and exclusion criteria not well described
  • Short treatment period
  • Sources of funding not described (assumed University of Brighton).
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? ???
  2.2. Were criteria applied equally to all study groups? ???
  2.3. Were health, demographics, and other characteristics of subjects described? No
  2.4. Were the subjects/patients a representative sample of the relevant population? ???
3. Were study groups comparable? Yes
  3.1. Was the method of assigning subjects/patients to groups described and unbiased? (Method of randomization identified if RCT) Yes
  3.2. Were distribution of disease status, prognostic factors, and other factors (e.g., demographics) similar across study groups at baseline? ???
  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? Yes
  4.1. Were follow-up methods described and the same for all groups? N/A
  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%.) N/A
  4.3. Were all enrolled subjects/patients (in the original sample) accounted for? Yes
  4.4. Were reasons for withdrawals similar across groups? N/A
  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? No
  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? N/A
  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? No
  10.2. Was the study free from apparent conflict of interest? Yes