Muscle Mass During Space Exploration: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for Muscle Mass During Space Exploration (NCT00968344), a randomized na study. The primary outcome is Lean Leg Mass.

2. Clinical Question

The biological rationale recorded by the investigators is that bed rest may blunt the anabolic response to a mixed nutrient meal and contribute to losses in muscle mass and function. The stated hypothesis is that leucine enrichment of meals may help maintain protein synthesis and the anabolic response, preserving lean muscle mass and function during bed rest and supporting recovery during rehabilitation.

The posted primary outcome focuses on lean leg mass. It does not directly report the broader outcomes described in the hypotheses, such as functional capacity, metabolic capacity, protein synthesis, or rehabilitation recovery. The registry record does not report a separate result for those outcomes. The clinical question that can be assessed from the posted primary results is therefore narrower: what mean lean leg mass values were recorded in the two groups around the bed-rest period, and how precisely are those group estimates described?

3. Trial Design

The record identifies this as a randomized, double-masked, parallel-group trial. The listed allocation is randomized, and the listed masking is double. The registry record does not report the randomization sequence method, allocation concealment procedures, or which trial roles were masked. Those details should not be inferred from the design labels alone.

The intervention groups are described as leucine and placebo. Participants in the leucine group received 3-4 g leucine added to daily meals during bed rest; the placebo group received 3-4 g alanine added to daily meals during bed rest. The registry lists the interventions as dietary supplements. The posted primary outcome describes DEXA scans of both legs before and after bed rest, with the assessment scheduled at baseline before bed rest and after 14 days of bed rest.

The record gives an enrollment of 40 and a completed status. The posted primary outcome summaries list 9 participants in the control group and 10 in the leucine group. The registry record does not explain the relationship between the enrollment figure and the group counts shown for this endpoint, or whether participants were excluded from this particular analysis. Enrollment, endpoint-specific counts, and analysis populations are different kinds of information and should not be treated as interchangeable.

The trial is listed with phase NA and was sponsored by The University of Texas Medical Branch, Galveston. The recorded start is 2009-08 and primary completion is 2015-04. The primary outcome is the only outcome measure indicated in the record, and no analyses are listed. Details of the protocol, follow-up beyond the posted primary outcome, and the statistical analysis plan are not reported in the registry record.

4. Participant Flow and Accounting

The registry reports how participants moved through each stage of the study. All counts below are exactly as posted; missing counts are not inferred from other stages.

Overall Study

Analysis population for the primary outcome, as defined in the registry: Healthy community-dwelling men and women aged 45-60 y

5. Baseline Characteristics

The table reproduces the baseline characteristics posted in the registry, which describe the participants analysed at baseline and help show whether the randomized groups were comparable.

Population: Healthy community-dwelling men and women aged 45-60 y old participated in this randomized, double-blind, placebo controlled study. Volunteers were recreationally active but athletically untrained.

6. Primary Endpoint

The registry lists the primary outcome as Lean Leg Mass, measured over At baseline prior to beginning bed rest and after 14 days of bed rest.

The primary endpoint is lean leg mass, measured in grams. The registry describes DEXA scans of both legs before and after bed rest, with the scheduled assessments at baseline and after 14 days of bed rest. DEXA is the stated measurement method; the registry record does not provide further technical details about the scan procedure or how the two assessments were converted into the reported endpoint value.

Lean mass is used as a body-composition measure relevant to the trial’s stated concern about muscle loss. Leg-specific measurement focuses the assessment on the lower limbs rather than reporting whole-body lean mass. That makes it relevant to the recorded hypothesis about maintaining muscle mass during bed rest, but it is not itself a direct measure of physical function, metabolic capacity, or recovery during rehabilitation.

The outcome is described as a continuous measure, summarized by a mean with standard error. The posted group values are -1.2 g for the control group and -0.9 g for the leucine group. Although the record describes measurements before and after bed rest, it does not explicitly state whether these negative values are change scores or another derived quantity. Accordingly, their exact interpretation as changes from baseline is not confirmed by the registry record.

The endpoint description names healthy community-dwelling men and women aged 45-60 y. The record does not report whether the endpoint was collected at additional time points or how missing DEXA measurements were handled.

7. Why No Re-analysis Is Shown

8. Statistical Interpretation

9. Statistical Methods Explained

What does the mean describe?

For a continuous outcome, a mean is a summary of the observed values in a group. It provides a central estimate in the outcome’s measurement units. Here the endpoint is lean leg mass in grams, and the registry reports a mean for each group. A group mean does not show how widely individual participants’ measurements vary, whether the distribution is symmetric, or whether a small number of observations strongly influence the average.

The endpoint record describes DEXA assessments at baseline and after 14 days of bed rest, but does not explicitly define the reported negative values as changes between those assessments. A reader should therefore distinguish the measurement schedule from the precise calculation underlying the posted mean. That calculation is not reported in the registry record.

What does the standard error describe?

A standard error describes the precision of an estimated statistic, such as a mean. In general, a smaller standard error indicates a more precisely estimated mean under the assumptions of the method used. It is not the same as the standard deviation, which describes variability among individual observations. The posted standard errors are 0.2 for the control group and 0.1 for the leucine group. The registry does not report standard deviations or individual-level values, so the spread of participant outcomes cannot be read directly from these summaries.

A confidence interval would express a range of values compatible with the data and the statistical procedure, under its assumptions. No confidence interval is posted for either group mean or for the between-group contrast. A confidence interval for a contrast would be especially useful because it would show both the estimated direction and the precision of the comparison. The registry record does not report that estimate.

Why is a group comparison different from two group summaries?

Comparing two means requires an estimate of the difference between groups and an assessment of its uncertainty. The descriptive group means alone do not supply the result of a statistical test or a confidence interval. A comparison may also depend on the prespecified analysis, including how baseline measurements are used and how the endpoint is defined. Those analysis details are not reported in the registry record.

The record states that values were missing or not estimable for a two-arm comparison of this continuous endpoint. Thus, no comparative result is available to interpret. There is no sponsor-reported contrast to take precedence over a recalculation, and no recalculated comparison is presented. The absence of a comparison should not be rephrased as a finding of no effect or equivalence.

Why can over-dispersion matter?

In general, over-dispersion means that observed variation is greater than a statistical model expects. When a model assumes less variability than the data contain, standard errors may be too small and intervals or tests may convey more certainty than is warranted. It is a relevant general consideration for analyses whose assumptions do not adequately represent the data. The posted primary outcome is continuous and summarized by means and standard errors; the registry record does not report an over-dispersion assessment or a model-based analysis. Over-dispersion should not be asserted as a feature of these results.

How should planning calculations be read?

The record describes sample-size illustrations based on the observed effect, a two-sided significance level of 0.05, equal allocation, and a normal approximation. The effect to detect is described as a stated fraction of the observed effect. Such calculations are planning illustrations: they indicate the participants per arm that a future design might require under those assumptions. They are not observed power and do not show the probability that this completed trial detected an effect.

The registry record does not provide numerical planning estimates in the information available for interpretation. It also does not report that these illustrations were the trial’s original design calculations. They should not be presented as protocol targets or as evidence that the completed study was adequately powered. The stated enrollment and the endpoint-specific counts do not establish a sample-size justification.

What information is needed to assess the analysis fully?

Interpretation of a randomized trial’s analysis would ordinarily benefit from knowing the prespecified estimand, analysis population, handling of baseline values, missing-data method, model specification, and treatment of protocol deviations. Those items are not reported in the registry record. The listing of randomized allocation and double masking describes broad design features, but it does not fill in the statistical analysis details. In general, these details matter because different reasonable choices can produce different estimates or uncertainty, particularly when endpoint data are unavailable for some participants.

10. Confidence Intervals for Each Arm

No two-arm comparison is shown for this trial, but each arm's mean can be given with a t-based 95% confidence interval computed from the posted mean, spread and number of participants.

R code and output

arm <- c("Control Group", "Leucine (Intervention) Group")
n   <- c(9, 10)
m   <- c(-1.2, -0.9)
sp  <- c(0.2, 0.1)
sd  <- sp * sqrt(n)      # standard errors converted to SDs
for (i in seq_along(n)) {
  se <- sd[i] / sqrt(n[i]); h <- qt(0.975, n[i] - 1) * se
  cat(sprintf("CARM|%s|%d|%.3f|%.3f|%.3f|%.3f\n", arm[i], n[i], m[i], sd[i], m[i] - h, m[i] + h))
}

CARM  Control Group  9  -1.200  0.600  -1.661  -0.739
CARM  Leucine (Intervention) Group  10  -0.900  0.316  -1.126  -0.674

11. Multiplicity, Missing Data and Analysis Populations

The registry lists one outcome measure and zero analyses. It does not list posted secondary outcomes. This means the record contains no reported set of multiple endpoint tests whose multiplicity adjustment can be assessed. It does not establish whether additional outcomes or analyses were prespecified in a protocol or conducted outside the posted record; those details are not reported in the registry record.

The primary endpoint’s analysis population is not reported in the registry record. The endpoint summary lists group counts of 9 and 10, while the overall enrollment is 40. The record does not explain the difference or say whether the endpoint counts reflect missing measurements, exclusions, or another reporting convention. No missing-data strategy, imputation method, or sensitivity analysis is reported. Missing outcome information should not be assumed to have been absent simply because a method is not described.

The record also does not report whether analysis followed intention-to-treat principles, whether participants were analyzed according to assigned group, or how withdrawals and protocol deviations were handled. These omissions limit assessment of how closely the posted endpoint summaries correspond to the randomized comparison. The registry’s note that a two-arm comparison was not estimable further means there is no inferential result to evaluate for this outcome.

12. Safety Results

The registry reports serious adverse events as affected participants divided by participants at risk.

Other (non-serious) adverse events

Participants with at least one non-serious event at or above the registry's 0% reporting threshold.

The safety record reports zero serious adverse events among 10 participants at risk in the leucine group and zero among 9 participants at risk in the placebo group. It also reports zero other adverse events among the participants at risk in each group. These are the figures recorded for the listed groups; they should not be interpreted as proof that no adverse events occurred outside the reporting scope or observation period.

Deaths are recorded as not reported, rather than as a count of zero. No top serious adverse event terms are listed. The registry record does not provide additional detail about the adverse-event collection period, definitions, severity grading, relatedness assessment, or completeness of reporting. Those details are needed to place the zero event counts in context.

Adverse-event counts describe reported events in the recorded risk groups. They do not, on their own, establish comparative safety, particularly when the registry does not provide an analysis of group differences or further follow-up information. These figures should be read as registry reporting, not as a recommendation about treatment.

13. What the Result Does, and Does Not, Mean

The posted results describe lean leg mass summaries in the leucine and control groups after a trial involving bed rest. The reported means are -0.9 g for the leucine group and -1.2 g for the control group, with standard errors of 0.1 and 0.2, respectively. The registry describes DEXA assessments before and after bed rest, but does not explicitly label the negative endpoint values as changes from baseline. The values are therefore best interpreted with that reporting limitation in mind.

The record does not include an estimable two-arm comparison, confidence interval, or statistical test for the primary endpoint. These summaries cannot establish that leucine preserved lean leg mass, that the groups differed, or that they were equivalent. Nor does the absence of an inferential comparison demonstrate that there was no effect. It means the posted registry results do not provide the comparative analysis needed to answer that question statistically.

The trial’s hypotheses extend beyond this endpoint to muscle function, protein synthesis, metabolic capacity, and recovery during rehabilitation. No posted results for those measures are included. The primary result therefore cannot be used as a substitute for evidence on those distinct outcomes.

More broadly, the result applies to the listed study context and outcome as recorded: healthy community-dwelling men and women aged 45-60 y, meal supplementation during bed rest, and DEXA-based lean leg mass. The record does not report enough detail to assess the endpoint calculation, analysis population, handling of missing data, or full statistical plan. Those limits constrain what can be concluded from the posted summaries, regardless of the biological rationale.

14. Why This Trial Matters Statistically

This trial illustrates why a randomized design and a posted endpoint summary are not sufficient by themselves to establish a treatment effect. Random allocation can support a fair comparison when the analysis preserves the randomized groups and accounts appropriately for the outcome data. However, the registry record does not provide an estimable comparison for the primary continuous endpoint. Without a contrast and its uncertainty, readers cannot determine how compatible the observed group summaries are with a meaningful difference or with little difference.

The distinction between a mean and its standard error is also important. The mean summarizes the group’s endpoint values, while the standard error describes precision of that estimated mean. Neither directly shows participant-to-participant variability. The absence of standard deviations, confidence intervals, and individual data limits assessment of distribution and precision beyond the reported standard errors.

The record also separates enrollment from endpoint-specific reporting. It lists enrollment of 40, but the primary outcome summaries give smaller group-specific counts without explaining the relationship. Transparent reporting of analysis populations and missing-data handling helps readers understand whether the analyzed groups remain comparable and how much outcome information contributes to the estimates.

Finally, the trial’s stated rationale encompasses several biological and functional questions, while the posted primary outcome is lean leg mass. Keeping the endpoint distinct from the broader hypotheses prevents descriptive results from being stretched into conclusions about function, recovery, or metabolic response. These are general lessons in interpreting clinical trial records: design labels, outcome summaries, and inferential analyses answer different questions.

15. Limitations

• The posted summary statistics are the only data these results rest on; patient-level data, full protocols and statistical analysis plans are not available.
• Recalculated results are simple, unadjusted calculations from posted summaries. They do not replace the sponsor's prespecified analysis.

The registry record provides limited information for judging the primary endpoint analysis. Although it specifies DEXA measurement of both legs at baseline and after 14 days of bed rest, it does not describe the exact calculation that produced the posted negative means. In particular, it does not explicitly identify those values as change scores. This limits interpretation of their direction and clinical meaning.

No two-arm comparison is reported for the continuous endpoint because values were missing or not estimable for that comparison. The record does not explain the reason. It also does not report confidence intervals, a statistical test, the analysis population, missing-data procedures, or an intention-to-treat approach. The endpoint-specific group counts are not reconciled with the overall enrollment.

The clinical outcomes posted do not cover all elements of the investigators’ stated hypotheses. Results for muscle function, protein synthesis, metabolic capacity, and rehabilitation recovery are not reported in the registry record. Follow-up beyond the stated bed-rest assessment is likewise not described for the primary result.

Information on the randomization process, allocation concealment, masking procedures, protocol deviations, and the trial’s statistical analysis plan is not reported in the registry record. The record’s sample-size assumptions describe planning illustrations based on the observed effect, but do not establish the original design rationale. These limitations make the posted summaries useful as a record of reported values, but insufficient for a complete assessment of the intended randomized comparison.

16. Related Tutorials

17. Related Statistical Calculators

18. Sources

• ClinicalTrials.gov: Muscle Mass During Space Exploration — NCT00968344.
• Registry record last updated 2019-05-31; status last verified 2017-12.
• Results first posted 2019-05-31.
• Data retrieved from ClinicalTrials.gov on 2026-11-09.
• Group sizes, means, event counts and sponsor-reported p-values are reproduced as posted. Effect estimates, confidence intervals and p-values not attributed to the sponsor are independently calculated from the posted summary statistics.

19. Record Summary

NCT00968344, titled “Muscle Mass During Space Exploration,” is listed as a completed, randomized, double-masked, parallel-group study sponsored by The University of Texas Medical Branch, Galveston. The record gives enrollment of 40 and describes a dietary comparison during bed rest: 3-4 g leucine added to daily meals versus 3-4 g alanine as placebo.

The primary outcome is lean leg mass in grams, assessed by DEXA at baseline and after 14 days of bed rest. The posted mean values are -1.2 g for control and -0.9 g for leucine, with standard errors of 0.2 and 0.1. A two-arm comparison is not estimable in the registry record, and no analyses are listed. The record does not clarify whether the negative means are change scores or explain the endpoint-specific counts relative to enrollment.