Reduction of Risk for Low Back Injury in Theater of Operations: Complete Statistical Analysis

1. Trial at a Glance

This page covers the registry results for Reduction of Risk for Low Back Injury in Theater of Operations (NCT01401842), a randomized na study. The primary outcome is Isometric Lumbar Extension Muscular Strength at 11 Weeks.

2. Clinical Question

The primary outcome is isometric lumbar extension strength measured in Nm. The comparison is between the strengthening and stabilization exercise arms. The registry reports a mixed models analysis and a p-value, but does not report the arm-specific outcome values, the estimated difference between arms, or a confidence interval. Consequently, the statistical result can be described, but its direction and size cannot be established from the registry record.

The broader study topic is risk for low back injury in a military training context. The primary endpoint, however, is a strength measure assessed at follow-up, not an injury event. The registry also lists muscular endurance outcomes at 11 weeks as secondary outcomes. Those measures should not be conflated with the primary strength result.

3. Trial Design

The registry describes a randomized, single-masked, parallel trial, with enrollment of 582. A note attached to the available analysis identifies the design as cluster-randomized and cautions that a simple individual-level re-analysis would be misleading. The registry record does not report what units were randomized, how many clusters participated, or how allocation was carried out. It also does not identify which people were masked.

The comparison is between lumbar extension high-intensity progressive resistance exercise and low-intensity core stabilization exercise. The study summary describes soldiers training to become combat medics and an 11-week high-intensity program targeting low back muscles using specialized equipment. It frames the comparison as high-intensity exercise versus lower-intensity general core stability exercise. Details about the comparator’s delivery, session schedule, adherence, or any co-interventions are not reported in the registry record.

The primary outcome was adjusted isometric lumbar extension muscular strength at 11 weeks. The registry defines it as a mean, adjusted by baseline score, measured in Nm with a validated physical performance test on a lumbar dynamometer. The registry also names isometric core muscular endurance and dynamic lumbar extension muscular endurance at 11 weeks as secondary outcomes.

The listed study status is completed. The record gives a start date of 2012-06 and a primary completion date of 2014-04. It identifies the University of South Florida as sponsor and lists the phase as NA. Further information about the trial protocol, recruitment setting beyond the soldier training population, and the conduct of follow-up is 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.

Before assignment: Assessed for eligibility: n = 698. Consented: n = 645. Deemed eligible to participate, completed baseline assessments for primary outcome, and randomized: n = 582. Reasons for ineligibility: declined to participate (n = 43), did not meet inclusion criteria (n = 28), and other or unknown reasons (n = 45).

Overall Study

Reasons for not completing this period:

Analysis population for the primary outcome, as defined in the registry: Adjusted (by baseline score) isometric lumbar extension muscular strength in Nm (mean ± SE) at follow-up

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: All enrolled participants who were deemed eligible, completed baseline assessments, and were randomized.

6. Primary Endpoint

The registry lists the primary outcome as Isometric Lumbar Extension Muscular Strength at 11 Weeks, measured over 11 weeks.

The primary endpoint is isometric lumbar extension muscular strength at 11 weeks, expressed in Nm. It represents torque produced during a standardized isometric test: the participant exerts force while the measured position is held, rather than moving through a dynamic range. The registry says the assessment used a validated physical performance test on a lumbar dynamometer.

The recorded outcome is a mean adjusted for baseline score. Adjustment for baseline is a way to account for starting strength when estimating follow-up group results. The registry specifies standard error as the dispersion measure for the endpoint, but does not include the actual adjusted means or standard errors in the reported analysis.

As a measure, lumbar extension strength is relevant to the study’s focus on low back muscular performance. It is not the same outcome as an occurrence of low back injury. A strength difference may describe performance on this test, but the primary endpoint alone does not establish whether injury risk changed. The registry lists muscular endurance measures as secondary outcomes, which are related but distinct from the primary strength measure.

The registry’s reported inferential result is a p-value for a mixed models analysis. Without the estimated effect and its confidence interval, the magnitude and precision of the difference in strength cannot be read from that result. The posted registry record also does not report the group-specific follow-up means needed to describe the observed strength levels.

7. Why No Re-analysis Is Shown

8. Statistical Interpretation

9. Statistical Methods Explained

The registry reports a mixed models analysis for the primary endpoint and gives p = 0.001. A mixed model is a statistical approach that can represent both systematic group differences and dependence among observations that are related. In general, this can be useful when participants are grouped by a shared unit, or when measurements have a structure that means they should not be treated as entirely independent. The registry does not describe the model’s full specification, including which terms were included or how the cluster structure was represented.

Why does cluster randomization affect analysis?

In a cluster-randomized trial, assignment occurs at the level of groups rather than by independently assigning each individual. Members of a group may share setting, training, or other influences, so their outcomes can be correlated. General statistical reasoning says that treating correlated observations as independent can make uncertainty estimates too small and can distort inference. The registry’s analysis note specifically warns that a simple individual-level re-analysis would be misleading. It does not report the identities or number of randomized clusters, or further detail about the methods used to account for clustering.

What does the reported p-value tell the reader?

A p-value describes how compatible the observed data are with a specified null model, under the assumptions of the analysis. It is not the probability that the null hypothesis is true, and it does not measure the size or importance of an effect. Here, p = 0.001 is reported for the mixed models analysis. The registry does not state the full null hypothesis or provide the estimated contrast, so the p-value should not be used to infer which arm had a higher mean or how large the difference was.

Why is an effect estimate important?

An effect estimate expresses the observed contrast on the outcome’s scale. For this endpoint, a group contrast would describe a difference in adjusted lumbar extension strength, measured in Nm. A confidence interval gives a range of values compatible with the data and model under the interval procedure. It helps readers assess uncertainty and the plausible magnitude of the effect. Neither the effect estimate nor its confidence interval is reported in the registry record. The endpoint’s mean and standard error are specified as the type of summary, but their observed values are not included.

What does baseline adjustment mean?

The primary outcome is described as adjusted by baseline score. In general, baseline adjustment uses starting measurements to account for differences in initial outcome levels when estimating follow-up results. It can improve the relevance of a group comparison when baseline outcome values are available. The registry does not provide the baseline values, the adjustment model, or enough detail to assess how the adjustment was implemented.

What is the role of over-dispersion?

Over-dispersion is a general statistical issue that arises when observed variability is greater than a model’s assumed variability. If a model understates variability, its uncertainty estimates may be too narrow and its inferential results may be overly optimistic. This concept can be relevant to clustered or grouped data, but the registry record does not report an over-dispersion assessment, a diagnostic, or a method used to address it. It would therefore be inappropriate to claim that over-dispersion was present or handled in a particular way in this trial.

Can the reported result be recalculated from the registry?

No simple unadjusted re-calculation is presented. The analysis note identifies the trial as cluster-randomized and warns that an individual-level re-analysis would be misleading. In addition, the registry record lacks the group outcome summaries needed to reconstruct an adjusted contrast. The sponsor’s mixed models result is therefore the only inferential analysis described in the available record. The model’s detailed assumptions and diagnostics are not reported in the registry record.

How should planning illustrations be distinguished from trial results?

Sample-size calculations based on an observed effect are planning illustrations, not evidence that a study had a particular probability of detecting an effect. They are not observed power. The assumptions described for such illustrations include a two-sided significance level of 0.05, equal allocation between the arms, a normal approximation, and effects expressed as fractions of the observed effect. Those assumptions do not supply an observed effect estimate for this trial. Because the registry record does not report the numerical effect needed to interpret the planning illustration, it cannot replace the missing estimate and confidence interval.

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("Strengthening Exercise", "Stabilization Exercise")
n   <- c(230, 217)
m   <- c(310.2, 282.7)
sp  <- c(6.1, 6.1)
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  Strengthening Exercise  230  310.200  92.511  298.181  322.219
CARM  Stabilization Exercise  217  282.700  89.859  270.677  294.723

11. Analyses Reported by the Sponsor

The registry’s reported result is the sponsor’s mixed models analysis, with p = 0.001 for the primary endpoint. The analysis note identifies the design as cluster-randomized and warns that a simple individual-level re-analysis would be misleading. Accordingly, there is no simple recalculated comparison to set alongside the sponsor’s result.

The registry record does not provide the sponsor’s estimated contrast, adjusted means, confidence interval, or the model’s full specification. The p-value therefore cannot convey the direction or magnitude of the difference. The study summary’s stated goal of a 25% increase is an aim, not the sponsor’s reported estimate of the observed effect.

The registry entries do not identify a discrepancy between a recalculated result and the sponsor’s analysis; no suitable individual-level recalculation is presented. The sponsor’s own analysis is the inferential result reported in the registry, while its interpretation is limited by the missing estimate and interval. The p-value should be attributed to the mixed models analysis and not expanded into claims about the amount of strength change, which arm performed better, or injury prevention.

12. Multiplicity, Missing Data and Analysis Populations

The registry lists one primary outcome, isometric lumbar extension muscular strength at 11 weeks, and two secondary outcomes: isometric core muscular endurance and dynamic lumbar extension muscular endurance at 11 weeks. It also reports three outcome measures and three analyses. The record does not explain how the analyses correspond to those outcomes or whether additional comparisons were made.

When several outcomes or contrasts are tested, the chance of at least one small p-value can increase. General statistical approaches to this issue include prespecifying a primary comparison or adjusting inference across tests. The registry record does not report a multiplicity strategy, a hierarchy among secondary outcomes, or whether any adjustment was applied. The reported p = 0.001 should therefore be read as the result given for the primary endpoint analysis, without assuming how other analyses were handled.

Information about missing outcome data is not reported in the registry record. The record does not state how many participants contributed primary outcome measurements, how missing follow-up values were handled, or whether sensitivity analyses were performed. It also does not report the analysis population, such as whether all randomized participants were included according to assignment or whether another population was used.

The registry describes the primary outcome as adjusted by baseline score, but does not report the model details or the handling of missing baseline or follow-up measurements. Because the design is identified as cluster-randomized, the number and sizes of clusters, and how missingness within clusters was treated, would also be relevant to a full assessment. Those details are not reported in the registry record. These omissions limit evaluation of the robustness and scope of the reported inferential result; they do not establish that a particular problem occurred.

13. 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.

Most frequent non-serious adverse event terms

The registry lists zero serious adverse events for both the Strengthening Exercise and Stabilization Exercise arms. For other adverse events, it lists 6 affected in the Strengthening Exercise arm and 1 affected in the Stabilization Exercise arm. The record gives the corresponding at-risk counts as 298 and 284, respectively. These are the registry’s listed counts; they should not be interpreted as a complete comparison of safety without additional information about collection and reporting.

The registry does not list any serious adverse event terms among the top serious terms. Deaths are not reported in the registry record. The record does not describe how adverse events were defined, how actively they were collected, or the observation period used for safety reporting. It also does not explain whether the other adverse-event counts refer to people with events or to events themselves.

These figures are descriptive safety entries, not proof that serious events could not occur or that the arms have equivalent safety. The absence of listed serious events in the record should be read as a reported count, not as a general guarantee. No causal interpretation of the other adverse-event counts is supported by the registry entries alone.

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

The reported finding concerns a physical performance outcome: adjusted isometric lumbar extension strength at 11 weeks. The registry gives p = 0.001 for a mixed models analysis. It does not provide the adjusted group means, the estimated difference, or a confidence interval. The result can therefore be described as statistically detectable in the reported analysis, but its direction, magnitude, and precision cannot be determined from the registry record.

The study summary’s target of a 25% increase is a stated objective, not an observed effect. It should not be interpreted as the amount by which strength changed or as a result achieved by the high-intensity program. The primary endpoint is not an injury count, and the registry result does not establish that low back injury risk changed. Endurance is listed as a secondary outcome, but results for it are not included in the registry entries.

General statistical interpretation distinguishes evidence against a model’s null hypothesis from evidence about practical importance. A small p-value does not by itself tell readers whether a difference is large enough to matter, whether the result is precise, or whether it reflects a causal effect. Those judgments require an effect estimate, an uncertainty interval, and relevant information about study conduct and outcome measurement. The registry does not report those details for the primary comparison.

The trial’s cluster-randomized structure also constrains interpretation of any re-analysis. The record cautions that treating individual observations as independent would be misleading and reports the sponsor’s mixed models analysis instead. Overall, the posted result supports only a limited statement about the reported statistical analysis of strength at follow-up. It does not establish the size or direction of the strength contrast, a change in injury risk, or the result of the secondary endurance outcomes.

15. Why This Trial Matters Statistically

This trial illustrates why the design and analysis structure matter as much as a p-value. The registry identifies the study as cluster-randomized and explicitly warns against a simple individual-level re-analysis. When allocation is grouped, dependence among participants can affect uncertainty estimates. An analysis that ignores the randomized structure may not provide a trustworthy comparison, even if the outcome data appear to be ordinary individual measurements.

It also shows the limits of reporting a p-value without an effect estimate. The primary endpoint is a continuous measure with a clinically interpretable unit, Nm, and the registry describes a baseline-adjusted mean. Yet the reported record gives no adjusted means, estimated contrast, or confidence interval. As a result, readers cannot judge the size, direction, or precision of the strength difference from the p-value alone.

The study’s stated aim and its recorded outcome need to be kept distinct. The aim refers to a 25% increase in low back muscular strength and endurance, while the primary outcome is strength at 11 weeks. The stated target is not a finding, and strength is not an injury event. Keeping those distinctions clear prevents a performance endpoint from being mistaken for evidence about injury occurrence.

Finally, the record illustrates how incomplete reporting can narrow what statistical interpretation is possible. Details about clusters, missing data, multiplicity, and the model specification are not reported in the registry record. That does not show that the analysis was inadequate; it means that readers cannot independently assess those aspects from the registry information. A reported result is most informative when its estimate and uncertainty are available alongside its p-value.

16. 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 most immediate limitation is that the registry reports a p-value for the primary analysis but not an effect estimate, adjusted arm means, or confidence interval. Without those quantities, the direction and magnitude of the strength contrast and its precision cannot be assessed. The stated 25% target does not fill this gap because it describes the study aim, not an observed estimate.

The registry identifies cluster randomization but does not report the randomized units, the number or sizes of clusters, or detailed methods for accounting for clustering. It names mixed models as the analysis method, but does not provide the model specification, diagnostics, or assumptions. The record itself cautions against simple individual-level re-analysis, so the sponsor’s reported analysis cannot be independently checked using the available summary information.

Other important analysis details are not reported in the registry record. These include the analysis population, missing-data handling, the multiplicity strategy, and whether sensitivity analyses were conducted. Although baseline adjustment is specified for the primary outcome, the adjustment model and baseline results are not provided. Results for the listed secondary outcomes are also absent from the registry entries.

The record does not identify which party was masked, describe adherence or intervention delivery in detail, or explain adverse-event definitions and ascertainment. The safety entries do not include deaths, which are not reported in the registry record. These reporting gaps limit assessment of conduct and interpretation; they do not demonstrate that bias, missingness, or safety problems occurred.

17. Related Tutorials

18. Related Statistical Calculators

19. Sources

• ClinicalTrials.gov: Reduction of Risk for Low Back Injury in Theater of Operations — NCT01401842.
• Registry record last updated 2015-05-25; status last verified 2015-05.
• Results first posted 2015-05-25.
• 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.

20. Record Summary

The registry describes a completed randomized, single-masked, parallel trial sponsored by the University of South Florida, with enrollment of 582. It compares lumbar extension high-intensity progressive resistance exercise with low-intensity core stabilization exercise in U.S. Army soldiers training to become combat medics. The study summary describes an 11-week intervention, while the primary endpoint is adjusted isometric lumbar extension strength at 11 weeks, measured in Nm.

The registry reports a mixed models analysis with p = 0.001. A note identifies the design as cluster-randomized and cautions that a simple individual-level re-analysis would be misleading. The registry record does not report the effect estimate, confidence interval, or direction of the primary comparison. It lists muscular endurance measures as secondary outcomes, but their results are not included in the registry entries.

Serious adverse-event counts are listed as zero in both arms; other adverse-event counts are 6 and 1. Deaths are not reported in the registry record. Important details about clustering, missing data, multiplicity, and the analysis population are also not reported.