Abstract
Background Limited evidence exists on how strength and endurance exercises commonly used to prevent low back pain affect muscle morphometry and endurance.
Objective The purpose of this study was to analyze the effects of 2 exercise regimens on the morphometry and endurance of key trunk musculature in a healthy population.
Design The study was designed as a randomized controlled trial.
Setting The study was conducted in a military training setting.
Participants A random subsample (n=340; 72% men, 28% women; mean [±SD] age=21.9±4.2 years; mean [±SD] body mass index=24.8±2.8 kg/m2) from the larger Prevention of Low Back Pain in the Military trial (N=4,325) was included.
Intervention The core stabilization exercise program (CSEP) included low-load/low-repetition motor control exercises, whereas the traditional exercise program (TEP) included exercises conducted at a fast pace, with the use of high-load, high-repetition trunk strengthening exercises.
Measurements Baseline and follow-up examinations included ultrasound imaging of the trunk muscles and endurance tests. Linear mixed models were fitted to study the group and time effect and their interactions, accounting for the clustering effect.
Results Symmetry generally improved in the rest and contracted states, but there were no differences suggestive of muscle hypertrophy or improved ability to contract the trunk muscles between soldiers receiving the CSEP or the TEP. Total trunk endurance time decreased over the 12-week period, but endurance performance favored soldiers in the CSEP group. Endurance time was not associated with future episodes of low back pain.
Limitations The lack of morphological changes may not be detectable in an already-active cohort, or a more intensive dose was needed.
Conclusions Although improved symmetry was noted, neither the CSEP nor the TEP resulted in muscle hypertrophy. Longer endurance times were noted in individuals who completed the CSEP but were not strongly predictive of future low back pain episodes.
Systematic reviews have demonstrated that exercise therapy reduces pain and improves function in individuals with chronic low back pain (LBP).1,2 In addition, researchers have demonstrated that exercise therapy is beneficial for secondary and tertiary LBP prevention by preventing recurrence and decreasing long-term disability.3–6 Trunk-strengthening exercises that are based on traditional strength training paradigms such as frequency and intensity have demonstrated improvements in strength and disability.7–9 Similarly, exercise regimens designed to enhance motor control of the deep trunk muscles (transversus abdominis [TrA], lumbar multfidus [LM], and pelvic-floor muscles) have demonstrated effectiveness for individuals with acute and chronic LBP,3,10 pregnancy-related LBP,11,12 and spondylolysis or spondylolisthesis.5 The ability of both of these exercise programs to help restore function and decrease disability for those with LBP demonstrates their effectiveness for tertiary prevention. However, evidence to date has failed to demonstrate that one of these approaches is superior to the other, and controversy persists.13–16 Understanding how trunk strengthening and motor control exercise regimens affect trunk muscular morphometry may assist in developing more effective interventions to prevent LBP.
Recent systematic reviews have supported the reliability17 and validity18 of using ultrasound imaging to assess trunk muscle morphometry. Assessment of muscular thickness, cross-sectional area (CSA), and muscle symmetry may have clinical utility for identifying underlying impairments associated with LBP. Smaller CSA and decreased thickness of the LM have been found in individuals with LBP.19–22 Hides et al23 demonstrated mean (±SD) asymmetry of the LM of 31%±8% in individuals with LBP compared with 3%±4% among individuals without LBP. The relative symmetry of the LM in individuals without LBP (<10%) was later confirmed by Stokes et al24 in a sample of 120 participants. However, in a study of 126 men who were healthy, asymmetry ranged from 0.1% to 44.3%, and asymmetry >10% was commonly found in men without a history of LBP.25 The clinical studies focusing on patients with LBP demonstrated that changes in muscular morphometry also have been associated with pain and disability.26 With respect to the abdominal muscles, the TrA, internal oblique (IO), and external oblique (EO) muscles appear to be asymmetrical (13%–24% asymmetry) in individuals without LBP.27 However, when the lateral abdominal muscles are measured as a group, the symmetry (8%–9%) was similar to that of some of the studies that assessed the LM muscle.27 Limited evidence exists for documenting muscle morphometry over time in a large group of individuals without LBP. Understanding how muscle symmetry and endurance changes in relationship to different trunk exercise regimens may assist in designing future LBP prevention protocols.
In addition to being able to assess muscle morphometry at rest, ultrasound imaging provides the opportunity to assess changes in thickness and CSA between the resting and contracted states. With respect to the lateral abdominal muscles, researchers have demonstrated a diminished response of the TrA muscle during the abdominal drawing-in maneuver and the active straight-leg-raise test in individuals with LBP.21,28,29 Other researchers19,30,31 have demonstrated diminished response of the LM muscle during a prone arm-lifting task and during a voluntary contraction in individuals with LBP or induced pain. Hebert et al32 demonstrated that a diminished response in thickening the LM during the prone arm-lifting task was a prognostic indicator predicting individuals who would succeed with motor control exercises. Finally, Hides and colleagues33,34 have demonstrated that motor control exercises can positively affect both symmetry and the ability to contract the LM muscle in individuals with LBP. Although preliminary evidence is encouraging that ultrasound imaging can be useful for assessing trunk muscle morphometry in individuals with and without LBP, scant evidence exists on the relationship between changes in muscle morphometry and trunk muscle exercise regimens intended for primary prevention of LBP in individuals who are healthy.
As part of the Prevention of Low Back Pain in the Military (POLM) trial,35,36 we sought, in part, to determine whether core stabilization was more effective in preventing the incidence of LBP compared with traditional trunk strengthening exercise. In addition to this primary purpose, the POLM trial provided a unique opportunity to gain insight into the underlying changes in trunk muscle morphometry and endurance that occurred on the basis of each form of trunk strengthening exercise. Knowledge of changes in trunk morphology and endurance in individuals without LBP may provide insight into underlying mechanisms that are associated with preventative exercise programs. Therefore, the purpose of this planned secondary analysis was to assess the effects of 2 exercise regimens designed to prevent LBP in individuals who are healthy on the morphometry and endurance trunk musculature believed to be important in limiting the onset of LBP. Although we did not have specifically stated hypotheses, it was our expectation that core stabilization exercise would have favorable effects on morphometry and trunk endurance in comparison to a traditional exercise program.
Method
Design Overview
Participants were US Army soldiers who were healthy and entering a 16-week training program at Fort Sam Houston, Texas, to become a combat medic and who were enrolled in the cluster randomized POLM trial (NCT00373009),36 which has been registered at ClinicalTrials.gov. In the primary trial, 4,325 volunteers from 20 companies were enrolled to determine the effectiveness of different exercise programs with or without a psychosocial educational program on the prevention of LBP. The 2-year follow-up results have been published elsewhere.35 Before initiating the different exercise regimens, ultrasound imaging assessments of the abdominal and LM muscles as well as trunk endurance times were performed on a random subset of 340 participants. A random subset was selected because it was not feasible to perform ultrasound imaging on all 4,325 participants, and a priori power estimates indicated that 340 participants would be adequate to detect meaningful differences in trunk muscle morphometry between the groups.36 The randomization schedule was prepared by computer and was determined before recruitment began. The randomization schedule was balanced to ensure equal allocation to each exercise regimen after 20 companies were recruited. The ultrasound imaging assessments from the first 2 companies of soldiers were used to establish reliability (n=21); a summary of the rater reliability is provided later in this article and has been presented in more detail in an earlier publication.37 The present analysis includes all 340 participants who were randomly assigned to receive the ultrasound imaging evaluation (eFigure).
Participants
Soldiers were eligible to participate if they were between 18 and 35 years of age (or a 17-year-old who was an emancipated minor), fluent in English, and were enrolled in combat medic training. Soldiers were excluded if they had a history of LBP that resulted in limited work or physical activity >48 hours, medical care associated with LBP, or prior surgery in the lumbar spine region. Soldiers also were excluded if they were unable to participate in unit physical training because of other musculoskeletal injuries, if they had a history of fracture (stress or traumatic) in the hip or pelvis, or were pregnant. Soldiers who were transferred from another training group, accelerated into a company already randomly assigned and recruited for the trial, or reassigned to an occupational specialty other than a combat medic also were excluded. All participants provided written informed consent before their participation.
Exercise Programs
All participants from the POLM trial were randomly assigned by cluster to receive either a traditional exercise program (TEP) or a core stabilization exercise program (CSEP) on the basis of which training company the soldier was assigned for combat medic training. Half of these participants also received a brief biopsychosocial education training program; additional details of the cluster randomization used in this study are provided elsewhere.38,39 As part of daily unit physical training, exercises were performed in a group setting under direct supervision by drill instructors. The exercise programs for both groups consisted of 5 to 6 exercises, each of which was performed for 1 minute. Exercise programs were performed 4 days per week for approximately 5 minutes per day over a 12-week period. In addition to specific exercises performed for this research trial, all participants performed standard physical training (eg, warm-up, aerobic training, strength and conditioning drills, and recovery) to US Army standards by both groups, resulting in an hour-long exercise regimen 4 days per week.
The TEP consisted of exercises that were performed quickly through a full range of motion and consisted of sit-ups, sit-ups with left and right trunk rotation, and abdominal crunches. The exercises selected for the TEP were based on the historic use of these exercises during unit physical training in this environment, with the goal to use the current exercise approach as the comparison. Therefore, the TEP did not include any exercise that focused exclusively on trunk extensor strength or endurance. Each exercise was performed for 1 minute, with sit-ups performed twice during the 5-minute exercise session. Instructions provided to the participants were to do as many repetitions possible. The CSEP consisted of exercises that were performed slowly around a relatively stable spine posture and consisted of abdominal drawing-in maneuver crunch, left and right horizontal side supports, squats, supine shoulder bridge, and quadruped alternate arm and leg. Each exercise was performed for 1 minute during the 5-minute session. Instructions provided to the participant were to perform each exercise slowly and to standard in a slow, controlled manner. The exercises were not progressed in frequency, intensity, time, or type over the 12-week training program. Additional details regarding each exercise regimen are provided in the eTable and published elsewhere.38
The drill instructors leading the exercise regimen were trained by the research staff before initiation of the study. Detailed training cards with additional details provided on a website were provided to the drill instructors. Performance of the exercise programs under direct supervision of drill instructors in a group setting helped to ensure adherence to the assigned program and dosage. Research staff supervised physical training for an average of 2 days per week to answer questions and monitor adherence to the assigned exercise regimen.
Endurance Measurements
The following 4 endurance tests were assessed: supine flexor endurance test, prone extensor endurance test, and right and left horizontal side support.40 The participants performed each endurance test for as long as possible, not to exceed 4 minutes. If the participant deviated from the test position, the examiner provided 1 verbal cue. If the participant could not return to the test position, the test was terminated. No other verbal cues were provided. All measures were obtained before and after the 12-week training program. Pictures and demonstration by the examiners were provided before testing.
Trunk flexor endurance was assessed with the participant positioned supine and arms at the sides.41 Participants were instructed not to push down with their arms during testing. The participant's feet were passively positioned 15 cm (6 in) above the table to demonstrate the starting position. If the participant raised his or her feet above 20 cm (8 in) or lowered them below 10 cm (4 in), a verbal cue was provided to return to the test position. If the participant went outside the zone a second time, the test was terminated. The test ended when the participant was unable to keep the feet within the target zone, the feet went out of the zone twice, or 4 minutes elapsed.
Trunk extensor endurance was assessed with the participant positioned prone and their anterior superior iliac spine positioned near the edge of the testing table and the upper body supported by a stool.42–44 Three adjustable straps secured the participant at the proximal ankle, popliteal fossa, and greater trochanters. Timing began when the participant released his or her hands from the resting position and folded the arms across the chest. The inclinometer was immediately placed between the inferior borders of the scapula, and the participants were instructed to position the upper body so that the inclinometer read zero. If the participant deviated more than ±10 degrees, he or she was instructed to return to the test position. The test was terminated on the second deviation, if the participant rested the body or hands on a supporting surface/stool, or 4 minutes had elapsed.
Trunk lateral flexor endurance was assessed with the participant in a side-lying position, supported on the elbow of the dominant arm, and the legs extended.43–45 The top foot was placed in front of the lower foot for support, and the opposite arm was placed on the shoulder of the dominant arm. Participants were instructed on how to support themselves by lifting their hips off the surface to maintain a straight line over their full body length while supporting themselves over the top of the dominant elbow and the sides of the feet. Timing began when the participant moved into the testing position. If participants deviated from the test position because of fatigue, they were given 1 correction. The test was terminated if the participant deviated a second time, could not return to testing position after first deviation, or 4 minutes had elapsed.
Ultrasound Imaging Measurements
Measurements of resting muscle thickness of the TrA, IO, EO, rectus abdominis (RA), and LM at L4–L5 were obtained by means of ultrasound imaging with previously described protocols.37 In addition to the resting thickness measures, the CSA of the RA muscle was obtained. Aggregate muscle thickness values were calculated for the lateral abdominal muscles (TrA, IO, and EO) and the total abdominal muscles (TrA, IO, EO, and RA). Measurements of contracted muscle thickness of the TrA, IO, and EO muscles were obtained by means of an active straight-leg-raise task.28,46–48 Measurements of contracted muscle thickness of the LM were obtained during a prone arm-lift task.30,48,49
Images were acquired in B-mode with the use of a portable ultrasound unit (Sonosite Titan, Sonosite Inc, Bothell, Washington) with a 5-MHz, 60-mm curvilinear array. To help avoid an order effect, images were obtained in a counterbalanced order. Three images of the resting and contracted states were obtained bilaterally. The measurements of muscle morphology were obtained from the right and left sides. The morphology measurements at rest and during the contracted state are provided for the right side only. Muscle asymmetry values are reported to assess the side-to-side differences obtained both at rest and during the contracted states. Thirty-six images were analyzed for each participant at baseline and at follow-up; 20,160 images were analyzed for this study.
Examiners
All assessors were blinded to group membership. In addition, assessors worked in pairs to minimize error and other sources of bias. One assessor was responsible for generating the ultrasound image. The second assessor would ensure image quality, perform measurements of the LM muscle thickness and RA CSA, and add administrative information on the image. Measurements of the lateral abdominal muscles were performed off-line by an assessor blinded to group membership. Both assessors had to agree on image quality before storing the imaging. Throughout the process, the assessor obtaining the images was blinded to all measurements. The pairs of raters used in this study were assigned to participants in a counterbalanced manner. Sample images obtained from the techniques used in this study are published elsewhere.37
Good to excellent reliability of the assessors was established in a sample of 21 participants with LBP (mean [±SD] age=21.5±4.4 years; 5 women and 16 men). Ultrasound measures of the TrA, IO, RA, and LM were obtained as outlined in this report. The intraclass correlation coefficients (ICC [1,3]) for the point estimates ranged from .86 to .94; the standard error of the measurement ranged from 0.04 to 0.16 cm for the thickness values and 0.67 cm2 for the cross-sectional area of the RA muscle. These data are reported in more detail in a previous publication.37
Outcome Measures and Follow-up
Study-related measurements were collected before and after training 12 weeks later by study personnel who were unaware of the randomization assignments. It was not possible to blind soldiers to their group assignment because they actively participated in their randomly assigned training program. However, no information was provided to the soldiers about which program was hypothesized to be more beneficial.
Data Analysis
Descriptive statistics, including measures of central tendency and dispersion for continuous variables, were calculated to summarize the data. Demographic and baseline levels of variables were compared between those who completed the 12-week training program and those who were withdrawn because of physical or academic requirements, through the use of a t test or chi-square test. The study was originally planned to randomly sample 100 soldiers from each of the exercise regimens, resulting in 200 participants, to ensure 90% power to detect a difference at the .07 level. The sample size was increased to 340 participants on the basis of historic dropout rates to ensure that we had 200 participants at baseline and follow-up.
The dependent measures of the TrA, IO, EO, and LM muscles consisted of muscle thickness values at rest and values while muscles were contracted. Percent change in muscle thickness was calculated by ([MUSCLEactive−MUSCLErest]/MUSCLErest) × 100%. Symmetry was calculated by ([MUSCLElarge−MUSCLEsmall]/MUSCLElarge) × 100%. The dependent measures for the RA muscle consisted of thickness and CSA values at rest. Lateral abdominal thickness was the sum of the TrA, IO, and EO muscles. Total abdominal muscle thickness was the sum of the lateral abdominal and RA muscle thickness values. The dependent measures for the endurance tests were measured in seconds. The sum of all 4 endurance times for all 4 tests was calculated.
For each outcome measure, we fitted a linear mixed model that included the main effects of group (TEP versus CSEP) and time (baseline versus 12 weeks), their interactions, and a random clustering effect. Such models allowed us to use the baseline data of participants with loss to follow-up. In addition, a paired t test was used to assess within-group changes (time effect comparing baseline and 12 weeks). The α level was set to .05 a priori. All statistical analyses were performed with the use of SAS software, version 9.1 (SAS Institute Inc, Cary, North Carolina).
Role of Funding Source
This study was funded by the Congressionally Directed Peer-Reviewed Medical Research Program (W81XWH-061-0564). The funding agency did not have a role in the design, conduct, or reporting of the study or in the decision to submit the article for publication.
Results
Three hundred forty participants (mean [±SD] age=21.8±3.9 years; 96 women and 244 men) were enrolled in this aspect of the POLM trial. The sample consisted of 229 (67.3%) whites, 48 (14.1%) black or African Americans, 34 (10.0%) Hispanics, and 29 (8.5%) other (eg, American Indian, Asian, Pacific Islanders). Of the 340 participants, 280 were available for follow-up analysis 12 weeks later. Sixty participants were removed from the military training program because they did not meet the academic or physical requirements. There were no differences in baseline data from those who completed the trial and those who were lost to follow-up, except those who were lost to follow-up were 1.4 years younger on average (P=.02). Demographic and baseline characteristics of the participants are provided in Table 1. In addition, there were no differences between the group sampled for this analysis and the sample of the larger POLM trial35 in regard to age, sex, height, weight, education level, time in the army, and baseline fitness values. The only statistical difference was that the current sample included 69% whites compared with 74% in the larger POLM trial (P=.034).
Demographics and Baseline Characteristicsa
Resting Muscle Thickness and CSA
In general, there were no significant group × time interactions for morphometry of the abdominal and LM muscles at rest (Tab. 2). There was a trend for decreased thickness of the EO, IO, and aggregate abdominal muscle thickness in both groups after the 12-week training program. However, only the decreased thickness in the EO muscle was statistically significant (P≤.05), but the absolute difference was <0.5 mm, which was within measurement error.
Resting Muscle Thickness and Area Valuesa
There were significant group × time interactions for symmetry of the EO muscle at rest (Tab. 3). Specifically, individuals in the TEP program demonstrated greater improvements in EO symmetry (6.9% versus 2.4%, P=.02) after training. There was not a significant group × time interaction for symmetry of the TrA muscle, the IO muscle, the lateral abdominal muscles, and aggregate abdominal muscle thickness; however, there was a main effect for time regardless of group membership (P≤.001). Before training, the asymmetry of the lateral abdominal muscles and aggregate abdominal muscle thickness values ranged from 6.3% to 16.8%; after training, the asymmetry of the lateral abdominal muscles and aggregate abdominal muscle thickness values ranged from 4.2% to 11.3%. There was no effect for time on symmetry of the RA and LM muscles (P≥.2).
Symmetry of Muscle Morphometry at Resta
Contracted Muscle Thickness Values
There were no significant group × time interactions for the contracted thickness of any of the muscles (Tab. 4). There was a significant main effect for time (P≤.001), demonstrating a decrease in contracted muscle thickness of the TrA, IO, and total lateral abdominal muscles after the 12-week training program. The mean differences ranged from 0.5 to 1.4 mm in thickness. There was not a main effect for time for the EO or LM muscles (P≥.17).
Thickness of Muscles During Contractiona
There were significant group × time interactions for symmetry of the aggregate lateral abdominal muscles during the contracted state (Tab. 5). Specifically, although both groups had greater symmetry of the aggregate lateral abdominal muscles during the contracted state at the end of the 12-week training program (P≤.001), individuals in the TEP program demonstrated greater improvements in symmetry (7.7% versus 3.7%, P=.01). There was not a significant group × time interaction for symmetry of the TrA, IO, and EO muscles during the contracted state; however, there was a main effect for time (P≤.001), demonstrating improvements of 10.6%, 7.3%, and 8.7% in the TEP group versus 5.7%, 2.9%, and 6.7% in CSEP group at the end of the 12-week training program, respectively. There was no effect for time on symmetry of the lumbar multifidus muscles (P≥.2).
Symmetry of Contracted Muscle Thicknessa
Endurance Measurements
There were no significant group × time interactions for the 4 endurance measurements (Tab. 6). However, there was a significant decrease in endurance hold times for the trunk extensor (mean difference of 13.5 seconds) and the total endurance time (mean difference of 25.3 seconds), and the decreases in times were significant for both groups (P<.002). In addition, baseline score of horizontal side support was found to be a significant predictor of LBP, with odds ratios (OR) of 1.008 (P=.04), but the area under curve (AUC) of the receiver operating characteristic curve had a very low value of 0.56. The OR (AUC) corresponding to baseline scores of trunk extensor endurance, trunk flexor endurance, and the total endurance time were 1.001 (0.52), 1.006 (0.56), and 1.002 (0.56), respectively. The maximal hold time used at baseline and follow-up was 4 minutes. This time limit did not create a significant ceiling effect because only 8 of the 1,360 (0.6%) baseline endurance tests and only 8 of the 1,120 (0.7%) follow-up endurance tests had to be terminated on the basis of a participant reaching the maximum time of 4 minutes.
Endurance Timesa
Discussion
The primary results from the POLM cluster-randomized trial demonstrated that a brief psychosocial education program in combination with either of the exercise programs resulted in lower 2-year incidence of health care–seeking for LBP.35 The results of this study are consistent with the primary results of the POLM trial in that there was not an advantage for either exercise group. The similar rates of LBP from both exercise regimens may be related to a lack of hypertrophy, no change in the ability to contract the trunk muscles, or the overall decrease in endurance times over the 12-week period. Although the constrained training environment limited the amount of time dedicated to these types of exercises, the inability to find differences over time may indicate that the doses of both exercise regimens were suboptimal. Specifically, 5 minutes of trunk-related exercises 4 days per week with no increase in intensity was not sufficient to demonstrate changes over time. Although exercise may be effective in preventing LBP, recent guidelines have indicated no benefit of one specific exercise approach for prevention of LBP.50 Future research should determine whether specific exercise regimens with greater frequency, intensity, duration, and specificity result in muscle hypertrophy, improved endurance, or are more effective in preventing LBP.
Improved symmetry was noted, but, as expected, these results differed from those of clinical trials that assessed the influence of exercises on muscle morphometry in patients with LBP. Danneels et al51 found hypertrophy of the LM when motor control exercises are performed similar to the CSEP used in this study. Additionally, Hides and colleagues34,52,53 found LM hypertrophy, improved symmetry, and enhanced ability to thicken the TrA muscle during the abdominal drawing-in maneuver in those who performed exercises similar to the CSEP. The main difference between these studies and the current study is that the participants currently did not have LBP or a history of LBP. Therefore, the benefits from these exercise programs and changes in muscle morphometry related to these exercises may be limited to secondary and tertiary prevention programs (ie, for patients who already have LBP). Another difference is that the current study used group exercise instruction in which all participants completed the same group of exercises. This approach differs from the tailored individualized approaches that have demonstrated efficacy for treatment of patients with chronic LBP1 but have not been tested in primary prevention settings. Although practical limitations would limit the ability of individualized LBP prevention programs for all individuals in a military setting or other settings with large numbers of participants, future research should try to identify those at highest risk for LBP and assess the impact of individualized prevention programs that can enhance specificity of training for modifiable risk factors.54
Regardless of group, endurance times decreased over the 12-week training program. This finding was unexpected and differed from the overall improvements in physical fitness test scores in this sample.38 However, the results are similar to the decrease in fitness scores in those who were in the top quartile of fitness at baseline.38 The decrease in performance on the endurance tests could be related to changes in biopsychosocial factors,55,56 which are known to vary during military training, or could be a regression to the mean. For example, in the POLM cohort, there was a general increase in anxiety and depressive symptoms during the 12-week training period,55 which could account for the decreased endurance times. The decrease in performance on the endurance tests also could be related to overtraining that may occur in an intense and prolonged military training environment.55,57 The difference in performance on the physical fitness test38 and the endurance measures are consistent with previous findings that suggest that intense and prolonged military training has different effects on strength and endurance.58
A pragmatic approach was used to determine the dosage of the exercise programs in this study. Current time constraints have historically limited trunk exercises to approximately 5 minutes per day during unit physical training. For example, the left and right horizontal side support used in the CSEP group was held for only 1 minute on each side. However, the endurance test at baseline and follow-up for both groups asked the participants to hold those postures for up to 4 minutes. Increased time to focus on longer hold times during the training period may have resulted in greater hold times at follow-up on the basis of specificity of training. Although our dosing parameters were consistent with expert recommendations,59 greater or increasing intensity and time may be necessary for changes in morphometry in adults without a history of LBP. This is consistent with other researchers who have suggested that more intensive lumbar resistance training may be necessary to demonstrate changes in morphometry.60,61 The differences in dosage may help to explain the contradictory evidence in the literature regarding the benefits of exercise for LBP prevention.50,62
The primary limitation of the current study is that these results may have limited direct application to civilian populations because of the trial implementation in a military setting. An alternate explanation of the lack of changes in muscle morphometry may be that the participants lacked a history of LBP and the function of the deep trunk muscles that are associated more with slow oxidative versus fast glycolytic muscles. Therefore, there may be a ceiling effect in which additional hypertrophy or enhanced ability to thicken these muscles during the low-level tasks performed are not likely to benefit from either exercise regimen. Although the TEP used in this study was similar to standard army training, we did not have a pure control group; therefore, we cannot comment on the absolute effects of either exercise regimen. Finally, future researchers should assess changes in muscle recruitment patterns associated with the exercise regimens.63
Conclusion
Although improved symmetry was noted, a 5-minute exercise regimen conducted 4 days per week for 12 weeks was not sufficient to generate hypertrophy of the trunk musculature. Few changes were noted for exercises that were conducted slowly with the use of a low-load, load-repetition paradigm and exercises conducted at a fast pace with the use of a high-load, high-repetition paradigm. It is unlikely that the results were adversely influenced by suboptimal adherence, given the regimented and consistent manner in which military training was conducted and monitored through the study. The lack of hypertrophy was associated with decreased trunk endurance. The decreased endurance time at the end of the 12-week training program could be indicative of overall fatigue associated with the military program in which the participants were enrolled. Despite the overall decrease in endurance in both groups, longer endurance hold times were noted at the 12-week assessment in individuals who completed the CSEP. Endurance times were not predictive of future LBP in this cohort.
Footnotes
Dr Teyhen, Dr Childs, Dr Mello, Dr Taylor, Dr Wu, and Dr George provided concept/idea/research design. Dr Teyhen, Dr Childs, Dr Wright, Dr Sorge, Dr Mello, Dr Marmolejo, Dr Taylor, Dr Wu, and Dr George provided writing. Dr Teyhen, Ms Dugan, Dr Wright, Dr Sorge, Dr Mello, Dr Marmolejo, and Dr Taylor provided data collection. Dr Teyhen, Dr Childs, Dr Wright, Dr Sorge, Dr Mello, Dr Marmolejo, Dr Taylor, and Dr Wu provided data analysis. Dr Teyhen, Dr Childs, and Ms Dugan provided project management. Dr Teyhen, Dr Childs, and Dr George provided fund procurement. Dr Teyhen provided study participants and institutional liaisons. Dr Teyhen and Dr Childs provided facilities/equipment. Ms Dugan provided clerical support. Dr Childs, Ms Dugan, Dr Mello, Dr Marmolejo, and Dr George provided consultation (including review of manuscript before submission).
The authors acknowledge the work of CPT Nicole Hall, Capt Sonrie Gervacio, CPT Joseph Lopez, CPT Jason Mitchler, Lt Joshua Shumway, Lt Brittany McCright, Dr John May, Dr Elizabeth Sampey, and Dr Alexandria Gentles for their contributions to image acquisition and measurement. They also acknowledge the physical therapist students at the University of Texas Health Science Center for their support of this research study.
The institutional review boards at the Brooke Army Medical Center (San Antonio, Texas) and the University of Florida (Gainesville, Florida) granted approval for this project.
Platform presentations of this research were given at the following meetings: 10th Annual Force Health Protection Conference, August 2007, Louisville, Kentucky; Combined Sections Meeting of the American Physical Therapy Association (APTA), February 6–9, 2009, Las Vegas, Nevada; Combined Sections Meeting of APTA, February 17–20, 2010, San Diego, California; Combined Sections Meeting of APTA, February 9–12, 2011, New Orleans, Louisiana; Combined Sections Meeting of APTA, February 8–11, 2012, Chicago, Illinois; Air Force Medical Service Research Symposium, 2010; and Air Force Medical Service Research Symposium, August 2011, Washington, DC.
The views expressed in this article are those of the authors and do not reflect the official policy or position of Brooke Army Medical Center, the US Army Public Health Command, the US Army Medical Research and Material Command, the US Army Medical Department, the US Army Office of the Surgeon General, the Department of the Army, the Department of the Air Force, the Department of Defense, or the US Government.
The Prevention of Low Back Pain in the Military trial is supported by the peer-reviewed medical research program of the Department of Defense (PR054098), Congressionally Directed Peer-Reviewed Medical Research Program (W81XWH-06-1-0564), Fort Detrick, Maryland.
- Received March 31, 2012.
- Accepted October 9, 2012.
- © 2013 American Physical Therapy Association
References
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