Lower Physical Activity, Article Critique Example

Introduction and Purpose

This study in Physical Therapy builds on previous research in the field focusing on the relationship between physical activity and individuals with diverse medical conditions such as diabetes and peripheral neuropathy.  Recent studies found that individuals with diabetes and similar conditions have an increased amount of adipose tissue in muscles particularly located in the lower extremity.  While these studies do not point to a particular causal mechanism why this condition forms, other studies have explored how this condition may amplify existing problems with the main medical condition.  For example, Hilton, Tuttle, Bohnert, et al. found that the existence of increased intermuscular adipose tissue (IMAT) is also associated with poor functional status: Poor functional status meaning the ability to get around and conduct every day activities.  At the same time, research also indicates that IMAT may have an association with causal mechanisms involved with the onset and progression of neural and diabetes (Goodpaster, Thaete, Simoneau,Kelley, 1997), potentially making the condition worse in some individuals.  This study is building upon earlier research findings, and also attempts to lend insight regarding the potential association of how physical activity and IMAT levels are related vis-à-vis individuals with diabetes and peripheral neuropathy.  While the methodology of the article prevents specific causal claims regarding the impact of physical activity on IMAT levels, it does open the door for more studies to be done on the relationship.

To be more specific, the authors’ stated research purpose of the article is: To assess how the physical activity level in certain individuals with diabetes and peripheral neuropathy differs in the variables of step count, intermuscular adipose tissue volume, muscle performance, and physical function (Tuttle, Sinacore, Cade, Mueller, 2011).

Study Population and StudyVariables

In order to conduct the study, the principal investigatorsidentified 27 individuals with type 2 diabetes and peripheral neuropathy.  The individuals were recruited as part of a larger study that assessed the impact of exercise for people with diabetes and peripheral neuropathy in St. Louis, Missouri.  Participants were excluded if they had any one of the following conditions: weighed more than 300 lb; had a history of severe foot deformity, or had any comorbidity of medications that would interfere with exercise (Tuttle, Sinacore, Cade, Mueller, 2011).

The authors run a number of different models in their analysis, but the main regression model uses step count as the dependent variable, and IMATvolume, age, BMI, HbA1c, durationof diabetes, muscle performance(dorsiflexion and plantar-flexionpeak torque and power), and physicalperformance (6MWT distanceand PPT score) as the independentvariables (Tuttle, Sinacore, Cade, Mueller, 2011).

The authors operationalize the dependent variable (step count) in the following manner: activity level, was measured using a watch activity monitor to measure the number of steps taken daily.  The participants were monitored for nine consecutive days, with average steps taken calculated for the first seven days. IMAT volume, another dependent variable, was operationalized in the following manner: IMAT were quantified using magnetic resonance imaging (MRI) equipment of the right leg of each participant (Tuttle, Sinacore,Cade, Mueller, 2010). The MRI measurements were performed with a 3.0 T superconducting MRI instrument; with a pulse sequence of TE_12 milliseconds, TR_1,500 milliseconds, matrix_256 _ 256(Tuttle, Sinacore, Cade, Mueller, 2011).  Volumes were then quantified using a computer to analyze the level of IMAT between muscle and adipose tissue- the measurements were performed seven days apart (Tuttle, Sinacore,Cade, Mueller, 2011).  Duration of diabetes was operationalized from the individual’s total number of months since diagnosis.  In order to assess physical function and walking endurance, the investigators used the “Six-minute Walk Test”- individuals walked back and forth in a hallway between 2 cones 100 feet apart, walking as far as they could in six minutes (Tuttle, Sinacore,Cade, Mueller, 2011).  In order to assess physical function in all participants, the investigators used the 9-item Physical Performance Test –the test’s results have been validated as a proxy for disability and frailty (Tuttle, Sinacore,Cade, Mueller, 2011). Muscle performance was measured via the Biodex Multijoint system 3 pro isokinetic dynamometer over a total of three different trials.  The independent variable of age was taken as part of a self-report survey.  BMI was operationalized as a function of existing clinical data for each patient.  HbA1c, also known as glycosylated hemoglobin, was measured at the clinic at baseline.  Finally, peripheral neuropathy was assessed using the Semmes-Weinstein monofilament on at least 1 point on the plantar surface of the foot and on a vibration perception threshold of greater than 25 V as measured with a biothesiometer(Tuttle, Sinacore, Cade, Mueller, 2011).

Statistical and Research Results

In order to assess how the independent variable physical activity (step count) was related to the disease status and condition of the study participants, the researchers ran the multiple regression with physical activity (step count) as a continuous, independent variable.  In the original model, although many variables had a relatively high correlation coefficient (r), only IMAT levels had a high correlation level and was statistically significant (r=44; p-value<.05)- IMAT also accounted for 19% of variance in the physical activity.  The authors then transformed activity steps per day into a categorical variable composed of three parts:  1) >5,000 steps a day; 2) 5,000-8,000 steps a day; 3> 8,000 day) (Tuttle, Sinacore, Cade, Mueller, 2011).  After converting the categorical variable, the researchers ran a one-way ANOVA analysis (stratified by age) with the null hypothesis that there was no difference (association) between the categories of physical activity vis-à-vis IMAP levels.  The group with the lowest amount of physical activity (> 5,000 steps) had 3 times the volume of IMAT in their calf muscles compared to other two groups- the differences between the three groups were statistically significant (p-value< .05) (Tuttle, Sinacore, Cade, Mueller, 2011).

The study had a number of important findings.  First, the study was the first to demonstrate an inverse relationship between physical activity levels and IMAT volume (lower legs) with diabetes and neuropathy (Tuttle, Sinacore,Cade, Mueller, 2011).  There was also a found correlation between the increase in IMAT volume, lower performance of muscles, and physical function (Tuttle, Sinacore,Cade, Mueller, 2011).

Main limitations to the study

The article pointed out three main limitations in the study.  First, the participants of the study were all interested in participating in the study.  This phenomenon is typically referred to as “selection bias”, in that individuals who participate in studies typically are different than individuals who don’t volunteer to participate in studies.  In the case of this study, it is likely healthier individuals volunteered in the study.  This means that the study’s results might be positively biased: that is, because the individuals were healthier, it is possible the relationship between physical activity and IMAP levels was more robust than found in the study.  This also means that the results of the study are not likely generalizable to the population writ large. Second, the study’s investigators excluded a number of individuals with pre-existing conditions: people who had a foot ulcer, peripheral arterial disease, or severe foot deformities.  This further limits the study’s generalizability to those populations.  Third, and perhaps most importantly, it should be noted that correlation does not equal causation.  This is particularly important to recognize in this study due to the findings and the number of variables at play.  Although the study found an association between physical activity and IMAT levels in the lower leg area, there is not a specific causal mechanism entailed to explain how this relationship may be a causal one.  Indeed, there may be several variables contributing to this relationship, but they were not included in the model and thus not seen to be significant.

This study has numerous implications for the practice of physical therapy.   First, the finding of a relationship between physical activity and IMAT levels is notable(Tuttle, Sinacore, Cade, Mueller, 2011).  Putting aside whether this finding reveals a causal mechanism or not, the study’s result means that individuals with diabetes and peripheral neuropathy may benefit from physical therapy in order to facilitate greater physical activity on a daily basis. The other implication is: individuals with these medical conditions might want to seek additional physical activity and therapy to deal with existing side effects. These results although preliminary, mean that physical therapists could play a greater role in the treatment of these diseases from a mobility and muscular perspective.

Overall, this study added to a burgeoning literature examining the relationship between diseases such as diabetes, IMAT levels, and the potential use of exercise as a therapeutic tool to control the disease and its effects.  One potential criticism of the study would be its extremely small sample size: the study’s analysis included only 22 individuals that is likely not adequately powered to detect a true main effect in this case.  Larger studies should be done to replicate these findings with a larger sample size.  Finally, the study’s methodology could be expanded to other diseases were mobility is an issue- particularly looking at how exercise therapy might help individuals in the pre- or initial stages of diabetes.

References

Tuttle, L.J., Sinacore, D.R., Cade, T., Mueller, M.J. (2010) Lower physical activity is associated with higher intermuscular adipose tissue in people with type 2 diabetes and peripheral neuropathy.  Physical Therapy, (91) 6, 923-930.

Hilton T.N., Tuttle L.J., Bohnert K.L., et al.Excessive adipose tissue infiltration in skeletal muscle in individuals with obesity,diabetes mellitus, and peripheral neuropathy: association with performanceand function. Physical Therapy. 2008; 88:1336 –1344.

Goodpaster BH, Thaete FL, Simoneau JA,Kelley DE. Subcutaneous abdominal fat and thigh muscle composition predictinsulin sensitivity independently of visceral fat. Diabetes. 1997;46:1579–1585.