Article Summary: Effect of Aerobic Exercise Training With and Without Blood Flow Restriction on Aerobic Capacity in Healthy Young Adults: A Systematic Review With Meta-Analysis

Effect of Aerobic Exercise Training With and Without Blood Flow Restriction on Aerobic Capacity in Healthy Young Adults: A Systematic Review With Meta-Analysis.

In this systematic review and meta-analysis, the authors' goal was to: "examine the effects of aerobic exercise training (ET) with and without blood flow restriction (BFR) on aerobic capacity (AC) and to compare the effect of low-to-moderate aerobic ET with and without BFR to high-intensity aerobic ET with and without BFR on AC."

Inclusion Criteria:

• Study was conducted in healthy individuals
• There was random allocation of study participants to training and control groups
• BFR was the sole intervention difference between the groups.

Total number of studies: 7

• 5 low-to-moderate intensity
• 2 high-intensity
• All 7 studies had a PEDro score of 6 which is pretty good for these types of studies since it is not really reasonable to expect: concealed allocation, blind Subjects, blind therapists, or blind assessors considering some data was recorded during or shortly after exercise.

Total number of subjects: 121

Analyses performed:

• Standardized mean difference (SMD) in AC between BFR and non-BFR groups of all 7 studies.
• Separate analyses of the 5 low-to-moderate studies.
• Separate analyses of the 2 high-intensity studies

What they found:  

• Aerobic ET with BFR elicits a significantly greater AC than aerobic ET without BFR
• Low-to-moderate intensity aerobic ET with BFR elicited a greater improvement in AC than aerobic ET without BFR
• High-intensity aerobic ET with BFR did not elicit an improvement in AC over high-intensity aerobic ET without BFR.

Some of the proposed mechanisms of adaptation due to BFR were listed by the authors and are provided below.

Potential Mechanisms During BFR that Contribute to Aerobic Capacity Improvements

• Decreased Arterial Blood Flow and Decreased Venous Return
• Increased Heart Rate (to accommodate for the decreased blood flow)
• Increased hypoxia and metabolites
• Increased Minute Ventilation: Tidal volume x breaths per minute
• Increased Vascular Endothelial Growth Factor (VEGF) - encourages the growth of new blood vessels

Potential mechanisms contributing to re-perfusion after the cessation of BFR

• Increased Arterial Blood Flow and Increased Venous Return
• Increased stroke volume and increased shear stress on blood vessels
• Increased nitric oxide production (vasodilation to accommodate for increased volume)
• Increased End Diastolic Volume (EDV)
• Increased A-VO2 difference

Aerobic Capacity was measured using a ramping protocol to exhaustion.  The authors used Aerobic Capacity (AC) rather than calling it a true VO2 max because only 3 of the studies used strict VO2 max criteria. Maximal AC was identified as "the highest oxygen consumption during the final 15-30 seconds of exercise testing."

All studies used the same modality for maximal exercise testing as their intervention modality with the exception of the study by Park et. al that utilized walking as their intervention but biking as their maximal exercise testing method.

Super quickly breaking down the statistics used here:

• Standardized Mean Difference (SMD):
• Low-to-moderate intensity: .57 (.12, 1.01) - moderate effect size
• High-intensity: -.01 (-.67, .64) - essentially no effect
• Total: .38 (.01, .75) - moderate effect size
• Chi square: takes the sum of: (Observed - Expected)² / Expected to provide a "chi square statistic" which can then be entered along with the degrees of freedom (n - 1) in order to get your "p-value"
• In both the low-to-moderate intensity group and the high-intensity group, the p-value for their chi square test was insignificant suggesting that there was not significant heterogeneity between the studies in each group.
• Z in this case is not the "z-score" which tells us how far individual values differ from the mean, but rather the overall effect that the intervention has.  
• In the case of low-to-moderate intensity training,  the p-value is .01 suggesting that the intervention was effective. 
• In high-intensity training, the authors found an overall effect with a p-value of .97 suggesting that the intervention was not effective; but with only 2 relatively small studies, this probably warrants further investigation before any significant conclusions are drawn.
• I² is representative of the percent of variation across studies due to heterogeneity, not chance.
• 0% - 40%: minimal heterogeneity
• 30% - 60%: moderate heterogeneity
• 50% - 90%: substantial heterogeneity
• 90% - 100%: considerable heterogeneity
• Both subgroups showed low heterogeneity across studies.

For more information on how to interpret a forest plot, view this video

For more information on heterogeneity, view this video

What conclusions can we draw from these findings?:

• SMD: Based on the studies provided, there is a clear advantage of low-to-moderate intensity exercise with BFR over all other interventions.
• Overall Effect: The overall effect is greatest in the low-to-moderate intensity BFR group.
• There is appropriate homogeneity in the pooled analyses of the low-to-moderate and high-intensity exercise groups.
• I am not an expert, but I chose to focus a lot more of my attention on the low-to-moderate intensity results based on the larger number of studies and subjects as well as consistency among the activity level of the participants.

Additional Findings from the Individual Studies:

The authors discussed the impact of the Fick equation on the observed results, MCID in aerobic capacity and low-to-moderate intensity vs. high intensity exercise with and without BFR.

• Fick Equation: VO2 = HR x SV x (a-VO2 difference)
• MCID for AC = 3.3 ml/kg/min
• Increase in AC of 3.4% is considered to be a MCID in healthy adults.

Abe :

• Heart rate during aerobic ET with BFR was significantly greater than aerobic ET without BFR
• Achieved MCID for AC

Park:

• Heart rate was significantly lower at the mid-point and maximal point of BFR ET after performing BFR 2x/day, 6 days/week, for two weeks.
• Increase in stroke volume (approximately 22%) during the last aerobic ET with BFR compared to the first session with BFR.
• Minute ventilation (VE) increased significantly in the BFR group (10%), but was unchanged in the non-BFR group.
• Achieved MCID for AC

Keramidas et. al: High-intensity; subjects were untrained in this study

• Similar heart rates during ET with and without BFR 
• Both the BFR and non-BFR groups increasing VE: 15.6% versus 13.7%, respectively. 
• Total hemoglobin and oxyhemoglobin increased in both the BFR and non-BFR groups during a sub-maximal exercise test
• ET oxygen consumption was significantly lower in both groups during the submaximal exercise test at the same relative workload reflecting greater muscular efficiency.

De Olivera et. al:

• Heart rate during aerobic ET with BFR was significantly greater than aerobic ET without BFR
• Onset of blood lactate accumulation (OBLA): BFR group improved 16% compared to the 6% improvement in the non-BFR group

Paton et. al: High-intensity

• Heart rate during aerobic ET with BFR was significantly greater than aerobic ET without BFR
• Both the BFR and non-BFR groups increasing VE
•  6.8% increase compared to the 0.8% increase in VE in the non-BFR group (not statistically significant)
• Improvement in running economy only in the BFR group despite AC improving similarly in both the BFR and non-BFR groups.

Esparaza et. al:

• BFR group experienced a 5% increase in stroke volume while the stroke volume of the non-BFR group was unchanged (this small increase was found to be insignificant)

Amani et. al:

General conclusions by the authors:

• Isometric knee extension strength tended to be greater post-ET with BFR when compared to ET alone, even though no significant standardized mean difference between the groups was observed in our supplementary analysis.
• Improvements in hypertrophy and power also seemed to be greater in the aerobic ET with BFR groups when compared to the non-BFR groups.
• Increased skeletal muscle strength, hypertrophy, and power may be partly responsible for the improvement in AC observed in this meta-analysis, but further investigation is warranted.
• All studies except for the work by Keramidas found an increase in AC that exceeded the MCID of 3.4% in the BFR group.
• Only Esparaza and Paton found increase > 3.4% in both BFR and non-BFR.
• Low-to-moderate intensity aerobic ET with BFR physiologically elicits more AC compared to high-intensity ET with BFR which appears to elicit more anaerobic capacity

Overall Conclusion:

"The results of this systematic review and meta-analysis reveal that aerobic ET with BFR elicits a greater improvement in AC than aerobic ET without BFR. Although high-intensity aerobic ET with BFR did not appear to elicit an improvement in AC over high intensity aerobic ET without BFR only two studies were available to be included in this analysis for which reason these results should be interpreted cautiously. Further investigation of the effects of low-to-moderate and high-intensity aerobic ET with and with BFR on AC as well as the components of the Fick equation and VE are needed."

Hopefully this summary helps you better understand this article as well as gave you a quick refresher on some of the important statistics needed to understand systematic reviews and meta-analyses.

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All information in this blog was derived from or reprinted from:

• Formiga, M. F., Fay, R., Hutchinson, S., Locandro, N., Ceballos, A., Lesh, A., ... & Cahalin, L. P. (2020). EFFECT OF AEROBIC EXERCISE TRAINING WITH AND WITHOUT BLOOD FLOW RESTRICTION ON AEROBIC CAPACITY IN HEALTHY YOUNG ADULTS: A SYSTEMATIC REVIEW WITH META-ANALYSIS. International Journal of Sports Physical Therapy, 15(2).