Physiological responses of human skeletal muscle to acute blood flow restricted exercise assessed by multimodal MRI

Efficacy of Blood Flow Restriction Exercise for Improving Lower Limb Muscle Strength and Function in Chronic Spinal Cord Injury: A Randomized Controlled Trial

The objective of the present study was to evaluate the efficacy of low-load (LL) blood flow restriction exercise (BFRE) for improving lower limb muscle strength, muscle thickness and physical function in individuals with spinal cord injury (SCI). In a randomized sham-controlled trial, 21 participants (age ≥ 18 years, SCI duration ≥ 1 year, knee extensor strength grade 2-4, ASIA A-D) were randomized to either 45-min LL-BFRE (n = 11) or sham BFRE (n = 10) twice/week for 8 weeks. The exercise protocol consisted of four sets (30 × 15 × 15 × 15 repetitions) of unilateral seated leg extensions and leg curls at 30%-40% of 1RM performed with pneumatic cuffs applied proximally on the trained limb and inflated to 40% of total arterial occlusion pressure (BFRE) or non-inflated (sham exercise). Maximal voluntary isometric quadriceps and hamstring muscle strength, quadriceps muscle thickness, thigh circumference, and physical function were assessed at baseline, after 4 and 8 weeks of training and at 4-week follow-up. No significant between-group differences were found between BFRE and sham exercise in quadriceps or hamstring muscle strength, 10-m walking test, timed up & go, 6-min walking test or the spinal cord independence measure. In contrast, a significant between-group difference favoring BFRE was present for muscle thickness and thigh circumference from baseline to 4-week follow-up (0.76 cm (95% CI: 0.32; 1.20, p = 0.002) and 2.42 cm (0.05; 4.79, p = 0.05), respectively). In conclusion, there was no significant difference in the effect of LL-BFRE and sham exercise on muscle strength and physical function in individuals with SCI. However, significant increases in muscle thickness and thigh circumference were observed in favor of BFRE. Trial Registration: ClinicalTrials.gov identifier: NCT03690700.

In vivo human medial gastrocnemius fascicle behaviour and belly gear during submaximal eccentric contractions are not affected by concentric fatiguing exercise

Changes in muscle geometry and belly gearing during eccentric contractions influence fibre strain and susceptibility to muscle damage. They are modulated by the interaction between connective tissues and intracellular-intrafascicular fluid pressures and external pressures from neighbouring structures. Fatiguing exercise triggers fluid shifts (muscle swelling) and muscle activation changes that may influence these modulators. Our purpose was to measure medial gastrocnemius (MG) geometric changes in vivo during eccentric contractions before and after maximal concentric muscle work to test the hypothesis that fatigue would reduce fascicle rotation and muscle gear and provoke greater fascicle strain. Submaximal eccentric plantar flexor contractions at 40% and 60% of maximal eccentric torque were performed on an isokinetic dynamometer at 5°.s-1 before and immediately after the fatiguing exercise. MG fascicles and muscle-tendon junction were captured using ultrasonography during contractions, allowing quantification of geometric changes, whole-MG length, and belly gear (Δmuscle length/Δfascicle length). Triceps surae (TS) activation was estimated using surface electromyography and the distribution of activations between synergistic muscles was then determined. After exercise, concentric torque decreased ∼39% and resting muscle thickness increased by 4%, indicating muscle fatigue and swelling, respectively. While soleus (Sol) activation and the Sol/TS ratio increased, no changes in MG, MG/TS ratio or fascicle rotation during the contraction were detected. Thus, fascicle lengthening and belly gear remained unaltered. Changes in muscle thickness during contraction was also similar before and after exercise, suggesting that changes in muscle shape were relatively unaffected by the exercise. Consequently, the muscle maintained mechanical integrity after the fatiguing muscle work.

Low-Load Blood Flow Restriction Squat as Conditioning Activity Within a Contrast Training Sequence in High-Level Preadolescent Trampoline Gymnasts

To investigate the effects of implementing low-load blood flow restriction exercises (LL-BFRE) instead of high-load exercises (HL-RE) in a contrast training program on strength and power performance of high-level young gymnasts. Fifteen high-level pre-pubescent trampoline gymnasts (national level, Tanner Stage II, intermediate experience in strength training) were divided into two groups to complete the same structure of a ten-week contrast strength training program differing only in the configuration of the first resistance exercise of the contrast sequence. The LL-BFRE group (n = 7, four girls, 13.9 ± 0.4 y) performed the first resistance exercise of the contrast with LL-BFRE (20%–30% 1RM, perceived pressure of 7 on a scale from 0 to 10). The HL-RE group (n = 8, four girls, 13.8 ± 0.5 y) trained the first resistance exercise of the contrast sequence with moderate-to-high load (60%–85% 1RM). Before and after the training period, isometric mid-thigh pull (IMTP), squat jump (SJ), counter movement jump (CMJ), and drop-jump (DJ) were performed to evaluate the effect of the intervention on strength and power capacities as primary outcomes. Changes in participants’ anthropometric measures, muscle mass, left and right thigh girth, IMTP relative to bodyweight (IMTP-R), eccentric utilization ratio (EUR), and reactive strength index (RSI) were assessed as secondary outcomes. There was no significant interaction (p > 0.05) between group x time in any power and strength outcome, although SJ and EUR showed a trend to significant interaction (p = 0.06 and p = 0.065, respectively). There was an overall effect of time (p < 0.05) in all power and strength variables (CMJ, SJ, EUR, DJ, RSI, IMTP, and IMTP-R). There was a significant interaction in muscle mass (MM) [β = 0.57 kg, 95% CI = (0.15; 0.98), t₁₃ = 2.67, p = 0.019], revealing that participants in LL-BFRE increased their muscle mass (6.6 ± 3.1%) compared to HL-RE (3.6 ± 2.0%). Anthropometric variables did not present any group or interaction effect. However, there was a time effect (p < 0.05). Implementing LL-BFRE in place of HL-RE as a conditioning activity in a contrast training sequence might be equally effective in improving lower-body strength and power in preadolescent trampoline gymnasts.

IVIM Imaging of Paraspinal Muscles Following Moderate and High-Intensity Exercise in Healthy Individuals

Background

Quantification of the magnitude and spatial distribution of muscle blood flow changes following exercise may improve our understanding of the effectiveness of various exercise prescriptions. Intravoxel incoherent motion (IVIM) magnetic resonance imaging (MRI) is a technique that quantifies molecular diffusion and microvascular blood flow, and has recently gained momentum as a method to evaluate a muscle's response to exercise. It has also been shown to predict responses to exercise-based physical therapy in individuals with low back pain. However, no study has evaluated the sensitivity of IVIM-MRI to exercise of varying intensity in humans. Here, we aimed to evaluate IVIM signal changes of the paraspinal muscles in response to moderate and high intensity lumbar extension exercise in healthy individuals.

Methods

IVIM data were collected in 11 healthy volunteers before and immediately after a 3-min bout of moderate and high-intensity resisted lumbar extension. IVIM data were analyzed to determine the average perfusion fraction (f), pseudo-diffusion coefficient (D^*), and diffusion coefficient (D) in the bilateral paraspinal muscles. Changes in IVIM parameters were compared between the moderate and high intensity exercise bouts.

Results

Exercise increased all IVIM parameters, regardless of intensity (p < 0.003). Moderate intensity exercise resulted in a 11.2, 19.6, and 3.5% increase in f, D^* and D, respectively. High intensity exercise led to a similar increase in f (12.2%), but much greater changes in D^* (48.6%) and D (7.9%).

Conclusion

IVIM parameter increases suggest that both the moderate and high-intensity exercise conditions elicited measurable changes in blood flow (increased f and D^*) and extravascular molecular diffusion rates (increased D), and that there was a dose-dependence of exercise intensity on D^* and D.

Exercise-induced fluid shifts are distinct to exercise mode and intensity: a comparison of blood flow-restricted and free-flow resistance exercise

MRI can provide fundamental tools in decoding physiological stressors stimulated by training paradigms. Acute physiological changes induced by three diverse exercise protocols known to elicit similar levels of muscle hypertrophy were evaluated using muscle functional magnetic resonance imaging (mfMRI). The study was a cross-over study with participants (n = 10) performing three acute unilateral knee extensor exercise protocols to failure and a work matched control exercise protocol. Participants were scanned after each exercise protocol; 70% 1 repetition maximum (RM) (FF70); 20% 1RM (FF20); 20% 1RM with blood flow restriction (BFR20); free-flow (FF) control work matched to BFR20 (FF20_WM). Post exercise mfMRI scans were used to obtain interleaved measures of muscle R2 (indicator of edema), R2' (indicator of deoxyhemoglobin), muscle cross sectional area (CSA) blood flow, and diffusion. Both BFR20 and FF20 exercise resulted in a larger acute decrease in R2, decrease in R2', and expansion of the extracellular compartment with slower rates of recovery. BFR20 caused greater acute increases in muscle CSA than FF20_WM and FF70. Only BFR20 caused acute increases in intracellular volume. Postexercise muscle blood flow was higher after FF70 and FF20 exercise than BFR20. Acute changes in mean diffusivity were similar across all exercise protocols. This study was able to differentiate the acute physiological responses between anabolic exercise protocols. Low-load exercise protocols, known to have relatively higher energy contributions from glycolysis at task failure, elicited a higher mfMRI response. Noninvasive mfMRI represents a promising tool for decoding mechanisms of anabolic adaptation in muscle.NEW & NOTEWORTHY Using muscle functional MRI (mfMRI), this study was able to differentiate the acute physiological responses following three established hypertrophic resistance exercise strategies. Low-load exercise protocols performed to failure, with or without blood flow restriction, resulted in larger changes in R₂ (i.e. greater T₂-shifts) with a slow rate of return to baseline indicative of myocellular fluid shifts. These data were cross evaluated with interleaved measures of macrovascular blood flow, water diffusion, muscle cross sectional area (i.e. acute macroscopic muscle swelling), and intracellular water fraction measured using MRI.