An observational safety and feasibility cohort study was conducted.
Ethical approval
Ethical approval was granted by the Queen Mary Ethics of Research Committee (QMREC2018/48/054).
Participants
Participants were recruited as a sample of convenience and provided written informed consent prior to study commencement using Google Forms (Google LLC, California, USA). Sample size was based on previous guidelines for safety and feasibility studies (18), with a minimum of 12 male and 12 female participants sought. Participants were eligible for inclusion if they were over the age of 18 and currently in good health, injury-free in their upper limbs and willing to perform two BFR sessions per week and cease other forms of upper limb exercise for the study duration. Participants were excluded if they had any history of deep vein thrombosis (DVT) or pulmonary embolism (PE), a previous diagnosis of rhabdomyolysis, haemorrhagic or thrombotic stroke, previous surgery in the past six weeks, were currently or recently pregnant, or had a family history of any blood clotting disorder. Participants were also excluded if they had any prior experience with BFR.
Demographics
Eligible participants self-reported their age (in years), height (to the nearest cm), mass (to the nearest kg) and activity level using the Tegner scale. Combining both work and sports activities, the Tegner scale is a reliable and valid measure to reflect the average activity levels of recruited participants (19).
Experimental protocol
The study was conducted online using Microsoft Teams (1.400.11161, Microsoft, Washington, USA) due to the SARS-CoV-2 pandemic. All included participants followed a five-week upper limb BFR programme consisting of two sessions per week and a total of ten sessions and were advised to avoid other forms of upper limb exercise for the duration of the study. Participants were provided with a garment with integrated BFR (Hytro Limited, London, UK). This uses a standardised elastane strap (width 4cm) located at the most proximal part of the upper limb and secured with a Velcro mechanism (YKK Fastening Corp, Tokyo, Japan; see figure 1) to allow for standardisation of compression stimulus. Participants were also provided with a standardised resistance band (male=red [tension 7-16kgs] and female=yellow [tension 2-9kgs]), a flexible tape measure, and a pulse oximeter (NHS Pulse Oximeter, London, UK). Participants had an initial familiarisation meeting with a researcher (BD or EM), where they were introduced to the BFR programme and protocol. Participants were then instructed to pull the BFR strap on their dominant arm to its maximal position (reflecting 100% compression stimulus), before releasing to a perceived 50% compression stimulus, noting the corresponding number on the Velcro mechanism. Participants then underwent a three-minute passive BFR session at 50% compression stimulus to familiarise them to the sensation of BFR, before releasing. Participants finally completed four sets of a single exercise (banded bicep curls) at 50% compression stimulus, resting for 30 seconds in between sets and taking a pulse oximeter reading before and after the exercise was completed with compression applied for familiarisation.
BFR programme
Each BFR session involved four exercises (push-ups, banded bent over rows, banded triceps extensions, and banded bicep curls). Male participants were instructed to complete full push-ups, whilst female participants were instructed to complete kneeling push-ups. Participants were required to complete four sets of each exercise (30/15/15/15 repetitions), taking a 30 second rest interval between sets and a two minute rest interval between exercises (1). If participants reached volitional failure prior to the prescribed number of repetitions in a set, they were asked to record their number of successful repetitions. Participants were instructed to tighten the BFR strap to a perceived compression stimulus of 50% for their first session and increase to 60% for their second session as a familiarisation week. Participants were then instructed to increase to a perceived compression stimulus of 70% for the remaining eight sessions (weeks two-five). The BFR strap was to be applied prior to commencing an exercise and remain secured for all four sets (i.e., 75 repetitions), releasing at the start of the two-minute rest interval between exercises.
Safety outcomes
Safety was determined by recording the occurrence of adverse events during and after BFR and by monitoring for potential arterial occlusive limb pressure using pulse oximetry.
Questionnaire
Potential adverse events that could reflect thrombosis or ischaemia (20) were monitored (see table 1). Participants were instructed to report the occurrence of these events by completing a safety questionnaire after each BFR session and attend a weekly virtual meeting with a researcher (BD/EM) to discuss any adverse events that may have occurred during the preceding week.
Table 1: adverse events monitored for the duration of the BFR programme
Adverse events during exercise
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Adverse events post exercise
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Excessive pain (subjective severity)
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Excessive pain (subjective severity)
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Chafing/abrasions
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Shortness of breath
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Bruising/pressure marks
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Whole arm swelling
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Chafing/abrasions
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Bruising/pressure marks
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Persistent tingling/paraesthesia
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Numbness/loss of sensation
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Pulse oximetry
Participants were required to confirm the presence of an upper limb pulse by taking a pulse oximeter reading before commencing each exercise (once their BFR strap had been applied at the required perceived compression) and once each exercise had been completed (prior to releasing their BFR strap and commencing their rest period). Pulse oximetry has been reported to be a valid method of ensuring sub-occlusive arterial pressure (i.e., the presence of a pulse) in the upper limb when compared to the gold standard of ultrasound doppler (21).
Feasibility outcomes
Successful recruitment was determined by the time within which a minimum of 24 participants could be recruited, with a maximum of three months defined a priori.
Successful adherence was determined by monitoring the number of sessions completed by each participant (x/10), with a minimum of 80% required a priori.
Successful data collection was determined by outcome measure capture, with a minimum of 80% required a priori.
Efficacy outcomes
Push-ups to volitional failure
All participants performed a push-ups to volitional failure test in their familiarisation meeting, but before their BFR familiarisation. Total push-ups were observed and recorded by the researcher during the video call and the test was ceased once participants failed to meet the minimum movement standard of 90˚ elbow flexion (i.e., volitional failure). This was then repeated in the final meeting after the five-week BFR programme. Push-ups to volitional failure was chosen as a proxy measure of strength as it has been reported to correlate well with a one repetition maximum bench press using an equivalent load (22). It could also be performed virtually and without requiring participants to attend a human performance laboratory during the SARS-CoV-2 pandemic.
Arm girth
Arm girth was measured by the participant using a flexible tape measure according to the International Society for the Advancement of Kinanthropometry (ISAK) guidelines (23). Participants were instructed to measure from their acromion process to their cubital fossa on their right arm, marking the midpoint. Participants were then instructed to take a circumferential measurement of their arm at this point to the nearest 0.5cm, with their arm relaxed in the anatomical position (23).
Number of prescribed repetitions completed
The total number of repetitions completed were compared from week two to week five, excluding week one as a familiarisation week, as a measure of muscular endurance (24).
Statistical analysis
Data were collected and collated using a customised spreadsheet (Microsoft Excel 16.0.13426320270, Microsoft, Washington, USA). Safety and feasibility data were analysed using Microsoft Excel. Safety outcomes were calculated by dividing the incidence of reported adverse events by the total number of BFR sessions (x/280) and expressed as a percentage. Adherence outcomes were calculated by dividing the number of completed sessions by the total number of prescribed sessions (x/280) and expressed as a percentage.
Efficacy data were analysed using JAMOVI (v.1.6.23, the JAMOVI project, Sydney, Australia). Mean change and associated standard deviation (SD) were calculated for push-ups to volitional failure and arm girth (follow up – baseline), and total number of repetitions completed (week five – week two). A Shapiro-wilk normality test was conducted to determine if data were normally distributed. As a feasibility study not powered apriori to detect statistical significance, dependent sample t-tests were not performed, and p-values not reported, because of the potential for type II error and to avoid giving the impression of there being robust findings from a feasibility design. Instead, mean change with 95% confidence intervals (CI) and effect sizes (25) were calculated. If normally distributed, Cohen’s d was interpreted as trivial (<0.2), small (0.2-0.49), medium, (0.5-0.79) and large (≥0.8; 26) and if non non-normally distributed, rank biserial correlation (RBC) was interpreted as strong positive correlation (1.0), no correlation (0), strong negative correlation (-1.0; 27).