Background: Patellofemoral pain (PFP) is the leading cause of lower extremity injury in female runners, significantly limiting running ability and becoming chronic in nearly 50% of the cases. This condition, which presents as pain around (peripatellar) or behind (retropatellar) the patella, is associated with significant weakness of the quadriceps and gluteal musculature, as well as altered movements mechanics. The challenge for clinicians is designing an effective treatment program to reduce pain and improve function without causing symptoms to flare. Blood flow restriction training (BFRT) is a relatively new training method within rehabilitation that involves placing a pressurized cuff to the proximal thigh to partially restrict arterial inflow and fully restrict venous outflow in working musculature during exercise. Unlike traditional strength training that relies primarily on mechanical stress (i.e. load) across the muscle, BFRT relies on metabolic stress (i.e. accumulation of metabolites) within the muscle. While BFRT holds great promise, minimal research has assessed the effects of BFRT in individuals with PFP.
Objective: To determine if low-load strength training with BFRT would lead to greater improvements in quadriceps and gluteal strength (Aim 1), knee and hip joint mechanics during running and a step down task (Aim 2), and patient-reported outcome measures (Aim 3) compared to low-load strength training without BFRT in female runners with PFP over a 10-week intervention period.
Participants: Twenty female runners with PFP completed the study protocol, 10 in the BFRT group (age: 31.2 ± 9.1 years; symptom duration: 26.3 ± 31.6 months; running mileage per week: 14.5 ± 6.6 miles) and 10 in the standard of care group (age: 27.9 ± 7.5 years; symptom duration: 27.9 ± 43.9 months; running mileage per week: 12.9 ± 7.3 miles).
Methods: Participants were randomly allocated to one of two treatment groups, low-load strength training with BFRT (BFRT group) or low-load strength training with sham BFRT (standard of care group), and performed knee and hip low-load strengthening exercises 2 times per week for 10 weeks. All outcome measures were recorded at baseline (pre-intervention) and after 10 weeks of treatment (post-intervention). For Aim 1, quadriceps strength was assessed on a Biodex dynamometer and gluteal strength was assessed with a handheld dynamometer. For Aim 2, three-dimensional motion analysis was used to assess self-selected speed running mechanics, standardized speed (3.0 m/s) running mechanics, and step down mechanics. For Aim 3, participants completed patient-reported outcome measures focused on self-reported pain, knee function, and running ability.
Main Outcome Measures: Outcomes were assessed across three domains. The first domain was muscular strength and included peak isometric and isokinetic knee extension, as well as peak isometric hip external rotation, hip abduction, and hip extension (Aim 1). The second domain was movement mechanics and included knee and hip joint kinematics and kinetics during running and a step down task (Aim 2). The third domain was patientreported outcome measures and included the Numeric Pain Rating Scale (NPRS), Knee Injury & Osteoarthritis Outcome Score (KOOS) subscales, KOOS Patellofemoral Subscale (KOOS-PF), and University of Wisconsin Running Injury and Recovery Index (UWRI) (Aim 3).
Statistical Analysis: Change scores for each outcome measure were calculated as the difference between post-intervention value and pre-intervention value (i.e. postintervention – pre-intervention). Differences between groups in change scores were assessed with independent samples t-test. Differences within each group from preintervention to post-intervention were assessed with paired samples t-test. For further exploration of the data, Pearson product moment correlation coefficients were used to assess the relationships between pre-intervention values, post-intervention values, and change scores of the outcome variables.
Results: Aim 1: There were no significant differences between groups for isometric and isokinetic quadriceps strength, hip external rotation strength, or hip abduction strength; however, the standard of care group demonstrated a significantly greater improvement in hip extension strength compared to the BFRT group (p<0.01). Improvements in quadriceps strength ranged from 6-10% and improvements in gluteal strength ranged from 23-71%. Aim 2: There were no significant differences between groups and no significant change within groups for hip and knee joint mechanics during running at a self-selected speed and at a standardized speed. There were no significant differences between groups for hip and knee joint mechanics during the step down task; however, there was a significant decrease in peak knee extensor moment (0.1 Nm/kg, p=0.02) in the BFRT group and a significant increase in peak hip adduction angle (2.1°, p<0.01) in the standard of care group. Aim 3: There were no significant differences between groups for all patient-reported outcome measures. Significant improvements were seen in both groups within the NPRS (worst), NPRS (average), NPRS (running), PSEQ, KOOS subscales, KOOS-PF, and UWRI, with all values exceeding their respective minimal clinically important difference.
Conclusions: Low-load strength training with BFRT was not more effective than lowload strength training without BFRT for improving quadriceps and gluteal strength, knee and hip joint mechanics during running and a step down task, and patient-reported outcome measures. These results indicate that low-load strength training with and without BFRT can be used to address quadriceps and gluteal strength deficits, as well as reduce pain and improve function in female runners with PFP. However, additional interventions, such as activity-specific neuromuscular re-education, are likely needed to improve hip and knee joint mechanics.