The main finding of this study was that LL-BFR exercise twice weekly for 4–8 weeks was feasible in patients suffering from knee pain due to various orthopedic knee disorders. The mean adherence rate was on average 95.6% for the eight participants, when related to the a priori adherence calculation. All eight participants completed ≥ 90% of the planned exercise sessions. Training adherence in the present study is comparable to Ferraz et al. (7) and Segal et al. (15) both reporting excellent adherence to LL-BFR in patients with knee OA. Additionally, we found that performing LL-BFR twice weekly without reaching concentric contraction failure, as done by the non-affected limb (work matched to the affected limb), was inferior to increase thigh circumference and knee extensor strength.
In the present study, one of the five KOOS subscales showed a significant improvement. However, a change of + 10 points was seen in three subscales (“pain”, “symptoms”, and “sport & recreation”). In general, the minimal important change is considered to be 8–10 points in each KOOS subscale (16), suggesting that the patients experienced a meaningful change from baseline to follow-up. Ferraz et al. (11) also demonstrated significant changes in patient-reported outcomes (WOMAC pain, stiffness and Physical activity) after 12 weeks of LL-BFR, suggesting that longer and more intensive training periods seem necessary to further improve patient reported outcomes.
The association between quadriceps muscle strength and functional performance is well-documented (17–19). Hence, it is crucial for elderly and patients suffering from disuse atrophy and strength deficits to increase knee extensor strength (17–19). In the present study, MVC knee increased for both legs. However, the strength increment of the affected limb (62%, p = 0.05) was higher than the non-affected limb (15%, p = 0.2), suggesting that LL-BFR protocols performed to volitional failure add strength gains. Increments in MVC knee after a period of LL-BFR have been demonstrated in several studies (20, 21).
In line with strength increments, functional performance assessed by 30-sec CST and TUG tests improved significantly in this present study. Improvements in 30-sec CST after 36 sessions LL-BFR was shown by Ferraz et al.(7), while Ozaki et al. demonstrated a faster TUG performance in old adults after 10 weeks of BFR walk training (4/weekly) (20). Thus, our results, combined with findings from other studies utilizing BFR exercise methods, indicate that functional performance can be improved with a minimal mechanical stress on the musculoskeletal system.
Strengths and limitations
This study is limited by the heterogeneous group of patients, the lack of a control group and the small sample size. The small sample size resulted in insufficient statistical power to detect changes. We cannot conclude if changes reflect the efficacy of LL-BFR or a result of participating in a physiotherapy-supervised training program. Also, we did not predetermine an acceptable adherence before starting the pilot project, hence, limiting our interpretation of the adherence. However, when comparing the adherence rate to other LL-BFR studies (7, 15), it seems that our patient population exhibited a high adherence to LL-BFR. Additionally, two different protocols were tested resulting in some variations in the total amount of work performed by the patients. However, all patients performed the exercise to failure, making the endpoint for each protocol identical. A strength of the present study was that all patients trained with an individualized restrictive pressure, hence increasing the safety and unifying the exercise stimulus to all participants.
Assessment of pain exacerbation during training was not included in this study. Thus, the amount of knee discomfort during training remains unknown. We did, however, not register any dropouts due to LL-BFR-related pain aggravation which suggests that LL-BFR was tolerable for patients involved in this project. At baseline, seven patients were able to perform the 5RM test while only two patients performed the test at follow-up. This might reflect a fear of pain aggravation and suggest that the patients included in the present study may not have been able to perform HRST.