Cross-over Muscular Adaptation to Blood Flow–Restricted Exercise

Abstract

Dear Editor-in-Chief: We read with interest the article published by Weatherholt et al. (7). The authors performed a well-controlled study that adds to our knowledge about blood flow–restricted exercise. With their within-subject design, the authors observed similar increases in upper arm muscle cross-sectional area (mCSA) and strength after 8 wk of low load unilateral elbow flexion and extension training with and without blood flow restriction (BFR). The authors suggest that the adaptations for both muscle strength and hypertrophy observed in the limb trained without BFR are attributable to a cross-education effect. Although unilateral training with a high load can increase muscular strength in the untrained, contralateral limb (6), we feel it is likely that the training adaptations observed in the limb trained without BFR are due to the local training stimulus itself. The authors’ reason for suggesting a neural cross-education effect is that training with a low load “is generally accepted as being too low to cause strength increases” (7). Although evidence suggests that high-load training does produce greater increases in strength compared with low-load training, it is clear that low-load training will increase maximal strength to an extent (1). Moreover, other studies using a within-subject design have shown greater increases in muscular strength in a limb trained under BFR compared with the contralateral limb trained without BFR (5). This suggests that increases in strength in one limb are not always equivalent in the contralateral limb. The similar increases in muscular strength observed in both the BFR- and non–BFR-trained limb in the present study are likely attributable to similar increases in mCSA (7). With respect to the author’s second conclusion, a systemic effect of BFR resistance training causing anabolic hormone-induced hypertrophy of the contralateral limb, we suggest that the increase in mCSA of the non–BFR-trained limb is likely due to the local training stimulus itself. Although BFR knee extensor and flexor exercise has been shown to enhance training-induced hypertrophy of the elbow flexors (3), the acute anabolic hormonal response causing such adaptation is questionable. In contrast to the aforementioned study, West et al. (8) have shown that exercise-induced increases in anabolic hormones do not enhance training-induced hypertrophy of the elbow flexors. Furthermore, elbow flexor exercise alone is unable to cause an acute rise in such hormones (8). In the present investigation (7), it is unlikely that such a low volume of unilateral elbow flexor and extensor exercise, even with BFR, would be able to increase such hormones. It is likely that 8 wk of low-load training without BFR was capable of inducing muscle hypertrophy because low-load training without BFR has been shown previously to induce muscle hypertrophy (2,4). This commentary should not be taken as criticism of the study; we offer another possible explanation for the muscular adaptations observed with low-load training without BFR. Similar training adaptations to different unilateral training protocols should not necessarily be attributed to a cross-over effect. C.A. Fahs has received a grant from the American College of Sports Medicine for doctoral research in Kaatsu methodology (#012012).