Author(s): RITSCHE, P., LEDERGERBER, R., LICHTENSTEIN, E., FAUDE, O., KELLER, M., Institution: UNIVERSITY OF BASEL, Country: SWITZERLAND, Abstract-ID: 2275

INTRODUCTION:
Low-load blood-flow restriction (BFR) training induces muscle hypertrophy comparable to high-load (HL) resistance training, but strength gains are typically smaller as neural adaptations appear attenuated. Yet, effects of BFR compared to HL on muscle contractile properties remain unknown. Therefore, we investigated how BFR affects muscle contractile properties and whether a sequential combination of BFR with subsequent short-term HL increases neural adaptations.
METHODS:
After familiarization, 37 healthy adults completed a randomized controlled trial comparing BFR (11 males: 47.3±6.0 years, 12 females: 49.7±7.2 years) and HL (5 males: 45.0±6.1years; 10 females: 47.3±5.9 years) knee extensor training over 8 weeks, followed by 2-weeks HL in both groups. Maximal voluntary contraction (MVC) was quantified during isometric knee extension using dynamometry (ScienceToPractice LtD, Ljubljana, Slovenia). Rate of torque development (RTD) was calculated within 250ms of force onset in 50ms windows. Voluntary activation (VA) of the knee extensor muscles was determined using the interpolated twitch technique. Vastus lateralis (VL) and Rectus femoris (RF) volume was calculated based on 5 anatomical cross-sections at 30-70% of thigh length spaced 10% apart using ultrasound (12L3, Acuson Juniper, Siemens, Erlangen, Germany). VL muscle thickness, fascicle length (FL) and pennation angle (PA) was assessed at rest at 50% of thigh length and 50% muscle width using ultrasound (LF9, ArtUS, Telemend, Milano, Italy). Maximum fascicle shortening velocity was determined within first 250ms of force onset.
RESULTS:
Linear mixed models were used to analyze group*time interactions and Cohen's d effect sizes are reported. After both training phases, the BFR group showed smaller improvements than HL in VA (d = −0.31 to −0.37) and knee extension MVC (d = −0.16 to −0.27). RF and VL volume increases did not favor of any group (d = −0.01 to 0.08). RTD did not change relevantly between groups. VL thickness, PA, and FL showed negligible differences between groups (d = −0.04 to 0.16) at rest. Maximum fascicle shortening velocity decreased more pronounced in the HL group (d = −0.66).
CONCLUSION:
While both training paradigms did not differ in muscle volume changes, a subsequent 2-week HL phase in the BFR group did not fully restore neural adaptations to the level of continuous HL training. Importantly, although more than 1200 architectural images were analyzed to assess reliability of manual and automated analysis, the magnitude of observed architectural changes remained within the range of measurement variability. Thus, ultrasound-based architectural measures appear insufficiently sensitive to detect intervention effects of the magnitude typically observed in resistance training studies. These findings underscore the central role of mechanical loading for neural adaptation and highlight the necessity of explicitly reporting and interpreting reliability metrics in longitudinal ultrasound research.