Neuromuscular and Metabolic Responses during Repeated Bouts of Loaded Downhill Walking

Introduction: The aim of this study was to compare vastus lateralis (VL) and rectus femoris (RF) muscles for their nervous and mechanical adaptations during two bouts of downhill walking (DW) with load carriage performed 2 wk apart. Moreover, we investigated cardiometabolic and perceived exertion responses during both DW bouts. Methods: Seventeen participants performed two 45-min sessions of loaded DW (30% of body mass; slope, −25%; speed, 4.5 km·h⁻¹) separated by 2 wk. Rating of perceived exertion, cost of walking, heart rate, and EMG activity of thigh muscles were assessed during the DW. Muscle shear elastic modulus (μ) of RF and VL were assessed before each exercise bout. Maximal voluntary contraction torque was assessed before (PRE), immediately after (POST), and 24 and 48 h after the two exercise bouts. Results: Maximal voluntary contraction torque decreased from POST (−23.7% ± 9.2%) to 48 h (−19.2% ± 11.9%) after the first exercise (Ex1), whereas it was significantly reduced only at POST (−14.6% ± 11.0%) after the second exercise (Ex2; P < 0.001). Rating of perceived exertion (Ex1: 12.3 ± 1.9; Ex2: 10.8 ± 2.0), heart rate (Ex1: 156 ± 23 bpm; Ex2: 145 ± 25 bpm), cost of walking (Ex1: 4.5 ± 0.9 J·m⁻¹·kg⁻¹; Ex2: 4.1 ± 0.7 J·m⁻¹·kg⁻¹), and RF EMG activity (Ex1: 0.071 ± 0.028 mV; Ex2: 0.041 ± 0.014 mV) were significantly decreased during Ex2 compared with Ex1 (P < 0.01). RF μ was significantly greater in Ex2 (0.44 ± 0.18) compared with Ex1 (0.56 ± 0.27; P < 0.001). Conclusions: The RF muscle displayed specific mechanical and nervous adaptations to repeated DW bouts as compared with VL. Moreover, the muscle adaptations conferred by the first bout of DW could have induced greater exercise efficiency, inducing lesser perceived exertion and cardiometabolic demand when the same exercise was repeated 2 wk later.