  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 two weeks apart. Moreover,
we investigated cardio-metabolic 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$\times$h -1 )
separated by two weeks. Rating of perceived exertion (RPE), cost of walking (C
w ), heart rate (HR), and EMG activity of thigh muscles were assessed during
the DW. Muscle shear elastic modulus ($\mu$) of RF and VL were assessed before
each exercise bout. Maximal voluntary contraction (MVC) torque was assessed
before (PRE), immediately after (POST), 24 and 48 h after the two exercise
bouts. Results MVC torque decreased from POST (-23.7 $\pm$ 9.2%) to 48 h (-19.2
$\pm$ 11.9%) after the first exercise (Ex1), whereas it was significantly
reduced only at POST (-14.6 $\pm$ 11.0%) after the second exercise (Ex2) (p <
0.001). RPE (Ex1: 12.3 $\pm$ 1.9; Ex2: 10.8 $\pm$ 2.0), HR (Ex1: 156 $\pm$ 23
bpm; Ex2: 145 $\pm$ 25 bpm), C w (Ex1: 4.5 $\pm$ 0.9 J$\times$m -1 $\times$kg
-1 ; Ex2: 4.1 $\pm$ 0.7 J$\times$m -1 $\times$kg -1 ) and RF EMG activity (Ex1:
0.071 $\pm$ 0.028 mV; Ex2: 0.041 $\pm$ 0.014 mV) were significantly decreased
during Ex2 compared to Ex1 (p < 0.01). RF $\mu$ was significantly greater in
Ex2 (0.44 $\pm$ 0.18) compared to Ex1 (0.56 $\pm$ 0.27; p < 0.001). Conclusions
The RF muscle displayed specific mechanical and nervous adaptations to repeated
DW bouts as compared to VL. Moreover, the muscle adaptations conferred by the
first bout of DW could have induced greater exercise efficiency, inducing
lesser perceived exertion and cardio-metabolic demand when the same exercise
was repeated two weeks later.
