2.2. Experimental Approach

Participants attended the laboratory on 6 different days (Figure 1). Two semi-recumbent cycle ergometers were used for steady-power CON (Ergoline GmbH, Lidenstrabe, Germany) and ECC (Cyclus 2, Cyclus GmbH, Leipzig, Germany) cycling. The knee and hip joint flexion angles were measured at the maximal knee extension possible on each bicycle, and the seat seatback and incline were adjusted so that there was less than 10° of difference between either of the two joint angles on the two bikes.

Overview of the protocol. In panel A, each of the six squares represents a session in the laboratory. Measurements were carried out in eccentric and concentric cycling during the first set of the first session (PRE-test) and during the sixth session (POST-test). EMG of the rectus femoris, vastus lateralis, biceps femoris, and soleus muscles, as well as perceptions of effort and of the difficulty to pedal at the instructed cadence (60 rpm), were recorded. Panel B shows an example of a raw and unfiltered EMG signal (at PRE during CON cycling at 60% PPO), with the timing of physiological (EMG and heart rate) and perceptual measurements (perceived effort and difficulty to pedal at the instructed cadence). PPO: Peak power output; CON: concentric cycling; ECC: eccentric cycling.

On the first day, volunteers performed a maximal incremental CON cycling test at 60 rpm (starting at 50 W with an increase of 1 W every 3 s). Power output (PPO) and heart rate (bpm) were recorded at exhaustion (i.e., when pedaling cadence dropped below 50 rpm for more than 10 s). The maximal test was followed by 3 min of ECC cycling at 40% PPO to introduce participants to the task. They then carried out a total of 50 min of both ECC and CON cycling in four sessions spread over two weeks. The first session took place a week after the incremental test, and a PRE-test was integrated in it. The second session was three days later, and the two other sessions were conducted the next week on the same days. A POST-test was carried-out on the sixth visit, two weeks after the PRE-test.

The first and second sessions consisted in pedaling 1 min twice at five different exercise intensities (30, 45, 60, 75, and 90% of PPO) on both bicycles, and recovered between bouts until heart rate had returned to baseline level ±10 bpm. Bouts of the third and fourth sessions lasted 1.5 min. On the sixth day, participants pedaled for 1 min at each power output on each bicycle. Power outputs and exercise modalities were performed in random order so as to avoid the confound of an order effect. Participants were asked to pedal at a cadence of 60 rpm and were oblivious to any objective marker of exercise intensity (i.e., power output or heart rate), across all sessions.

Physiological and perceptual measurements (Figure 1) were carried out during the PRE- and POST-tests, after a warm-up consisting of 2.5 min of CON and ECC cycling at 40% of PPO.

Participants were asked to keep their usual diet and physical activities, but to restrain from any intense physical activity for two days before the testing days.

Since the knee extensor, the hip extensor, and the ankle plantar flexor muscles are all heavily involved during ECC cycling [19], we recorded the EMG of the RF, the VL, the BF, and the soleus (SOL) muscles.