3.2 Methods 57
3.2.2 Study protocol 57
Each participant performed two separate tests within one week of each other which were carried out at the same time of day. In the first session participants were familiarised with the testing equipment, warm-up protocol and the testing procedures they would perform during the F-V test in the main testing session. Previously it has been shown that one familiarization session is adequate to obtain reproducible measurements of maximal power in young non-cyclist adults (Doré et al., 2003). Participants were also asked to refrain from consuming caffeinated beverages and food 12 hours prior to each test.
3.2.2.1Force-velocity test
An electro-magnetically braked cycle ergometer (Dynafit Pro Velotron, RacerMate Inc., Seattle, WA, USA), equipped with 170 mm scientific SRM® PowerMeter cranks (Schoberer Rad Messtechnik International, JÜlich, Germany) connected to TorxtarTM data logging system was used to run the F-V test, similar to methods used previously (Barratt, 2008; Yeo et al., 2015). The analog torque signal was recorded by TorxtarTM via strain gauges positioned within the spider of the SRM powermeter at a frequency of 250 Hz. A static calibration of the SRM cranks while connected to TorxtarTM was performed prior and after data collection, following procedures previously described (Wooles et al., 2005). Additionally, TorxtarTM was used to detect left-top- dead centre (LTDC) and right-top-dead centre (RTDC) crank positions and identify the start/end of each pedal cycle (i.e. LTDC to LTDC and RTDC to RTDC) completed during each sprint of the F-V test.
The external resistances used during the F-V test (including the warm-up) were adjusted and controlled using the Velotron Wingate software (v1.0, RacerMate Inc., Seattle, WA, USA).
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The cycle ergometer was fit with clipless pedals (Shimano, PD-R540 SPD-SL, Osaka, Japan) and participants wore provided cleated cycling shoes (Shimano SH-R064, Osaka, Japan). Saddle height was set at 109% of inseam length (Hamley & Thomas, 1967), while the handlebars were adjusted vertically and horizontally to the requirements of each subject.
At the beginning of both sessions, participants performed a standardised warm-up which included 8-min of cycling at 80 to 90 rpm, and two 7-s sprints at a workload of 1.2 W·kg1, controlled by Velotron Coaching software (RacerMate Inc., Seattle, WA, USA). Following 5-min of passive rest, participants performed a F-V test that consisted of six all-out, 6-s sprints interspersed with 5-min rest periods, in accordance with methods previously described (Arsac et al., 1996; Dorel et al., 2005). More specifically, the different sprints completed by each participant were as follows: 1) a sprint from a stationary start against an external resistance of 4 Nm·kg-1 using an 85 tooth front sprocket and 14 tooth rear sprocket; 2) a sprint from a stationary start against an external resistance of 1 Nm·kg-1 using a 62 tooth front sprocket and 14 tooth rear sprocket; 3) a sprint from a stationary start against an external resistance of 2 Nm·kg-1 using an 85 tooth front sprocket and 14 tooth rear sprocket; 4) a sprint from a rolling start with an initial cadence ~80 rpm against an external resistance of 0.5 Nm·kg-1 using a 62 tooth front sprocket and 14 tooth rear sprocket; 5) a sprint from a rolling start with an initial cadence ~100 rpm against an external resistance of 0.3 Nm·kg-1 using a 62 tooth front sprocket and 14 tooth rear sprocket; 6) a sprint from a stationary start against no external resistance (the chain was removed) in order to obtain an experimental measure of the participants maximal cadence (Cmax). All sprints were performed on the same cycle ergometer, with the front sprocket changed from the 85 tooth to the 62 tooth and vice versa, as required during the five minute rest period given between sprints. The external resistances listed for the different sprints above correspond to the torques exerted on the flywheel of the cycle ergometer. The order of the sprints was randomized for each subject. Rolling starts were implemented for sprints performed against low external resistance in order to enable participants to reach high cadences within the 6-s sprint duration. To achieve the rolling starts, the flywheel was accelerated by the experimenter immediately prior to the sprint so that participants could initiate their sprints at the target cadence without prior effort. Participants were instructed to remain seated on the saddle, keep hands on the dropped portion of the handlebars and to produce the highest acceleration possible throughout the sprint. Participants were vigorously encouraged throughout the duration of each sprint.
Surface electromyography (EMG) signals were bilaterally recorded from seven muscles of the lower limbs: gluteus maximus (GMAX); rectus femoris (RF); vastus lateralis (VAS); semitendinosus and biceps femoris (HAM); gastrocnemius medialis (GAS); tibialis anterior (TA). These muscles were selected as they are considered to be the main lower limb muscles used in the pedalling movement (Raasch et al., 1997; Zajac et al., 2002). Disposable pre-gelled Ag-
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AgCl surface electrodes (Blue sensor N, Ambu, Ballerup, Denmark) were used to record the EMG signals. Electrodes were positioned at an inter-electrode distance of 20 mm apart (centre to centre), aligned parallel to the muscle fibres in accordance with the recommendations of SENIAM (Hermens et al., 2000). Prior to placement of the electrodes, the skin was prepared by shaving, light abrasion and cleaned with alcohol swabs. Electrodes and wireless sensors were secured with adhesive tape to ensure good contact with the skin and to reduce movement artefact. EMG signals were recorded continuously and sent in real-time to a wireless receiver (Telemyo DTS wireless Noraxon Inc., AZ, USA) connected to a PC running MyoResearch software (Noraxon Inc., AZ, USA) at a sampling rate of 1500 Hz. Closure of a reed switch generated a 3-volt pulse in an auxiliary analogue channel of the EMG system which synchronised crank position (i.e. LTDC) with the raw EMG signals.
3.2.2.2Data processing
All mechanical and EMG signals were later analysed using Visual3D software (version 5, C- Motion, Germantown, MD, USA). First crank torque signals were low-pass filtered (10 Hz, 4th order Butterworth filter). Then, using the time synchronised events of LTDC and RTDC, average cadence was derived from time duration of the pedal cycle (i.e. LTDC-LTDC for left leg and RTDC-RTDC for right leg). Average crank torque values were calculated over the same time interval, while average power was computed using Eq. 1 below (Martin et al., 1997):
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Eq. 1
Raw EMG signals were processed using the following steps: i) removal of low-frequency artefact by using a 20 Hz high-pass Butterworth filter, ii) rectified using a root mean squared (RMS) with a 25-ms moving rectangular window and iii) smoothed using a low-pass Butterworth filter with a 10 Hz cut-off. The amplitude of the RMS of each muscle was normalised according to the methods previously defined by Rouffet and Hautier (2008).
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