Participants
Twelve young adult men (age: 25.0 ± 2.3 years, height: 1.73 ± 0.05 m, body mass: 65.7 ± 5.9 kg) participated in this study. Each participant was a healthy undergraduate or graduate student who had not participated in a regular resistance training program. Each participant received explanation about the procedures of the experiment before voluntarily participating in this study. This study was approved by the Ethics Committee on Human Research of Waseda University. During the day of experiment, the participants did not take caffeine that is known to affect the extent of PAP (MacIntosh and Gardiner 1987).
Experimental setup
Plantar flexors were selected as the target muscles in this experiment because previous studies that reported an increase in the maximal voluntary concentric torque selected plantar flexors as the target muscles (Fukutani et al. 2012); (Miyamoto et al. 2011). Each participant lay on an isokinetic dynamometer (CON-TREX, CMV AG, Switzerland) with the right knee and hip joints fully extended. The right foot was tightly secured to the dynamometer’s footplate, and the right thigh was fixed to the dynamometer’s bench with a non-elastic strap to minimize unwanted movements. The center of rotation of the footplate and center of the ankle joint were aligned visually. During the twitch and conditioning contractions, the ankle joint angle was set at 0° (anatomical position). The joint angular velocity of the maximal voluntary concentric contraction was set at 30°/s and 180°/s (slow and fast conditions, respectively). The range of motion of the maximal voluntary concentric contraction was set from -15° (dorsiflexion) to 30° (plantar flexion) in the fast velocity condition and from -15° to -5° in the slow velocity condition to minimize the difference of the contraction duration between the two velocity conditions. In a preliminary study, we confirmed that peak torque during the maximal voluntary concentric contraction in each velocity condition was recorded within these ranges of motion. Subjects were instructed to perform the fast and slow maximal voluntary concentric contraction as forcibly as possible.
Surface electromyography (EMG) signals were obtained from the medial gastrocnemius (MG), lateral gastrocnemius (LG), soleus (SOL), and tibialis anterior (TA). After shaving, abrasion and cleaning with the alcohol, we placed the pre-amplified bipolar differential electrodes (Ag/AgCl, input impedance: 1 MΩ, electrode bar size: 1 × 10 mm each, DE-2.1, DELSYS, USA) on the surface of each muscle with an inter-electrode distance of 10 mm (CMRR: 92 dB, Gain: 1000, Band-pass filter: 20–450 Hz, Bagnoli 8 EMG System, DELSYS, Boston, MA, USA). The reference electrode was placed over the left lateral malleolus (11 mm diameter, Blue sensor, N-00-S, Ambu, Denmark). The analog data of EMG signals, torques and joint angles were converted into the digital data using a 16-bit analog-to-digital converter (Power-lab/16SP, ADInstrument, Australia). The sampling frequency was set at 4 kHz.
The posterior tibial nerve was stimulated percutaneously to evoke an isometric twitch contraction of the plantar flexor muscles. A cathode (11 mm diameter, Blue sensor, N-00-S, Ambu, Denmark) was placed over the popliteal fossa, and an anode (40 × 50 mm, VIASYS, Healthcare, USA) was placed over the ventral aspect of the thigh near the patella. Single rectangular pulses of 500 μs duration were delivered from a high-voltage stimulator (SEN-3301, Nihon Kohden, Japan) with a specially modified isolator (SS-1963, Nihon Kohden, Japan). The stimulus intensity was determined prior to the experiment by increasing the voltage until the corresponding torque reached a plateau with joint angle at 0°. The stimulus intensity was set at 20% above the intensity, which no further increase in twitch torque was confirmed. The stimulation intensity ranged from 40 to 90 volts.
Protocol
First, an isometric twitch contraction of the plantar flexors was elicited and the peak torque value was recorded as the isometric twitch torque with no potentiation. This procedure was repeated two times to confirm the reproducibility. Next, the participant performed a familiarization task before the main experiment. The familiarization task consisted of several voluntary concentric contractions (more than three times in each condition) of the plantar flexors at 30°/s and 180°/s with submaximal and maximal effort. After the completion of the familiarization task, the participant rested for more than 10 minutes (Baudry et al. 2008) to avoid the effect of PAP caused by the familiarization task on the main experiment. The protocol of the main experiment is shown in Figure 1. First, an isometric twitch contraction was elicited in a similar fashion for the two velocity conditions to calculate the extent of increase in isometric twitch torque as an index of the positive effect of the conditioning contraction. After that, the participant was asked to perform three-consecutive maximal voluntary concentric contractions of the plantar flexors. To avoid the effect of PAP induced by three-consecutive maximal voluntary concentric contractions on the outcomes of the following trials, a rest period of approximately five minutes was allowed after the three-consecutive maximal voluntary concentric contractions. After confirming that the difference of the isometric twitch torque was within ±10% deviation and within ±2 Nm compared to that with no potentiation, the participant performed the maximal voluntary isometric contraction of the plantar flexors for six seconds as a conditioning contraction. Immediately after the conditioning contraction (approximately three seconds later), an isometric twitch contraction was elicited and then (approximately five seconds later), three-consecutive maximal voluntary concentric contractions were performed at 30°/s or 180°/s. In addition, the isometric twitch contraction and the three-consecutive maximal voluntary concentric contractions were conducted one and five minutes after the conditioning contraction. Each velocity condition was separated by a rest period of more than 10 minutes to avoid the effect of PAP caused by the conditioning contraction (Vandervoort et al. 1983) and three-consecutive maximal voluntary concentric contractions which were performed immediately, and 1 and 5 min after the conditioning contraction on the next velocity condition. The second condition was performed after confirming that the difference of the isometric twitch torque was within ±10% deviation compared to that with no potentiation. The order of the two velocity conditions was randomized.
Measurements
Typical waveforms of torque and EMG obtained during isometric twitch and the maximal voluntary concentric contractions are shown in Figure 2. Peak torque and the peak-to-peak amplitude of the M-wave during the isometric twitch contraction were adopted as the isometric twitch torque and M-wave amplitude, respectively. The contraction in which the highest peak torque was attained among the three-consecutive maximal voluntary concentric contractions was adopted for subsequent analyses. The peak torque attained during the maximal voluntary concentric contraction was recorded as the maximal voluntary concentric torque. The root-mean-square value of the EMG signal (RMSEMG) of each muscle was calculated over 200 ms period in the middle of the maximal voluntary concentric contraction (from 100 ms before to 100 ms after the instance of peak torque). A 20-Hz high-pass filter was used to minimize the effect of motion artifact on the RMSEMG value (De luca et al. 2010). Each parameter recorded at three time points (i.e., immediately, 1 and 5 minutes after the conditioning contraction) was expressed as the value relative to that recorded before the conditioning contraction (%change). The RMSEMG normalized by the M-wave was also calculated as an index of central fatigue. Furthermore, the ankle joint angle at peak torque during the maximal voluntary concentric contraction occurred was measured.
Mean value of torque and RMSEMG of plantar flexors during the conditioning contraction with a 500 ms duration that torque signal was stable were calculated in the first and second conditions to confirm whether subjects exerted conditioning contraction with the same intensity in first and second conditions.
We confirmed the reproducibility of the twitch torque between the two trials conducted at the beginning of the experiment and the maximal voluntary concentric torque among three-consecutive trials performed before the conditioning contraction in each velocity condition. For the twitch torque, the coefficient of variance was 1.5 ± 1.2% and the intra-class correlation was 0.99. For the maximal voluntary concentric torque, the coefficient of variance was 5.6 ± 2.2% at the 180°/s condition and 3.8 ± 2.6% at the 30°/s condition. The intra-class correlation was 0.78 at the 180°/s condition and 0.92 at the 30°/s condition.
Statistics
Descriptive data are presented as mean ± SD. The isometric twitch torque, M-wave amplitude, maximal voluntary concentric torque, RMSEMG during the maximal voluntary concentric contraction, the values of RMSEMG normalized by the M-wave and ankle joint angle at peak torque during the maximal voluntary concentric contraction occurred were tested by repeated-measures two-way ANOVA (velocity × time point). If the interaction was significant, additional repeated-measures one-way ANOVAs with a subsequent post-hoc test with Bonferroni’s correction was used for examining the time-course changes in each velocity condition. In addition, paired t-test was used for examining the difference between the two velocity conditions at each time point. Mean value of torque and RMSEMG of plantar flexors during the conditioning contraction was compared by paired t-test. The level of significance was set at p < 0.05. A software package (IBM SPSS Statistics 20.0, IBM, USA) was used for the statistical analyses.