Subjects
Subjects were 13 healthy young adults (7 men, 6 women) and 12 healthy elderly adults (6 men, 6 women). Mean age, height, weight, and foot length (FL) were 22.2 years [standard deviation (SD) = 4.8], 168.8 cm (SD = 7.3), 60.5 kg (SD = 6.8), and 25.0 cm (SD = 1.6) in young subjects and 65.5 years (SD = 3.6), 157.8 cm (SD = 6.9), 62.4 kg (SD = 5.8), and 23.7 cm (SD = 1.0) in elderly subjects, respectively. No subject had any history of neurological or orthopedic impairment. All young subjects had belonged to ball-game clubs, such as basketball and soccer, for more than 6 consecutive years. Elderly subjects, who could perform activities of daily living without assistance, were recruited from the suburban areas of Kanazawa and Suzu city of Japan. Many of them were relatively healthy people who performed daily work, such as farming, or participated in exercise programs several times a week. Informed consent was obtained from all subjects in accordance with the Declaration of Helsinki following an explanation of our experimental protocols, which were approved by the Institutional Ethics Committee of Kanazawa University.
Apparatus and data recording
A platform (FPA34; Electro-design, Japan) was used to measure CoPy. The CoPy electronic signals were sent simultaneously to one computer (PC9801BX2; NEC, Japan) to determine the CoPy position and to another computer for analysis. The former received CoPy data via an A/D converter (PIO9045; I/O-Data, Japan) at 20 Hz with 12-bit resolution and could generate a buzzer sound when the CoPy was located within ±1 cm of the required position. The CoPy position was calculated and shown as the percentage distance from the heel in relation to FL (%FL). The force platform was fixed to a handmade table that was movable horizontally in an anteroposterior direction by a computer-controlled electric motor (VLA-ST-60-60-0300; THK, Japan). Direction, velocity, and amplitude of platform movement were adjusted using Cutey Wave II software (Sanmei-Denshi, Japan). S1 was an auditory stimulus delivered via earphones with frequency, intensity, and duration of 2,000 Hz, 35 dB above the threshold, and 50 ms, respectively. S2 was a transient forward floor translation. Onset of translation was detected by an accelerometer (AG-2 GB; Kyowa, Japan) fixed to the force platform.
Ag-AgCl cup electrodes (diameter, 8 mm) for recording EEG were affixed to the scalp at Fz, Cz, and Pz in accordance with the international 10–20 system, and referenced to linked ear lobes. A ground electrode was placed at Fpz. Electrooculography (EOG) was recorded from a pair of electrodes placed above and below the left eye. To fix eye position, subjects were instructed to gaze at a fixation point presented on an Eye-trek face-mounted display (FMD011F; Olympus, Japan). Surface electrodes (P-00-S; Ambu, Denmark) were used in bipolar derivation to record surface electromyography (EMG) of the following muscles on the left side: the rectus abdominis (RA) erector spinae (ES), rectus femoris (RF), biceps femoris (BF), tibialis anterior (TA), medial head of the gastrocnemius (GcM), and soleus (Sol). For each muscle, electrodes were fixed after shaving and cleaning the skin with alcohol. The electrodes were aligned along the long axis of the muscle with an inter-electrode distance of about 3 cm. Electrode input impedance was < 5 kΩ. Signals from electrodes were amplified (EEG, ×40,000; EOG, ×4,000; EMG, ×4,000) and band-pass filtered (EEG, 0.05–100 Hz; EOG, 0.05–30 Hz; EMG, 5–500 Hz) using an amplifier (Biotop 6R12; NEC-Sanei Japan).
For subsequent analyses, all electrical signals were sent to a computer (Dimension E521; Dell, Japan) via an A/D converter (ADA16-32/2(CB) F; Contec, Japan) at 1 kHz with 16-bit resolution.
Procedure
All measurements were carried out while subjects were standing barefoot, with feet 10 cm apart and parallel on the force platform, and upper limbs crossed in front of the chest. To prevent falls due to floor translation, subjects wore a harness around the chest.
First, the mean positions of CoPy were measured while subjects maintained a quiet standing (QS) posture for 10 s. The mean value of the five trials was adopted as the QS position. Next, the mean CoPy position during EBL posture was measured twice. Subjects gradually leaned backward from QS for approximately 5 s, pivoting at the ankles with the rest of the body kept aligned, and then maintained this EBL posture for 3 s. The more posterior CoPy mean position of two trials was adopted as the EBL position, and the posterior peak position of the CoPy in the adapted trial was defined as the EBL peak position.
Next, the intensity of floor translation (amplitude and velocity) was set for each subject, and then the experimental session was carried out (Figure 1). During both the setting of the intensity and the experiment, subjects maintained the CoPy position within the QS position ± 1 cm, which was presented by a buzzer sound for at least 3 s, until S2 onset. S1 was randomly presented in 1–2 s after the experimenter stopped the buzzer sound, and then S2 started 2 s after S1. Subjects were instructed to avoid changing of the initial foot position or falling in response to S2.
The intensity of floor translation was set according to the following two processes[9]. First, the translation velocity was determined as 25, 20, 15, or 10 cm/s. To begin, a floor translation of 20 cm/s was applied at 5- and 10-cm amplitudes. If the posterior peak of CoPy after the translation in either of these amplitudes was located between the EBL and EBL peak positions, 20 cm/s was adopted as the translation velocity. If not, the velocity was reduced or increased until the posterior peak of CoPy in either of these amplitudes was located between those positions. Second, the translation amplitude was determined. A linear regression line was drawn through the two coordinates of the floor translation amplitude and the posterior peak of CoPy at the velocity determined in step 1. Based on the line, the translation amplitude, at which the posterior peak would be located midway between the EBL and EBL peak positions was determined. The mean ± SD of the adopted translation velocity and amplitude was 20.4 ± 2.5 cm/s and 8.6 ± 2.1 cm in the young, and 12.9 ± 3.3 cm/s and 5.6 ± 1.5 cm in the elderly, respectively.
In the experimental session, two sets of trials (initial and second sets) were conducted, each set with at least 20 trials. Trials were excluded if foot position changes were observed or if deviation of CoPy over ± 1 cm from the QS position was noted before S2. A 30-s standing rest and a 3-min seated rest were taken between trials and sets, respectively.
Data analysis
All data analysis was performed using BIMUTAS II software (Kissei Comtec, Japan). To evaluate the magnitude of backward disturbance in response to the floor translation, the posterior peak of CoPy after S2 was identified in each trial, and the distance from EBL position to this peak position was defined as the displaced position of CoPy. If more than 20 trials were conducted within a set, the mean value of the displaced positions in the trials after the 20th was defined as the value in the 20th trial. Displaced positions from the 16th to 20th trials in the second set were averaged, and the value was defined as the position in the final phase. Moreover, all trials in each set were averaged to investigate the change in displaced position of the CoPy corresponding to the change in CNV. To check whether the posterior peak was between the EBL and EBL peak positions, the difference between these positions was calculated and compared with the displaced position.
EEG, EMG, and CoPy waveforms from 300 ms before S1 to 3,000 ms after S2 were averaged for each set. The mean amplitude for the 300-ms period before S1 was used as a baseline of averaging. Preceding EMG averaging, all EMGs were 40-Hz high-pass-filtered to exclude electrocardiographic and movement artifacts, and then full-wave-rectified (rEMG). Trials with eye blinks or movement artifacts (voltage at EOG or any EEG electrode exceeding ± 100 μV) between 300 ms before S1 and S2 were excluded from the averaging, and at least 12 trials were adopted for each set. In each averaged waveform, mean amplitudes for every 100-ms period were calculated. Waveforms recorded from Cz, in which late CNV was the maximum in both sets, were used for CNV analyses. The mean amplitude of the 100-ms period just before S2 was defined as the late CNV amplitude.
In order to investigate the relationship between the CNV peak and EMG activity before S2, averaged waveforms of EEG and rEMG were 4-Hz low-pass-filtered[12]. The EEG waveform exceeded baseline negatively in about 300 and 500 ms after S1 in the young and elderly, respectively. Therefore, the maximal negative potential from 500 ms after S1 to S2 was defined as the CNV peak, and its latency relative to S2 and amplitude from baseline were calculated (CNV peak latency and amplitude). Around the CNV peak, a continuous increase of TA background activity started, especially in the young. Some elderly subjects showed a transient increase of TA around the peak. Accordingly, the muscle increasing timing was identified as the point in which the amplitude first exceeded the mean amplitude of the S1-S2 period for >100 ms from around the CNV peak to S2, and the start time relative to S2 was calculated. In 4 of 26 cases, combining the initial and second sets for the young, and in 6 of 24 cases for the elderly, the increases of background activity were observed in other anterior muscles (RA: 8 cases; RF: 2 cases), and thus these muscles were analyzed.
Observing rEMG waveforms after S2 (Figure 2), the earliest burst activation was found in TA. Therefore, the following analyses were conducted for TA after S2 in the trials adopted for averaging. In each trial, the envelope line of the TA burst continuing at least 50 ms was identified by visual inspection of the EMG trace on a computer. The time point at which the TA burst deviated more than the mean +2 SD from the background activity of the standing posture before S1 was defined as the burst onset of TA, and the onset time from S2 was measured. To analyze the burst activity level, rEMG waveforms of TA in the period from −500 to +1,000 ms with respect to the burst onset were then averaged for each set. The averaged waveforms were smoothed using a 40-Hz low-pass filter, and then a peak was identified. Peak amplitude from the baseline and peak time with respect to burst onset were measured. Peak amplitude was normalized by the EMG mean amplitude for 3 s during EBL posture.
Statistical analysis
Shapiro-Wilks and Levene’s tests confirmed that all data satisfied the assumptions of normality and equal variance, respectively. To evaluate group differences in CoPy parameters, and CNV peak and late CNV amplitudes, Student’s t-test was used. To compare QS and EBL positions in this study with those in previous studies, and to investigate whether EMG activity before S2 was significantly increased relative to baseline, the one-sample t-test was used. A two-way analysis of variance (ANOVA) was used to assess the effects of trial and group (40 trials × 2 groups) on the displaced position of CoPy. When a significant main effect of trial was shown, the differences between the first trial and the other trials by group were evaluated using Dunnett’s test to further assess the adaptability in response to the floor translation. Two-way ANOVA was also performed to assess the effects of set and group (2 sets × 2 groups) on mean displaced position among 20 trials, CNV peak latency, start time of TA increase before S2, and onset time, peak amplitude, and peak time of TA burst. In order to evaluate the magnitude of correlation among adaptive changes of measurement parameters, differences between initial and second sets were calculated, and Pearson’s correlation was used between the differences in each parameter. The alpha level for all tests was set at p < 0.05. All statistical analyses were performed using SPSS 14.0 J software (SPSS Japan, Japan)