Introduction 2004 2004 2002 position task in static conditions 2002 2003 2003 2005 2007 1997 position tasks in dynamic conditions 2005a 2003 1981 1985 1985 2002 1985 2002 2002 1973 1987 2002 2003 1986 1997 In the present study, the sEMG signal was described with four parameters derived from the IPL method: (1) the mean muscle CV which was the average of the obtained MUP velocities; the two statistical distribution variables, which were: (2) the within-subject MUP velocities’ skewness [Sk-peak velocity (PV)] and (3) the within-subject MUP velocities’ standard deviation (SD-PV), and (4) the peak frequency (PF), a variable expressing the amount of MUP activity (number of peaks = MUPs) per second. prolonged dynamic position tasks dynamic position tasks short static position tasks Methods Subjects The study involved short static and prolonged dynamic experiments. Forty-one healthy and physically active males (24.8 ± 6.7 years, from 17 to 48) (mean ± SD) volunteered for the first experiment and 30 randomly chosen subjects from that group (25.4 ± 7.6 years, from 18 to 42) participated in both experiments. Exclusion criteria were drug abuse and the practice of bodybuilding. Three from a total of 44 subjects were excluded because of the impossibility to obtain a required correlation coefficient between the sEMG signals used to estimate the parameters’ values. The experimental protocol was conducted according to the Helsinki Declaration and approved by the local ethics committee. All participants gave written informed consent. Experimental set up 1991 2004 During the experiment subjects were sitting in a chair. The upper arm was slightly abducted and comfortably supported at 45° of the shoulder flexion, the forearm was free. When the elbow was stretched, the line of the upper arm-forearm was at 45° in relation to horizontal. When the elbow was flexed to the angle of 135°, the forearm was horizontal. The forearm was supinated during static and dynamic tests. Static tests Subjects were asked to hold the forearm horizontally (elbow angle was then 135°). A visual bar helped to maintain the correct (horizontal) position of the forearm. In the loaded tests a sack filled with lead and sand was placed in the palm. Three levels of force were applied in blocks that were 3 min apart: unloaded, loaded 10 and 20% MVC. A block consisted of three tests (three repetitions at the same level of force); every test lasted for 3.8 s and was within a block separated 30 s from one another. Dynamic tests All participants of the dynamic tests underwent previously static tests, separated by 5 min. Subjects were asked to swing the forearm from the stretched (elbow angle 180°) to horizontal position (elbow angle 135°), thus moving over an angle of 45°. They did it within a rate of 40 beats per minute (one up-and-down movement in one beat), given by a metronome sound. The visual bar indicated the horizontal position to which the lower arm returned after being stretched. Four force levels were applied: unloaded, and loaded 5, 10 and 20% MVC. The tests lasted for 4 min and were separated by 5 min. EMG recording 1985 1985 1985 1987 r r r 1998 r Data processing The signals were simultaneously A/D converted (sampling 10 kHz, 12 bits acquisition). Data were stored on a personal computer. The signal was analyzed with LabVIEW 6.1 that also facilitated a partial on-line analysis. The peak selection and the correlation coefficient assessments for both static and dynamic tests were performed on 0.2 s epochs. In the static tests, measurements were taken every second during 0.8 s (comprising 4 epochs of 0.2 s). A static test was of 3.8 s duration and was repeated three times for each force level. The statistical analyses were performed on the data of these three repeated tests taken together. In the dynamic tests, the data were assembled every 30 s during 14.4 s (comprising 72 epochs of 0.2 s). The test duration was 4 min. Peak selection 2002 The parameters The following calculations were performed: (1) basic calculation of PV, following the IPL method; (2) mean CV, expressed as an average value of the PVs; (3) within-subject skewness of the peak velocities (Sk-PV), expressed as a skewness of a PVs’ population of a subject; (4) within-subject standard deviation of the peak velocities (SD-PV), expressed as a standard deviation of a PVs’ population of a subject; and (5) peak frequency (PF), expressed as a number of peaks per second. Statistics 2005 P Results 1 r P P Table 1 Characteristics of 41 participants to the static tests; 30 of them participated in the dynamic tests as well  N Minimum Maximum Mean SD Age (years) 41 16 48 24.7 6.7 Height (cm) 41 166.0 197.0 183.2 8.0 Weight (kg) 41 60.0 95.0 74.3 9.2 Force (N) right 38 148.5 346.5 246.1 41.9 Force (N) left (for left-handed) 3 262.3 267.3 265.1 2.5 Skin thickness (mm) 41 1.4 4.2 2.4 .7 Circumference upper arm (cm) 41 24.0 31.0 27.5 2.0 a 41 30.50 39.50 35.4 2.0 b 33 24.0 30.0 26.4 1.6 c 33 30.0 36.0 33.1 1.6 Initial skin temp. (˚C) 41 30.8 33.8 32.3 .8 Temp. upper arm before dyn. tests (˚C) 30 30.8 34.2 32.3 .9 Temp. upper arm after dyn. tests (˚C) 30 32.2 35.7 33.9 1.1 Room temp. (˚C) 41 20.0 24.0 22.3 1.0 a b  c Static tests Mean muscle fibre conduction velocity (CV) 1 2 −1 P r P Fig. 1 Distribution of peak velocities of one subject during short static tests at three force levels: unloaded, and loaded 10 and 20% MVC. Note the shift of the velocities as a whole to the higher values with increasing level of force  Fig. 2 a CV b c SD d PF SD  Skewness of peak velocities (Sk-PV) 2 1 P 1 Standard deviation of peak velocities (SD-PV) 2 Peak frequency (PF) 2 P Dynamic tests Muscle fibre conduction velocity (CV) 3 4 P r P −1 −1 P P P −1 −1 −1 P P P Fig. 3 PVs 1 MVC  Fig. 4 PVs a CV b c SD d MVC MVC  Skewness of peak velocities (Sk-PV) 3 4 P 3 3 P P P P P P 3 Taken together, in the initial phase of activity, the proportion of fast peaks increased with increasing force. In the prolonged tests loaded up to 10% MVC, the proportion of fast peaks declined again over the first 2 min and then stabilized at about the level of the unloaded test. During the 20% MVC test, however, the proportion of fast peaks still tended to decline up to the end of the test, accompanied with a growing amount of slow peaks. Standard deviation of peak velocities (SD-PV) 4 P t P P P P P P P P P P P P P 3 Peak frequency (PF) 4 P P P P P Discussion Changes in the distribution of MUP velocities as an effect of (low) force and duration were described with four parameters: (1) the global parameter of mean CV, (2) the within subject skewness of a population of MUP velocities; (3) the within-subject standard deviation of MUP velocities and (4) the amount of MUP activity, expressed as MUP frequency. First we will comment on the four parameters. Next, using these parameters, we will discuss the main findings. The four parameters 1966 1986 1984 1986 1989 position tasks static and dynamic force 2002 Skewness is used as an sEMG parameter in the present study for the first time. This statistical measure of deviation from a normal distribution, in this case expresses the proportion between slower and faster MUPs within an individual. It will increase with the growing proportion of slow/tonic/fatigue resistant MUs and will decrease with the augmenting proportion of fast/phasic/fatigable MUs. All the estimates were moderately positively skewed, which indicates a relative excess of lower MUP velocities. 2002 static position tasks 2002 static force tasks −1 2005 1992 1997 1972 1973a 1997 1979 1989 1979 1982 1981 1986 1991 2002 static force tasks static position tasks 2005a The initial changes on increasing force levels (in the static and dynamic tests) 2 1 3 in force tasks 1983 1985 1987 1988 2002 1965 1973b 1992 2001 1986 1989 1994 2002 in force tasks short static 2005 In short, increases of mean CV with increasing forces in the initial phase of muscle activity may be a result of both recruitment of fast/phasic motor units and faster membrane propagation. Changes in the prolonged dynamic tests Tests loaded below 20% MVC 4 4 3 4 4 position tasks study static force tasks isometric 1988 1989 1991 1992 1993 1997 1987 1968 1991 position character 2005a b 2003 The MUP frequency variable, expressing the amount of MU activity produced as a result of recruitment and rate coding did not change during these tests. This suggests that all the changes in recruitment and rate coding do not, in principle, affect the total amount of MU activity.  All subjects were able to complete the tests, and the sEMG parameters became stable in the course of time as well. Thus, one can assume that the three tests at lowest force levels were non-fatiguing. Taken together, the results of these apparently non-fatiguing dynamic position tasks suggest that following the initially increased activation of fast MUs, the proportion shifts after about 2 min in favour of slower MUs. The amount of activity seems to remain stable throughout the duration of the tests. The test at 20% MVC 4 3 4 2004 dynamic force tasks 1986 1987 1990 1999 1981 1991 In short, during these apparently fatiguing dynamic position tasks, a global slowing of MUP velocities appears, suggesting a fatigued muscle membrane. The amount of MU activity seems to diminish progressively and finally the recruitment stops. 1 2004 In conclusion, we present a set of parameters derived from the interpeak latency method, which yields information about changes in MUP velocities’ distribution and amount of MUP activity. Skewness, standard deviation and peak frequency parameters appear to corroborate the results of a global muscle conduction velocity. Together they could contribute to quantifying the dynamics of motor unit activity and membrane fatigue. The interconnected results may be useful in ergonomics (for assessment of fatigue) and in sports (for eliciting specific capabilities, such as explosive or endurance capabilities).