Effects of repetitive dynamic contractions upon electromechanical delay. Accepted: 20 March 1998 ... result of changes in motor unit recruitment during the.
Eur J Appl Physiol (1998) 79: 37±40
Ó Springer-Verlag 1998
ORIGINAL ARTICLE
David A. Gabriel á Jean P. Boucher
Effects of repetitive dynamic contractions upon electromechanical delay
Accepted: 20 March 1998
Abstract The eect of repeated maximal eort isotonic contractions on electromechanical delay was studied. Over 4 days, 17 male subjects performed 400 rapid elbow ¯exion trials. The kinematics and surface electromyographic (EMG) activity of the biceps brachii of these subjects were recorded. The period from the onset of the EMG until the beginning of movement was de®ned as the electromechanical delay. The period from the beginning of movement until the end of the EMG was de®ned as the second component of the contraction. Over the 4 day period there was an increase in the speed of limb movement. The mean power frequency and the duration of the EMG during the electromechanical delay did not change, while the root-mean-square amplitude increased. The duration of the EMG during the second component of the contraction remained stable. The mean power frequency and the root-mean-square amplitude of the EMG during the second component of the contraction increased with the speed of limb movement. We conclude that the faster contractions were a result of changes in motor unit recruitment during the second component of the contraction, rather than in the electromechanical delay. Key words Maximal isotonic contractions á Ballistic elbow ¯exion á Adaptations in muscle activity á Electromyography á Dynamic exercise
D.A. Gabriel (&) Physical Therapy Department, School of Allied Health Science, Belk Building, East Carolina University, Greenville, NC 27858-4353, USA J.P. Boucher DeÂpartement de Kinanthropologie, Universite du QueÂbec aÁ MontreÂal, Case postale 8888, succursale Centre-Ville, MontreÂal, QueÂbec, Canada H3C 3P8
Introduction Ballistic limb movement is associated with a large burst of electromyographic (EMG) activity from the agonist muscle, which precedes limb movement, and the interspike interval of activated motor units is minimal and increases as the contraction progresses (Desmedt and Godaux 1977; Maton and Bouisset 1975). The period from the onset of the EMG until the beginning of movement is the electromechanical delay. This portion of the EMG represents the activation of motor units and shortening of the series elastic component of skeletal muscle. The period from the beginning of movement until the end of the EMG is de®ned as the second component of the contraction (Grabiner 1986). The electromechanical delay is aected by mechanical factors that change the rate of series elastic component shortening, such as the initial muscle length (Grabiner 1986; 1988) and muscle loading (Ives et al. 1993; Simmons and Richardson 1993). Fatiguing exercise lengthens the electromechanical delay (Zhou 1996). Power-trained athletes and other individuals who have a high percentage of fast-twitch muscle ®bers exhibit a short electromechanical delay (Kamen et al. 1981; Taylor et al. 1997). The work described in this paper explored the adaptability of the electromechanical delay to repetitive dynamic contractions. Repetitive dynamic contractions can train the motor units to ®re in synchrony (Hayes 1977, 1978; Moritani and Shibata 1994), which may decrease the electromechanical delay time.
Methods Subjects Seventeen untrained right-handed males signed an informed consent document in accordance with the American College of Sports Medicine guidelines concerning the rights and welfare of human subjects. None of the subjects were paid for their services.
38 Experimental design and apparatus
Electromyographic activity
Maximal eort isotonic training consisted of 100 ballistic elbow ¯exion trials to a target in the horizontal plane on four sessions. Twenty-four hours separated sessions one and two and sessions three and four, with a 1-week interval between sessions two and three. The subjects sat at a table with the shoulder abducted to 90°, the upper arm restrained, and the lower arm free to rotate about the elbow joint. The forearm was secured in a semiprone position to a horizontal angular displacement device with a ®xed axis of rotation, in line with the elbow joint. With the elbow in full extension (0°), subjects rotated their forearm by 75° about the elbow joint to reach the target as fast as possible. The target was an area between 75° and 90° of elbow ¯exion, and was marked on the table as a 15° colored wedge. The large target emphasized that the objective was to move as fast as possible. Angular displacement was recorded by a potentiometer at the axis of rotation of the horizontal angular displacement device, and event markers activated by microswitches established the time taken to move between 0° and 75°.
The electromechanical delay was de®ned as the dierence between the onset of the EMG and the initiation of movement (Fig. 1). To determine the onset of the EMG, a Computer algorithm calculated the mean and standard deviation of the ®rst 50 ``points'' of the EMG signal. The onset of the EMG was the ®rst point to rise above the 99% con®dence interval of baseline for 20 ms (DiFabio 1987). The ®rst detectable movement was measured by rotation of the forearm o a microswitch, which activated the ®rst event marker. The electromechanical delay was de®ned as the dierence between the onset of the EMG and the ®rst event marker. The second component of the contraction was de®ned as the time between the onset of movement and the termination of the EMG, as veri®ed by visual inspection using an interactive computer graphics program. The duration, the root-mean-square amplitude, and the mean power frequency of the EMG during both the electromechanical delay and the second component of the contraction were measured (Basmajian and De Luca 1985).
Data recording
There was no signi®cant trial eect, and the means of the ®rst and last ®ve trials of each training session were used for analysis. Two means represented the two blocks of ®ve trials (i.e., trials 1±5 and 96±100) within each session. A repeated measures analysis of variance was used to test for signi®cant dierences at a probability level of a 0.05, with F-ratios greater than F[1,16] 4.49 (Kirk 1968).
The EMG of the biceps brachii was recorded with surface electrodes (standard Beckman Ag/AgCl) placed 3 cm apart, in line with the muscle belly at the middle of the upper arm (Delagi and Perotto 1980). There is one motor point at the inner border of the center of the muscle belly, and two other points distal and lateral to this (Walthard and Tchicalo 1971); the electrode position was distant from these motor points. The EMG was ampli®ed (Grass P-511), band-pass ®ltered between 20 and 300 Hz, and digitized (MDAS 7000 A/D with an IBM PC) on a 2000-Hz channel. The displacement and event signals were also sampled at 2000 Hz. The displacement signal was low-pass ®ltered at 10 Hz with a fourth-order Butterworth digital ®lter, then dierentiated using ®nite dierences to obtain velocity.
Fig. 1 Partitioning the agonist electromyographic (EMG) burst: electromechanical delay (EMD) and the second component of the contraction (END). The top graph is a representative example of biceps brachii EMG activity. The bottom graph is a representative elbow angular velocity trace
Statistical analysis
Results The performance of 400 dynamic contractions resulted in a 32% (57-ms) decrease in elbow ¯exion time, while the peak angular velocity increased by 29% (122 deg á s)1). The increase in speed was associated with altera-
39 Fig. 2 Representative biceps brachii EMG activity. The ®rst 10 waveforms were produced during the ®rst session, the second set of 10 waveforms (11± 20) were generated during the second session until the last session, for a total of 40 waveforms
tions in biceps brachii EMG activity (Fig. 2). There was also an increase in the amplitude of the EMG as elbow ¯exion time decreased. There was a mild increase (4 Hz, or 3%) in the mean power frequency of the EMG during the electromechanical delay, but it was nonsigni®cant (F[1,16] 3.33; P < 0.09). The root-mean-square amplitude of the EMG during the electromechanical delay increased by 41% (0.311 mV). Long-term training (i.e., 2 weeks) had no aect on the duration of the EMG during the electromechanical delay. However, the performance of 100 dynamic contractions decreased the electromechanical delay time by 4% (3 ms) within each session. Before training, the mean power frequency of the EMG during the electromechanical delay was 6% (7Hz) greater than the second component of the contraction. The performance of 400 dynamic contractions increased (6%, or 7 Hz) the mean power frequency of the EMG during the second component of the contraction. This increase resulted in a mean power frequency that was 5% (6 Hz) greater than during the electromechanical delay. The root-mean-square amplitude of the EMG during the second component of the contraction increased by 46% (0.318 mV). The duration of the EMG during the second component of the contraction was not aected by training.
Discussion A short training schedule (i.e., 2 weeks) was used to minimize the possibility of changes in the EMG due to alterations in either the size or type of muscle ®bers. The same protocol has been shown to improve the speed of elbow ¯exion through neural control mechanisms and not strength (Lagasse 1979). The present study revealed
increases in the speed of limb movement in association with changes in biceps brachii EMG activity. We hypothesized that repetitive dynamic contractions would train the motor units to ®re in synchrony (Hayes 1977, 1978; Moritani and Shibata 1994) and shorten the electromechanical delay. Therefore, a decrease in the mean power frequency of the EMG during the electromechanical delay was expected. After 400 dynamic contractions, there was a tendency towards an increase. This ®nding corroborates research where subjects performed 240 dynamic contractions over 8 days (Boucher and Flieger 1985). The mean power frequency of the EMG during the electromechanical delay increased by 9% (8 Hz), but failed to reach the 0.05 probability level of signi®cance. Both ®ndings suggest that the mean power frequency during the electromechanical delay can change. Adaptation may require a chronic training program, such as the 1,400 contractions utilized by Corcos et al. (1993). This is consistent with the observation that the interspike interval is already minimal during the electromechanical delay (Maton and Bouisset 1975). The present study showed that the performance of 100 dynamic contractions decreased the duration of the electromechanical delay within each session. A potentiation of muscular contractions may have occurred through alterations in cellular mechanisms which aect the rate of excitation and contraction coupling, such as an increase in calcium sensitivity and availability (Small and Stokes 1992). In support of contraction-induced potentiation, it has been shown that maximal voluntary contractions can produce a substantial increase in the rate of isometric twitches (Vandervoort et al. 1983). Another possibility is that the performance of 100 consecutive maximal eort contractions resulted in an increase in muscle temperature. A mean rise of 3.1°C has been shown to decrease the time taken to reach peak
40
tension in the human triceps surae muscle (Davies and Young 1983). Before intensive training, the mean power frequency and the root-mean-square amplitude of the EMG during the electromechanical delay was greater than that of the second component of the contraction. This is consistent with the observation that the interspike interval of activated motor units increases during ballistic contractions (Desmedt and Godaux 1977; Maton and Bouisset 1975). After 400 dynamic contractions, the mean power frequency of the EMG during the second component of the contraction increased to a level that was greater than that of the electromechanical delay. The data presented here suggest that the second component of the contraction was more responsive to training than was the electromechanical delay. To overcome the increase in the interspike interval that occurs during the second component of the contraction, more fast-twitch muscle ®bers must be recruited as part of the force gradation process (Mariani et al. 1980). Therefore, as the speed of limb movement increases, the second component of the contraction must exhibit an adaptive response. Changes in the EMG during the second component of the contraction suggest that training resulted in the recruitment of more fast-twitch motor units, or that the subjects learned to increase the ®ring frequency of already active motor units. The results of this study have revealed that the performance of 400 dynamic contractions produced no change in the mean power frequency and the duration of the EMG during the electromechanical delay, but caused an increase in the root-mean-square amplitude. The duration of the EMG during the second component of the contraction remained stable. However, both the mean power frequency and the root-mean-square amplitude of the EMG during the second component of the contraction increased with the speed of limb movement. We conclude that the faster contractions were a result of changes in motor unit recruitment during the second component of the contraction, rather than in the electromechanical delay.
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