Eur J Appl Physiol (1991) 63:444-448
Applied Physiology European Journal of
and Occupational Physiology © Springer-Verlag 1991
Recovery of the human biceps electromyogram after heavy eccentric, concentric or isometric exercise G. W. Kroon and M. Naeije Academic Centre Dentistry Amsterdam, ACTA, Louwesweg 1, NL-1066 EA Amsterdam, The Netherlands
Accepted July 8, 1991
Summary. Five men performed submaximal isometric, concentric or eccentric contractions until exhaustion with the left arm elbow flexors at respectively 50%, 40% and 40% of the prefatigued maximal voluntary contraction force (MVC). Subsequently, and at regular intervals, the surface electromyogram (EMG) during 30-s isometric test contractions at 40% of the prefatigued MVC and the muscle performance parameters (MVC and the endurance time of an isometric endurance test at 40% prefatigued MVC) were recorded. Large differences in the surface EMG response were found after isometric or concentric exercise on the one hand and eccentric exercise on the other. Eccentric exercise evoked in two of the three EMG parameters [the EMG amplitude (root mean square) and the rate of shift of the EMG mean power frequency (MPF)] the greatest (P< 0.001) and longest lasting (up to 7 days) response. The EMG response after isometric or concentric exercise was smaller and of shorter duration (1-2 days). The third EMG parameter, the initial MPF, had already returned to its prefatigued value at the time of the first measurement, 0.75 h after exercise. The responses of EMG amplitude and of rate of MPF shift were similar to the responses observed in the muscle performance parameters (MVC and the endurance time). Complaints of muscle soreness were most frequent and severe after the eccentric contractions. Thus, eccentric exercise evoked the greatest and longest lasting response both in the surface EMG signal and in the muscle performance parameters. Key words: Muscle fatigue - Recovery - Surface electromyogram - Biceps muscle
Introduction Intense muscle exercise leads to deterioration of muscle performance and to changes in the surface electromyoOffprint requests to: M. Naeije
gram (EMG) signal. This has been reflected in a decrease in muscle strength (Edwards et al. 1977) and in muscle endurance capacity (Funderburk et al. 1974), in an increase in EMG amplitude (Bigland-Ritchie 1981) and in a shift to lower frequencies of the EMG power spectrum (Lindstrom et al. 1970; Petrofsky and Lind 1980; Naeije and Zorn 1981). The changes in the EMG power spectrum especially have been suggested to be closely related to the processes leading to fatigue (Lindstrom et al. 1977; Naeije 1984). Due to the biochemical processes in the muscle tissue during exercise there is a decrease in conduction velocity of the muscle fibre action potentials along the membrane. This, together with changes in the discharge frequencies of the motor units (Lippold et al. 1960) and the recruitment of new, nonfatigued motor units (Maton 1981) have probably accounted for the gradual shift to lower frequencies of the EMG power spectrum during sustained isometric contractions. The rate of frequency shift to lower frequencies especially has been correlated to the endurance capacity of the muscle and has been advocated as a measure of local muscle fatigue (Lindstrom et al. 1977; Naeije 1984). However, studies have indicated a discrepancy between the times needed for the maximal voluntary contraction force (MVC) and for the EMG signal to recover to their normal values after exercise (Hara 1980; Moxham et al. 1982; Hultman and Sjoholm 1983; Kuorinka 1988). The recovery in MVC is said to take longer, ranging from 10 min to a few hours depending upon the experimental protocol, than the few minutes needed for the EMG signal to recover. This would contra-indicate the use of surface EMG spectral parameters as indicators of muscle fatigue. However, in most muscle fatigue studies the recovery in either muscle performance or the EMG signal has been investigated. Recently, simultaneous recordings of muscle performance and of the surface EMG signal have been made up to 25 h after dynamic exercise of the human biceps muscle to exhaustion (Kroon and Naeije 1988). This study has indicated that after heavy dynamic exercise the recovery rate of the EMG
445 signal was s i m i l a r to the rate o f recovery o f m u s c l e perf o r m a n c e . H o w e v e r , c o m p l e t e recovery was n o t a c h i e v e d i n the 25 h. E c c e n t r i c c o n t r a c t i o n s are k n o w n to be m o r e h a r m ful to the m u s c l e t h a n c o n c e n t r i c c o n t r a c t i o n s . This has b e e n s h o w n to be true b o t h for m o r p h o l o g i c a l a n d hist o c h e m i c a l c h a n g e s w i t h i n the m u s c l e ( F r i d e n et al. 1981; J o n e s et al. 1986) a n d for i n d u c i n g l o n g - l a s t i n g fatigue effects u p o n m u s c l e strength ( S a r g e a n t a n d D o l a n 1987; J o n e s et al. 1989). Thus, for the E M G signal to be a n i n d i c a t o r o f m u s c l e fatigue, eccentric contractions s h o u l d evoke the greater a n d l o n g e r lasting c h a n g e s in the E M G signal as well. W h e t h e r eccentric c o n t r a c t i o n s i n d e e d evoke these c h a n g e s has n o t yet b e e n investigated. The a i m o f this s t u d y was to c o m p a r e the recovery in surface E M G characteristics after isometric, c o n c e n t r i c or eccentric exercise to e x h a u s t i o n . F u r t h e r m o r e , the E M G recovery rate was c o m p a r e d to the rate o f recovery o f the m u s c l e p e r f o r m a n c e p a r a m e t e r s .
Subjects. Five healthy untrained male subjects (20-35 years) participated in this experiment. They were not amongst the subjects participating in the previous investigation (Kroon and Naeije 1988). Informed consent was obtained prior to the experiments. Experimentalprocedure. EMG recordings were made using a pair of silver amalgam surface electrodes. The electrode diameter was 3 mm and the interelectrode distance was 12 mm (centre-centre). The electrodes were applied to the skin parallel to the main direction of the muscle fibres of the common belly of the left biceps muscle. Before applying the electrodes the skin was thoroughly cleaned with alcohol and the positions of the electrodes were marked, enabling accurate repositioning of the electrodes on subsequent days. Measurements were performed before and after inducing muscle fatigue. During the measurements the subjects were seated upright in a chair with a rigid back support. The left arm was flexed to an angle of 90° during the isometric test contractions providing the same muscle length for all test contractions (Okada 1987; Inbar et al. 1987). The upper arm was supported horizontally. A force transducer was connected to the subject's wrist by a nonextensible strap, To control the arm position an alarm signal sounded if the arm touched a small metal bar surrounding the wrist at a few millimeters distance. The MVC and the endurance time for an isometric contraction at 40% of the pre-exercise MVC were used to evaluate muscle performance. The EMG signals were recorded during 30-s isometric test contractions at 40% of the pre-exercise MVC. The EMG signals were amplified 200-500 times. At every second of contraction an EMG record of 0.7 s was digitized by a computer (Declab 11/ 03, Digital Equipment) at a sampling rate of 2048 samples-s -1 and with a 12 bits resolution. The samples were stored on a Winchester disk. Before sampling, the signals were low-pass filtered (Krone-Hite filter, cut-off frequency 1000 Hz). The mean power frequency (MPF) and the root mean square (RMS) of each record were calculated (Kwatny et al. 1970). The MPF and the RMS at the start of each contraction (MPFo and RMSo, respectively) and the rate of change to lower frequencies of the MPF value [d(MPF)/dt] during the contraction were calculated by applying the linear least squares approximation to the 30 data points. Muscle fatigue was induced by either isometric, or concentric or eccentric muscle contractions to exhaustion. The interval time between experiments was at least 3 weeks. This particular se-
quence of experiments was chosen because eccentric contractions are known to induce the greatest and longest lasting muscle fatigue effects. Each experiment had three phases. In the first phase, the preexercise phase, reference values for the muscle performance parameters and the EMG parameters were recorded. For the MVC the subject had to perform three MVCs for 3 s each at 5-rain intervals. The highest value was used as the reference. Then the subject exerted three isometric contractions at 40% of the MVC at 5 min intervals. On two occasions the subject sustained the contraction for only 30 s, the third time he was asked to sustain the contraction until exhaustion. The endurance time of this test was used as the reference value of the endurance capacity at the 40% level, whereas the first 30 s were used to obtain values for the EMG parameters. The average EMG values of the three 30-s contractions were used as references. In the second phase, the exercise phase, muscle fatigue was provoked by either isometric, concentric or eccentric contractions. Each contraction lasted 3 s and was followed by a 2-s rest period. In between sets of ten contractions there was an extra 1-min rest period. The concentric and eccentric contractions were performed by lifting or lowering a load with the angle between the upper and the lower arm varying between 30° and 105° . To obtain comparable durations of muscle exercise, the isometric exercise was performed at 50% of the pre-exercise MVC and the concentric or eccentric exercise at 40% MVC. At exhaustion the load was reduced to 40% and 30%, respectively, and once again the contractions were performed until exhaustion. The subjects were considered to be exhausted when they could not perform the desired contractions more than three times in a row. During the third phase, the recovery phase, the MVC was recorded 0.5 h, 1.5 h and 3 h after exhaustion and then after 24 h, 48 h and so on until recovery. At 0.75 h and 1.75 h after exhaustion the EMG signal was recorded during a 40% MVC 30-s test contraction. At 3.25 h after exhaustion and then daily, 0.25 h after the MVC measurement, a 40% MVC endurance test was performed. The first 30 s of each endurance test were also used to record the EMG parameters. Muscle soreness was evaluated up to 10 days after the exercise. Each subjects was asked whether his arm felt heavy, the muscle was tender upon palpation, muscle pain was experienced during movement of the elbow joint and the muscle soreness limited the range of motion.
Statistics. Data were subjected to paired Student's t-tests. Probability levels P