Motor Control, 2014, 18, 55-75 http://dx.doi.org/10.1123/mc.2012-0056 © 2014 Human Kinetics, Inc.

Effects of Submaximal Eccentric Exercise on Muscle Activity at Different Elbow Joint Angles Katarzyna Kisiel-Sajewicz, Anna Jaskólska, Damian Janecki, Renata Andrzejewska, Jarosław Marusiak, and Artur Jaskólski Our study aimed to determine whether electrical and mechanical factors contributing to acute or long-term maximal torque reduction and muscle soreness due to submaximal eccentric exercise (ECC) are elbow-joint-angle specific and to what extent the joint angle affects the contribution of antagonist coactivation to this torque reduction. Maximal isometric torque (MIT), muscle soreness assessment, agonist electromechanical activities, and antagonist coactivation during the maximal voluntary contraction (MVC) were measured at elbow joint angles of 60°, 90°, and 150° before ECC, immediately after exercise, and 24, 48, 72, and 120 hr after exercise. ECC causes an immediate decrease in MIT as well as increased antagonist coactivation at three angles. Antagonist coactivation returned to its baseline level at 24 hr regardless of joint angle. The most rapid torque recovery and the highest force level at which pain occurred were found after ECC at a joint angle of 60°. During the recovery period, no mechanomyographical changes were observed when measuring surface mechanomyography changes at three angles, while the electrical activity differed between angles.

Eccentric exercise (ECC), which has been found to be more effective at increasing force output than isometric or concentric muscle contractions, is becoming popular in sports and rehabilitation training (Di Monaco, Vallero, Tappero, & Cavanna, 2009). However, improper periods of rest of the muscles involved frequently during ECC precede a far more disabling injury when the muscles are used again. An unaccustomed ECC regimen results in a prolonged decrease in muscle force output (Nosaka, Clarkson, McGuiggin, & Byrne, 1991) and pain (delayed onset muscle soreness, DOMS) within 24–48 hr after ECC (Fridén, Sjöström, & Ekblom, 1983). The force reduction induced by submaximal ECC may be due to alterations in excitation-contraction coupling related to damage of the fiber membrane (Chen, Nosaka, Sacco, 2007a; Corona, Balog, Doyle, Rupp, Luke, & Ingalls, 2010; Jones, Newham, Round & Tolfree, 1986), changes in the capabil-

The authors are with the Dept. of Kinesiology, University School of Physical Education, Wroclaw, Poland.   55

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56  Kisiel-Sajewicz et al.

ity to generate tension at the sarcomere level (Morgan & Allen, 1999; Newham, Mills, Quigley, & Edwards, 1983; Brockett, Morgan, Gregory & Proske, 2002), and a decrease (central modulation) in voluntary motor drive, perhaps related to the presence of muscle soreness (Racinais, Bringard, Puchaux, Noakes, & Perrey, 2008). The damage can lead to a shift in the muscle active length–tension relation in the direction of increasing muscle length (Morgan, 1990), which may cause angle-specific acute and prolonged muscle force decrease due to ECC. The force impairment after submaximal (Prasartwuth, Allen, Butler, Gandevia, & Taylor, 2006) and maximal (Saxton & Donnelly, 1996) ECC is joint-angle specific, with the greatest force reduction at the most acute elbow angle (50°), followed by 90° and 150° (Saxton & Donnelly, 1996) and with voluntary drive limiting muscle performance, especially at short lengths (Prasartwuth et al., 2006). This relationship can be attributed to a different contribution of antagonist muscle activation (antagonist muscle coactivation). The mechanism behind this link remains unclear. Considering a protective role of antagonist muscles (Baratta, Solomonow, Zhou, Letson, Chuinard, & D’Ambrosia, 1988), we expected the findings of this study to expand the current understanding of the effect of ECC on the drive to agonist and antagonist muscles in controlling a motor task at different joint angles. Based on the studies of Mullany, O’Malley, St Clair Gibson, and Vaughan (2002) and Jaskólski, Andrzejewska, Marusiak, Kisiel-Sajewicz, and Jaskólska (2007), which support the existence of a functional coupling (common drive; Jones, Newham, Round, and Tolfree 1986), we hypothesized a similar antagonist coactivation independent of joint angle. Moreover, in the flexed and extended positions, the sarcomere system may not be at its optimal length, electromechanical coupling failure can be amplified (Sacco, McIntyre, & Jones, 1994), and the level of excitation of motoneurons can be influenced directly by the Golgi receptors and indirectly by CNS motor commands modified by the afferent proprioceptive information at a given position (Christova, Kossev, & Radicheva, 1998). Consequently, the decrease in electromyographic (EMG) and mechanomyographic (MMG) parameters due to submaximal ECC may be greater than those measured at a 90° angle. The difference in ECC-related MMG changes at various joint angles might result from active and passive stiffness changes at dissimilar muscle lengths (Whitehead, Weerakkody, Gregory, Morgan, & Proske, 2001). Although Bajaj, Madeleine, Sjogaard, and Arendt-Nielsen (2002) used surface MMG and EMG measurements of the first dorsal interosseous muscle to assess DOMS, they only analyzed the amplitude of both signals for an agonist muscle at one joint angle immediately, 24 hr, and 48 hr after maximal ECC. Kroon and Naeije (1991) analyzed recovery (up to 7 days) of the biceps brachii electromyogram after maximal ECC; however, they did not measure the antagonist muscle activity, and all measurements were performed at one joint angle. Our study aimed to determine whether electrical and mechanical factors contributing to acute or long-term maximal torque reduction and muscle soreness due to submaximal ECC are elbow-joint-angle specific and to what extent the joint angle affects the contribution of antagonist coactivation to this torque reduction. This information is relevant to improving the efficiency of training and rehabilitation programs and avoiding injuries arising from an attempt to perform unaccustomed ECC.

Effects of Submaximal Eccentric Exercise   57

Methods

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Subjects Eighteen untrained men volunteered to participate in this study (mean age, 23 ± 1.3 years old; mean height, 178 ± 3.8 cm; mean body mass, 79 ± 10.5 kg; 16 right-handed). In accordance with the Helsinki Declaration, all subjects gave written informed consent for participating in the study after being told about the nature of the experiment in detail. A health history questionnaire was given to all subjects to ensure they were free of any neurological, muscular, or skeletal disorders and not taking anti-inflammatory medication that might have interfered with the investigation. None of the subjects were involved in any specific training program before or during the study. Subjects reported they were not following a specific diet or taking dietary supplements and were instructed to maintain their normal diet and level of daily activity. The experimental procedure was approved by the Research Ethics Committee of Wroclaw Medical University.

Experimental Design To evaluate the effect of ECC exercise on muscle activity immediately after ECC exercise and during recovery (up to 120 hr), all measurements were performed during five testing sessions: before beginning ECC (baseline), immediately after ECC (first session), and 24 (second session), 48 (third session), 72 (fourth session), and 120 hr (fifth session) after ECC (Figure 1). A within-subject repeated-measure design was used to assess the effects of submaximal ECC on all muscle activity measures at three elbow joint angles (60°, 90°, and 150°). Preliminary testing was carried out to design a protocol to induce DOMS and reduce force significantly using one series of submaximal ECC over a short time period. The elbow joint angles of 60°, 90°, and 150° (with 180° being full elbow extension) were chosen based on the joint angle-torque relationship (Kulig, Andrews, & Hay, 1984) for the shortest (60°) and longest muscle lengths (150°), providing comfortable positions for the measurement of the maximal voluntary contraction (MVC).

Experimental Procedures The study consisted of five sessions at the same time of day (± 2 hr, 3:00–5:00 p.m.; Figure 1). To ensure that the subjects maximally activated a muscle at each measurement time, they were familiarized with the isokinetic and isometric dynamometer and testing procedure. Two days before the testing day (day 0), subjects were trained to exert the maximal isometric torque (the torque of two consecutive maximal isometric contractions was not to differ by more than 5%). The average of the two trial scores was used for analysis, as this value is usually the best indication of a subject’s typical performance and is more reliable than the best score (Heinonen, Sievanen, Viitasalo, Pasanen, Oja, & Vuori, 1994). In addition, we checked the offline reproducibility (intraclass correlation coefficient [ICC]) of the MIT and found very high ICC values (the lowest value was 0.925 at 48 hr post-ECC, and the highest was 0.972 at the baseline). The EMG/MMG probes were placed in the center of the belly of the BB (short head) and TB (lateral head) to ensure the quality of the MMG signals. The position of the surface electrode was marked on the skin with a waterproof permanent marker to ensure the same electrode placement throughout the study.

58

Figure 1 — Schematic representation of the testing protocol. Control = measurements before ECC (baseline); Im post = measurements immediately after ECC (acute effect); post 24, 48, 72, 120 h = measurements at the respective hours after ECC (long lasting effect); MVC = maximal voluntary contraction. Two trails of MVC with simultaneous recordings of EMG and MMG were taken at each joint angle during each session, and there was 3-min rest between the two MVC at each joint angle, a 2-min rest between measurements at different joint angles.

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Effects of Submaximal Eccentric Exercise   59

The first testing session (session 1) consisted of measurements taken before and immediately after 25 submaximal ECC (acute effect; Figure 2). Sessions 2, 3, 4, and 5 consisted of measurements conducted during the recovery period (Figure 1). Before each session, the subjects performed a standard warm-up (10 repetitions of upper limb bends at the elbow joint in the sagittal plane without an external load and with 1.5 kg and 2.0 kg, followed by a 6-min rest). The EMG and MMG activities of BB (and EMG of TB) and the corresponding MIT were assessed at three elbow joint angles (60°, 90°, and 150°) during each of the five sessions (the same order of joint angles was used before and after ECC, Figure 2) and analyzed in the time and frequency domains. The three signals were continuously and simultaneously acquired and saved on a personal computer connected to the Biodyna system (data acquisition and analysis system for EMG, MMG, and force signals, University of Physical Education, Wrocław, Poland). Two MVC trials were used, with a 3-min rest between the two MVC tests at each joint angle, a 2-min rest between measurements at different joint angles, and a 4-min rest between the MVC measurement and ECC. The laboratory temperature was maintained at between 20 °C and 23 °C. The subject and two test monitors were in the testing room.

MIT Measurement The MIT of the elbow flexors of the nondominant hand was measured by a Biodex System 3 instrument (Biodex Medical Systems, Shirley, NY, USA; Felici, Colace, & Sbriccoli, 1997) at the three joint angles. The force signal was amplified and recorded (sampling rate 10 kHz) on a personal computer connected to the Biodyna system (Jaskólska, Kisiel, Brzenczek, & Jaskólski, 2003; Jaskólska, Kisiel-Sajewicz, Brzenczek-Owczarzak, Yue, & Jaskólski, 2006).

Figure 2 — Experimental setup.

60  Kisiel-Sajewicz et al.

Subjects were instructed to exert MIT as fast and hard as possible when an auditory signal was emitted by a computer and to release (relax) the force as fast as possible at when a second auditory signal was emitted (3 s later). The peak torque of each 3-s test was determined.

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Eccentric Exercise Each subject was seated in a Biodex System 3 dynamometer with his nondominant arm flexed to approximately 40°, forearm in a supine position, and hand holding the handle of the arm attachment (Figure 2). The rotational axis of the dynamometer was aligned to the epitrochlea–epicondyle axis of the arm. Subjects were required to resist elbow extension against the force generated by the dynamometer, which was set to yield at 50% of the subject’s MIT recorded at the 90° joint angle. The same absolute force was used throughout the entire range of motion (the average relative force during exercise was ~72% MVC for 60° and ~74% MVC for 150°). The movement speed was set at 60° x s–1. Subjects were instructed to exert just enough force to resist the movement within a range of 60° to 150° (180° = full extension), not so much force as to stop the machine. Subjects were instructed to relax their arms as soon their arms reached the final position (150°), after which the operator helped each subject move his arm back to its initial position within 2 s. The process was repeated 25 times, for a total exercise time of approximately 75 s.

Surface EMG and MMG Recording During the MVC, the EMG and MMG signals were recorded using probes placed on the BB and TB. An air-coupled condenser microphone (BCM 9765, BeStar Acoustic, Jiangsu, China; diameter 9.7 mm, weight 18 g, 54-dB sensitivity, 1 Pa = 7 mV) placed between the active EMG electrodes was used to record the MMG signal, as described in Jaskólska et al. (2006). The distance between the microphone and the surface of the skin was 9 mm. The EMG was measured using two custom-made bipolar active surface Ag/ AgCl electrodes, 4 mm in diameter, with an interelectrode distance of 25 mm. The EMG electrodes were connected to a Texas Instruments (Dallas, USA) differential preamplifier built into the probe (with electrical characteristics at specified Vcc ± 5 V, gain 54 dB, noise 18 nV/ÖHz, total harmonic distortion 0.003%, bandwidth 7 kHz [5 Hz—7 kHz], input resistance 1012 W, CMRR [common mode rejection ratio] 100 dB, supply voltage rejection ratio 100 dB, supply current 5 mA). Electrodes were placed on the belly of the muscle, along its longitudinal axis, and attached to the skin with double-sided adhesive tape. The ground electrode was fixed to the dominant hand wrist. The skin was cleaned with alcohol before electrode placement. The EMG signal was amplified by 500 (S/N ratio ~8.0 × 103), and the MMG signal was amplified by 1–20 (S/N ratio ~3.43 × 106). The EMG and MMG signals were then sampled at a rate of 10 kHz, band-pass filtered at 20–500 Hz and 3–120 Hz, respectively, and converted to a numerical format with a 14-bit A/D board (Analog Devices, Norwood, MA, USA). The period with the highest MIT value was selected for averaging and analyzing the EMG and MMG signals using Biodyna software. The countermeasures used to avoid crosstalk and their reliability have been described elsewhere (Jaskólska et al. 2003; Jaskólska et al. 2006).

Effects of Submaximal Eccentric Exercise   61

Muscle Soreness Assessment The force levels at which any pain occurred (expressed as a percentage of a subject’s MIT recorded before ECC with no pain at 60°, 90°, and 150° joint angle) was used to assess muscle soreness. We used the same set-up as for MIT recording, but the subjects were instructed to develop the force slowly and to stop (relax) immediately when any pain was experienced.

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Data Analysis The EMG and MMG signals from the BB were analyzed in both the time (RMS) and frequency domains. Within the 5-s recordings of the EMG and MMG signals, the most stable 1024-ms epoch (covering the most steady force signal after achievement of maximal force) was chosen based on visual inspection. This 1024-ms epoch was used to analyze the EMG signal of the TB. The EMG and MMG amplitudes in the 1024-ms epoch were quantified by calculating the RMS for each muscle. The median frequencies (MdF) of the EMG and MMG signals were determined offline with 1.22-Hz frequency resolution without overlap within the same epoch using a fast-Fourier transform (FFT) algorithm. The RMS EMG TB/BB ratio (as a coactivation index), MIT/EMG RMS ratio (as an index of neuromuscular efficiency), and MdF MMG/MIT ratio (as an index of mechanical output) were then calculated. The MMG reflects both the contractile properties of a muscle in the longitudinal direction and the viscoelastic properties in the perpendicular direction (Uchiyama and Hashimoto, 2011). In the longitudinal direction, the stiffness increases during contraction (Hatta, Sugi & Tamura, 1988), and the longitudinal component of the stiffness change affects the measurement in the transverse direction (Tsuchiya, Iwamoto, Tamura & Sugi, 1993). The longitudinal and transverse stiffness changes during an isometric tetanus decreased linearly with increasing sarcomere length, indicating that the stiffness changes during contraction reflect the formation of cross-links between the myofilaments (Hatta et al., 1988). Nevertheless, Tsuchiya et al. (1993) found that the stiffness decreased in the transverse direction but increased in the longitudinal direction in frog skeletal muscle. Based on these mechanical models of the MMG system and stiffness concepts, the MdF MMG/MIT ratio (an index of the transverse mechanical output) is thought to refer to the transverse muscle oscillation related to the transverse stiffness and could be related to the fact that an increase in the passive stuffiness during contraction may reduce the muscle fiber shortening capacity. Therefore, the changes in the MdF MMG/MIT ratio post-ECC could be attributed to the specific influence of the passive elements of the muscle mechanical model on each of the two mechanical outputs (force and muscle dimensional changes).

Statistical Analyses In our study, the same subjects participated in all conditions of the experiment. We compared two within-subject factors: measurement time and elbow joint angle. To test for changes from the baseline to the postexercise data, a factorial repeated-measures ANOVA with time after ECC (six levels: before ECC, immediately after ECC, and 24, 48, 72, and 120 hr after exercise) and joint angles (three levels: 60°, 90°, and 150°) as within-subject factors was conducted after the normal distribution of the data were verified. All variables satisfied normality assumptions in

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a Shapiro-Wilk test (p < .05).  A simple contrast was used to find the specific difference between the joint angle and measurement time. We calculated the confidence interval for all variables to determine the reliability of our estimations (Tables 1 and 2). In addition, we checked the offline reproducibility using the ICC of the MIT. Table 1  Group Average, Standard Deviation (SD), and Confidence Interval (CI) of Absolute Values of MVC Torque (MIT), RMS Amplitude, and Median Frequency of MMG and EMG Signals (RMS MMG, RMS EMG, MdF MMG, and MdF EMG, Respectively) for the Biceps Brachii and the Triceps Brachii (TB) Coactivation at Three Tested Elbow Joint Angle Before Eccentric Contractions Elbow Joint Angle Parameters

60°

90°

150°

109

162

112

MIT [Nm] SD 23.5

SD 37.8

SD 33.5

CI 10.86

CI 17.46

CI 15.47

RMS MMG [mV] 18.2

22.6

17.4

SD 15.9

SD 16.3

SD 11.7

CI 7.35

CI 7.53

CI 5.41

MdF MMG [Hz] 26.4

25

27

SD 4.9

SD 5.2

SD 6.3

CI 2.26

CI 2.40

CI 2.91

RMS EMG [μV] 693

718

619

SD 262

SD 316

SD 264

CI 121

CI 146

CI 122

MdF EMG [Hz] 92.2

92.8

78.0

SD 7.4

SD 9.7

SD 11.5

CI 3.4

CI 4.5

CI 5.3

0.17

0.15

0.16

SD 0.10

SD 0.08

SD 0.09

CI 0.05

CI 0.04

CI 0.04

TB coactivation

Note. Biceps brachii and triceps brachii (TB) coactivation expressed as a ratio of RMS EMG TB/BB muscle.

Effects of Submaximal Eccentric Exercise   63

Table 2  Confidence Interval (CI) of Relative Values of MVC Torque (MIT), RMS Amplitude, and Median Frequency of MMG and EMG Signals (RMS MMG, RMS EMG, MdF MMG, and MdF EMG, Respectively) for the Biceps Brachii and the Triceps Brachii (TB) Coactivation (Expressed as a Ratio of RMS EMG TB/BB Muscle) Immediately After Eccentric Contractions and During 120 Hr of Recovery at Different Joint Angle Parameters

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MIT

RMS MMG

MdF MMG

RMS EMG

MdF EMG

TB coactivation

Angle

Im post

24 h

48 h

72 h

120 h

60°

9.3

8.9

12.3

13.7

11.7

90°

9.2

8.8

11.5

14.7

12.4

150°

6.9

8.6

14.2

13.4

11.3

60°

147

85.7

190

267

163

90°

49.4

25.7

68.4

24.2

82.2

150°

26.4

23.3

34.4

26.3

132

60°

9.1

14.1

8.9

10.5

10.3

90°

15.4

15.4

10.8

12.6

12.8

150°

7.4

17.2

14.1

15.0

14.6

60°

8.5

16.9

10.8

11.3

28.7

90°

16.1

13.8

17.9

13.2

35.3

150°

5.2

6.1

11.2

10.4

6.34

60°

3.7

3.9

3.9

2.9

3.2

90°

4.3

2.7

3.5

3.1

3.6

150°

3.1

4.1

3.5

4.1

3.9

60°

16.2

16.2

13.8

15.2

20.3

90°

6.0

10.2

10.2

9.2

12.9

150°

7.8

10.2

14.3

17.1

6.9

Note. Biceps brachii and the triceps brachii (TB) coactivation (expressed as a ratio of RMS EMG TB/ BB muscle). Im post = immediately after ECC; h = hours after ECC.

The data were presented as the mean and standard deviation of values normalized with respect to the baseline (pre-ECC) at each respective joint angle and expressed as percentage change. The level of statistical significance was set at p < .05. Data were analyzed using SPSS 14.0 (IBM Corp., Armonk, NY, USA).

Results Table 1 contains the absolute values of the group average values (standard deviation and confidence interval) of MIT, EMG, and MMG parameters and antagonist coactivation considered as 100% (pre-ECC).

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Maximal Isometric Torque Factorial repeated-measures ANOVA revealed a significant main effect of angle (F[2,34] = 4.25, p < .05), measurement time (F[2.311,39.28] = 13.99, p < .05), and the interaction between them (F[10,170] = 2.66, p < .05). The acute effect of ECC on MIT was similar in all three joint angles: compared with the baseline, there was a significant decrease in MIT immediately post-ECC, with no difference between angles (Figure 3, Figure 4a). However, the MIT recovery (long-term effect) differed between angles: it was faster at 60° (after 72 hr) than at 150° (after 120 hr) and 90° (did not recover within 120 hr; p < .05). In addition, the values at 90° were lower than those at 60° (p < .05). On the last day of recovery (120 hr), the MIT value at 90° was also lower than that at 150°.

Muscle Soreness For the force level at which any pain occurred, statistically significant effects were found for joint angle (F[1.634,27.28] = 6.05, p < .05) and measurement time (F[2.747,46.694] = 37.08, p < .05), while no effect for the interaction between them (p > .05) was found (Figure 4b). The lowest level of torque at which pain was sensed occurred at 24, 48, and 72 hr post-ECC and at a joint angle of 150°, with statistically significant differences at 48 hr (150° vs. 60° and 90°) and 72 hr post-ECC (150° vs. 90°).

Electrical Activity of BB We found only the effect of session on RMS EMG to be statistically significant (F[2.463,41.87] = 4.26, p < .05). The RMS EMG decreased immediately post-ECC at 60° and 150° (p < .05) and then recovered to the baseline at 60° within 24 hr and at 150° within 48 hr (Figure 3, Figure 5a). The effects of angle (F[2,34] = 26.45, p < .05), measurement time (F[5,85] = 3.48, p < .05), and the interaction between them (F[10,170] = 5.05, p < .05) were significant for MdF EMG, indicating that the changes over time are related to joint angle. As an acute effect, there was a decrease in MdF EMG at 90° but an increase at 60° and 150°, with significantly lower values at 90° compared with 60° and 150° (p < .05). During the recovery period, the MdF EMG values were lower at 90° than at 60° and 150°. The MdF EMG did not recover to baseline at 90° within 120 hr (p < .05) but did return to the pre-ECC value within 24 hr post-ECC at 60° and 150° (Figure 5b). As an index of neuromuscular efficiency, the MIT/RMS EMG ratio was calculated. A significant effect of angle (F[2,34] = 3.81, p < .05) and measurement time (F[5,85] = 12.82, p < .05) was found, but no effect was found for their interaction (p > .05; Figure 5c). However, the contrast method showed statistically significant differences in the measurement time only. Compared with the baseline, the MIT/ RMS EMG ratio decreased immediately post-ECC with no statistically significant change at 60° (p > .05) and remained significantly depressed until the end of the recovery period, except at 150°, for which the ratio reached the baseline value at 120 hr Although the values at a joint angle of 90° were lower than those at 60° and 150°, the differences were not statistically significant.

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Effects of Submaximal Eccentric Exercise   65

Figure 3 — An illustration of raw EMG for the biceps brachii muscle and force of the right-arm elbow flexion recordings from a subject before and following ECC at 60°, 90°and 150° elbow joint angles (180° as full elbow extension). Pre = before ECC, Im post = immediately after ECC, h = hours after ECC.

Mechanical Activity of BB Joint angle (p > .05), measurement time (p > .05), and the interaction between them (p > .05) had no significant effect on RMS MMG and MdF MMG (Figure 6a, b). A significant effect of angle (F[2,34] = 3.07, p = .05) and measurement time (F[5,85] = 9.48, p < .05) was found for the MdF MMG/MIT ratio (Figure 6c). The MdF MMG/MIT ratio increased due to ECC in all three joint angles, with the values at 60° being significantly lower than those 90° (at 24, 48 and 72 hr) and 150° (at 48 hr).

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66  Kisiel-Sajewicz et al.

Figure 4 — Changes in the MIT (maximal isometric torque; panel A) and DOMS measured at the level of force (% MIT) at which pain was felt (panel B) following ECC at different elbow joint angles. Pre = before ECC, Im post = immediately after ECC, h = hours after ECC. Values are means. ◆, ■, ▲ above SD bars = significant difference compared with pre-ECC values for 60°, 90°, 150°, respectively (p ≤ .05). Significant differences between elbow joint angles are expressed above the x-axis: 1 = significant difference between 90° and 60° at the elbow joint; 2 = significant difference between 90° and 150°; 3 = significant difference between 60° and 150°.

Coactivation of TB Significant effects of angle (F[2,34] = 3.35, p < .05), measurement time (F[5,85] = 4.23, p < .05), and the interaction between them (F[4.714,80.136] = 3.15, p < .05) were noted for TB coactivation, indicating that the changes over time are related to joint angle. The TB coactivation was similar at the three joint angles before ECC (Table 1) and increased significantly (p ≤ .05) immediately post-ECC at all angles, with a significantly larger increase at 60° compared with 90° and 150° (p ≤ .05;

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Figure 5 — Changes in the RMS EMG (panel A), MdF EMG (panel B) and MIT/RMS EMG ratio (panel C) following ECC at different elbow joint angles. Pre = before ECC; Im post = immediately after ECC; h = hours after ECC. Values are means. ◆, ■, ▲ above SD bars = significant difference compared with pre-ECC values for 60°, 90°, 150°, respectively (p ≤ .05). Significant differences between elbow joint angles are expressed above the x-axis: 1 = significant difference between 90° and 60° at the elbow joint; 2 = significant difference between 90° and 150°.   67

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Figure 6 — Changes in the RMS MMG (panel A), MdF MMG (panel B), and MdF MMG/ MIT ratio (panel C) following ECC at different elbow joint angles. Pre = before ECC; Im post = immediately after ECC; h = hours after ECC. Values are means. ◆, ■, ▲ above SD bars = significant difference compared with pre-ECC values for 60°, 90°, 150°, respectively (p ≤ .05). Significant differences between elbow joint angles are expressed above the x-axis: 1 = significant difference between 90° and 60° at the elbow joint; 3 = significant difference between 60° and 150°. 68

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Effects of Submaximal Eccentric Exercise   69

Figure 7 — Changes in antagonist coactivation expressed as a ratio of RMS EMG of the triceps brachii (TB) to RMS EMG of the biceps brachii (BB) muscle (RMS EMG TB / BB) following ECC at different elbow joint angles. Pre = before ECC; Im post = immediately after ECC; h = hours after ECC. Values are means. ◆, ■, ▲ above SD bars = significant difference compared with pre-ECC values for 60°, 90°, 150°, respectively (p ≤ .05); 1 = significant difference between 90° and 60°; 3 = significant difference between 60° and 150°.

Figure 7). The long-term effect (throughout recovery) of the antagonist coactivation returned to pre-ECC values at 24 hr post-ECC (except 48 hr post-ECC at 150°). At 48 hr post-ECC, the coactivation at 60° was greater than that at 90°; however, these values did not differ from their pre-ECC values (Figure 7).

Discussion The present study shows that the effect of ECC on the elbow flexor is different when the functional consequences are tested at different joint angles immediately and several days after the exercise. Antagonist coactivation returned to the baseline level at 24 hr regardless of joint angle, and the fastest torque recovery with the highest force level at which any pain occurred was found after ECC for a joint angle of 60°.

Indirect Indicators of Post-ECC Muscle Damage: MIT Decrease, Muscle Soreness, and Changes in Muscle Activity The submaximal ECC-related MIT decrease is consistent with previous findings that employed maximal ECC (load during ECC was 100% MVC [Nosaka et al. 1991] 70–120% MVC [Felici, Colace, & Sbriccoli, 1997]) and submaximal ECC [~22% MVC, (Prasartwuth, Taylor, & Gandevia, 2005)]. The factors contributing to post-ECC MIT decrease, muscle soreness, and changes in muscle activity can result from alterations in excitation-contraction, ultrastructural changes in muscle (Chen et al., 2007b; Corona et al., 2010), and changes in the ability to generate

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tension at the sarcomere level (Morgan & Allen, 1999; Newhamet al., 1983; Brockett et al., 2002). The torque deficit immediately postsubmaximal-ECC recorded in the current study may partly result from increased antagonist (TB) coactivation at the three joint angles, but this is not an issue throughout the recovery time. The much higher coactivation immediately post-ECC suggests that coactivation serves as an acute joint protective mechanism to support maintaining joint stability during maximal voluntary contraction to decrease the risk of injury. The observed increases in TB coactivation at the three joint angles support our hypothesis that there is a functional coupling (common input) to motor neurons innervating agonist motoneuron pool and subpopulation of the antagonist motoneuron pool (Jongen, Denier van der Gon, & Gielen, 1989). The significantly larger TB coactivation at a 60° elbow joint angle after ECC compared with other angles may be a result of reduced inhibition of antagonist muscle motoneurons. The decrease in the RMS EMG of BB at 60° angle after ECC provides some indirect support for this hypothesis. Interestingly, the fastest torque recovery with the highest force level at which any pain occurred was also found after ECC at joint angle of 60°. Because the MIT recovery was fastest at 60° and long-term changes in the MIT due to submaximal ECC are muscle-length dependent, we suggest that the functional consequences of the submaximal ECC are altered at different joint angles. The faster MIT recovery at 60° can be partially related to smaller changes in the length of damaged fibers during contraction and smaller changes in transverse stiffness of the sore muscle (as expressed by the MdF MMG/MIT ratio). This could result in milder pain sensations and thus lower output from pain-sensitive afferents, which could affect muscle activation by inhibiting spinal motor neuron activity (Avela, Kyröläinen, Komi, & Rama, 1999). The results of indirect indicators of post-ECC muscle damage suggest that ECC can induce a larger and longer reduction of force-generating capacity at 90° and 150° joint angle.

Combined Analysis of Electrical and Mechanical Activities of BB Muscle After ECC In general, the results of the combined analysis of electrical and mechanical activities of the BB muscle after submaximal ECC indicated that electromechanical factors are the main contributors to MIT reduction. The MIT/RMS EMG ratio reduction post-ECC recorded in the current study (as well as in Sayers, Knight, & Clarkson, 2003) indicates a failure of muscle-contracting elements to respond to electrical signals. The present study reveals for the first time that the EC failure contributes to acute and postponed MIT reduction after ECC at each muscle length, as we have not found significant interactions of angle and measurement time in the MIT/ RMS EMG ratio. The electromechanical failure can be related to the mechanical disruption and disorganization of sarcomeres (Newham, Mills, Quigley, & Edwards, 1983), sarcolemma (Jones et al. 1986), transverse tubular system (Corona et al. 2010), and sarcoplasmic reticulum (Chen et al., 2007b). Because, based on the EMG and MMG, other factors contributing the MIT reduction post-ECC appeared to be more dependent on the tested joint angle, they were discussed separately at the 90° joint position and at the 60° and 150° positions.

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Joint Position of 90° Immediately post submaximal-ECC as well as throughout the recovery period at 90°, we recorded no changes in the RMS EMG and RMS MMG signals and a decrease in the MdF EMG. The lack of change in the RMS EMG at 90° observed in this study is consistent with the results of Sayers et al. (2003) but in contrast to those of Kroon and Naeije (1991). However, the latter set of authors applied ECC until exhaustion and tested the effects on EMG during submaximal (40% MVC) isometric contraction. At this level of force, fatigue-related increases in the RMS EMG can be expected as a result of the recruitment of new motor units, synchronization, and grouping of active motor units, and/or slowing of action potentials with larger RMS values (Orizio, 2000). Several factors can explain the lack of RMS EMG and MMG changes and lower MdF EMG observed herein. First, the dimensions of the contracting MU fibers did not change enough to affect their oscillation pattern, and there was no alternation in MU synchronization and grouping (Krogh-Lund & Jorgensen, 1992). Second, FT fibers selectively damaged during ECC may not have been present or their derecruitment due to damage may have been accompanied by a different pattern of firing rate of the still recruitable (with lower conduction velocity) motor units providing similar RMS but lower MdF. The MdF EMG reduction might also be due to the mean CV reduction after ECC (Piitulainen, Boatts, Komi, Linnamo, & Avela, 2010), which may result from the above mentioned potential derecruitment of FT fibers and/or impaired sarcolemmal action potential conduction. The fact that the costameres system is not damaged by the ECC may be supported by the MMG consistency. Indeed, if MMG is related to fiber shortening transmitted to the muscle surface, acto-myosin interaction may still be present (producing shortening), but less force/cross-bridge may be generated or some alterations in the transmission properties of the myotendinous junction may be involved. Based on the mechanical model of the MMG system proposed by Uchiyama and Hashimoto (2011) and the longitudinal and the transverse stiffness during contraction in the frog skeletal muscle (Hatta et al., 1988; Tsuchiya et al., 1993), we can suggest that an increase in the MdF MMG/MIT due to ECC (without significant changes in the MdF MMG) could be related to muscle (transverse) stiffness changes attributable to the specific impairment of the viscoelastic structures, which had a more direct influence on the force signal than on the MMG signal because of its series placement between the contractile machinery and the force transducer.

Joint Position of 60° and 150° As for the joint angle of 90°, there were no changes in RMS MMG at 60° and 150°. However, in contrast to the 90° angle, the RMS EMG (and MIT/RMS EMG ratio) decreased and the MdF EMG increased for these angles. The difference in the myoelectric signals at 90° compared with 60° and 150° could be explained by the different relative intensities of ECC (over 70% MIT for 60° and 150° and 50% MIT for 90°). Furthermore, the elbow angle has a significant effect on the motor unit discharge pattern during voluntary isometric contraction (Christova et al., 1998). In the biceps brachii, the discharge rate of single motor units is higher for shorter muscle lengths (Christova et al., 1998). Therefore, the functional consequences of the ECC of the elbow flexors could be different for short and long muscle lengths.

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Amplified failure of the EC coupling is a probable cause of acute MIT reduction for short and long muscle lengths. As we assumed, for nonoptimal muscle lengths, the excitation-contraction coupling failure can be amplified (Sacco et al., 1994). Potential mechanisms contributing to acute MIT reduction accompanied by a decrease in EMG amplitude may include increased inhibitory cortex neuron activities, reduced spinal motor neuron excitability, and decreased force-generating ability per MU/ muscle. These mechanisms may lead to the activation of a lower number of MUs during MVC (FT fibers selectively damaged during ECC were absent) with different firing rate patterns. Furthermore, EMG amplitude can be indirectly influenced by negative feedback from group III, IV afferents, which inhibit both cortical and spinal motor neurons (Garland & Kaufman, 1995). To compensate the failure at these angles, the central nervous system may increase the synchronization of active motor units to develop the desired force to increase the MdF of the EMG (Mesin, Cescon, Gazzoni, Merletti, & Rainoldi, 2009). However, it should also be noted that the increased MdF EMG at these two ´ joint positions may also result from a partial recovery (Cifrek, Medved, Tonkovic, ´ 2009), as the measurement started at an angle of 90° (most acute angle) & Ostojic, immediately post-ECC, followed by 150° and 60°. Because there were generally no differences in RMS EMG throughout recovery, no changes in the MMG signal and MdF EMG, and a lower MIT/RMS EMG ratio (in both angles), the previously mentioned failure of the electromechanical coupling can also largely account for lower MIT at these angles compared with pre-ECC values. The lack of long-term changes in MdF EMG due to submaximal ECC at 60° and 150° is in contrast to those found at 90° (decrease). This difference can be explained by the weaker BB muscle contribution to MIT development at the 60° and 150° joint positions due to differences in activation patterns in elbow flexor muscles during isometric and eccentric contractions and at different joint positions (Nosaka & Sakamoto, 2001). As DOMS develops, the central nervous system may activate synergistic muscles according to their mechanical advantages and muscle architecture. The lack of RMS MMG changes due to ECC contradicts the results of Bajaj et al. (2002), who reported such changes related to the different fiber composition, function of the tested muscle, and intensity of ECC contractions. In the current study, we tested a large proximal limb muscle of mixed fiber composition; in contrast, Bajaj et al. (2002) examined the first dorsal interosseous muscle, a small hand muscle comprised mainly of type I muscle fibers, which may result in a larger oscillation of fibers (increasing RMS MMG) of the hand muscle after ECC. The higher intensity of ECC used by Bajaj et al. (2002; 116% MVC) compared with our protocol (50% MVC) may also contribute to the difference in findings. The difference in the myoelectric signals at 90° compared with 60° and 150° could be explained by different functional consequences of the ECC of the elbow flexors for short and long muscle lengths caused by the different relative intensities of ECC (over 70% MIT for 60° and 150° and 50% MIT for 90°). The present study showed that ECC induced different changes in EMG than in MMG immediately after ECC and during 120 hr of recovery at different joint angle, which suggests that the simultaneous recording and combined analysis of EMG and MMG signals provides a more precise assessment of the effects of submaximal eccentric exercise on muscle activity at different elbow joint angles.

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Conclusions These results are first documented measurements of the effect of submaximal eccentric exercise on antagonist cocontraction at different joint angles. Regardless of the elbow joint angle submaximal ECC (at 50% MVC), increased antagonist coactivation immediately post-ECC can contribute to acute post-ECC MIT reduction, while it is not an issue throughout recovery. The combined analysis of MIT, EMG, and MMG indicates that electromechanical failure is a main contributor to acute and long-term MIT reduction after submaximal ECC regardless of elbow joint position. At a joint angle of 90°, the submaximal ECC reduced the MdF EMG, indicating reduced muscle fiber conduction velocity and FT fiber derecruitment. In contrast, at 60° and 150°, amplified excitation-contraction coupling failure of the biceps brachii muscle occurred immediately after ECC. Our results show that torque recovers more rapidly and the pain sensation is reduced for shorter muscle lengths; thus, sports coaches and therapists can use the flexed elbow joint position at least 72 hr after submaximal eccentric exercise to obtain adequate muscle forcegenerating capacity with minimal pain sensation. Moreover, we suggest that an increase in the coactivation of the antagonist immediately after ECC is necessary to support maintaining joint stability during isometric, maximal voluntary contraction to decrease the risk of injury.

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