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HIGHLIGHTED TOPICS
Neural Control of Movement
1Department of Physiology, National Institute of Occupational Health, DK-2100 Copenhagen; and 2Institute of Exercise and Sport Sciences, University of Copenhagen, DK-2100 Copenhagen, Denmark; and 3Department of Orthopaedics, University of California, San Diego, California 92103
Submitted 3 July 2003 ; accepted in final form 28 November 2003
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ABSTRACT |
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 TOP ABSTRACT METHODSRESULTSDISCUSSIONREFERENCES |
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shoulder; static muscle contraction; dynamic muscle contraction; prolonged muscle contraction
During dynamic contractions, some studies are in support of IMP as well as EMG to serve as indexes of force. For example, IMP in the human tibialis and soleus muscles was reported to be dependent only on ankle torque but independent of contraction mode (static vs. isokinetic) and velocity of contraction during both concentric and eccentric exercise (2, 30). In contrast, in the rabbit tibialis muscle during isotonic contractions, the highest values of IMP were found at the lowest force levels corresponding to the highest shortening velocity, and IMP appeared to be related to the speed of shortening (8). According to physics, a larger force must be developed during dynamic contractions because the velocity is increasing during each contraction, i.e., an acceleration occurs that is larger the higher the velocity attained; therefore, correspondingly larger values for EMG and IMP were to be expected. Furthermore, during dynamic contractions of the human vastus lateralis muscle, the IMP-to-torque ratio was lower during eccentric than concentric contractions (6). In the human shoulder muscles, only very slow concentric movement of 
1°/s has been studied that may be considered to be a quasi-static contraction (27). Similarly, for sustained, prolonged fatiguing contractions, EMG and IMP have only been investigated systematically in a few studies, and the results are contradictory. It is generally accepted that prolonged fatiguing contractions at constant submaximal static force level induce a significant increase in the EMG amplitude, and a close correlation between increasing EMG and increasing IMP was reported in vastus lateralis muscle during prolonged contractions at 35% maximum muscle strength (29). Also, these authors found a constant IMP during a prolonged fatiguing contraction where the EMG amplitude was kept constant, despite a decreasing external force development. However, an increased IMP during prolonged constant-force development could not be confirmed by others (31, 33). Moreover, differential responses were reported in IMP and EMG during low- vs. high-level static contractions to fatigue in terms of IMP increasing during 25% MVC, but not during 70% MVC, despite increases in EMG amplitude throughout both contraction levels (5). One possible explanation of these different findings is that the physiological mechanisms for increases in EMG and IMP have no common route. Therefore, differences in contraction force level may account for different responses, such that the increase in IMP during muscle contraction level is abolished at high (short-term, due to exhaustion) as well as at very low contraction forces.
Additionally, local physiological and anatomic factors may affect the relation between EMG amplitude and IMP during prolonged contractions, and results may, therefore, depend on the anatomic region and/or the species investigated. Due to the complex anatomy of the shoulder, and corresponding complex shoulder function, it is not known if a change in the EMG amplitude is indicative of a change in muscle force or development of muscle fatigue. IMP is influenced by fascial compliance and the actual fluid content of the muscle. Specifically, a certain degree of muscle edema affects IMP, dependent on the local anatomy. Thus, in the quadriceps muscle, no measurable effect of an increased fluid content of 10% was seen on the IMP; whereas, in other muscles, e.g., in the lower leg compartment, swelling may raise IMP due to a low compliance of the muscle compartment (9, 11, 33). The anatomy of the supraspinatus muscle region is, to a large degree, comparable to that of the lower leg compartments (16). Furthermore, for supraspinatus muscle, a significant thickening of the muscle, indicating muscle edema, was found in a previous study during submaximal static contractions (17), but it remains unknown whether there is an effect of increased fluid content on the IMP during prolonged contractions of supraspinatus muscle. Interestingly, during repeated maximal isokinetic contractions as well as submaximal isometric contractions of vastus lateralis muscle, an increase in IMP during the relaxation periods was reported and may be causally related to an increase in muscle fluid content (4, 5). Based on the above, we presumed IMP in the supraspinatus muscle to increase with time, in line with the EMG during sustained static contractions. During dynamic as well as fatiguing shoulder muscle activity, the load sharing between the different fractions of the shoulder girdle anatomy may change. A classic subdivision of shoulder muscle function is into stabilizers and movers (for references, see Ref. 21). However, such strict subdivision is not always in accordance with actual in vivo activities (25, 26, 34). Also, via biofeedback, subjects can voluntarily change the load sharing (28). Our knowledge regarding load sharing among shoulder muscles is still limited, and, by measuring EMG in combination with IMP, possible changes in load sharing may be elucidated during dynamic as well as fatiguing contractions.
The purpose of the present study was to elucidate whether the constant IMP-EMG relationship seen with increased shoulder abduction torque may be extended to dynamic contractions and separately to fatigued muscle. More specifically, the hypotheses examined for supraspinatus muscle were as follows: 1) during dynamic contractions, both IMP and EMG amplitudes increase with increasing velocity, and 2) both IMP and EMG increase with time during a constant static contraction sustained for a prolonged period of time. Additionally, with inclusion of EMG from two more shoulder muscles, the aim was to study recovery as well as shoulder muscle load sharing between rotator cuff muscles (e.g., supraspinatus muscle) and other shoulder girdle stabilizers (e.g., trapezius muscle descending part) relative to shoulder joint prime movers (e.g., deltoideus muscle medial part), during brief static, dynamic, and sustained static contractions. The underlying hypothesis regarding load sharing was that stabilizers were relatively more active during static and prime movers during dynamic contractions.
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METHODS |
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TOPABSTRACTMETHODSRESULTSDISCUSSIONREFERENCES |
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Experimental Setup and Protocol
The subjects were sitting in an adjustable experimental chair, with their back vertical, thighs horizontal, and lower legs vertical. An IMP transducer-tipped catheter as well as intramuscular wire electrodes for EMG recording were inserted into the right side supraspinatus muscle under the guidance of ultrasound imaging B scan. Images were taken to also measure distances from skin surface as well as muscle thickness. Furthermore, surface EMG electrodes were mounted above deltoideus and trapezius muscles, and, for heart rate (HR) and blood pressure (BP) recordings, a small cuff was placed around the third finger of the left hand. The subjects then, in the sitting position, performed arm abductions in the frontal plane with the elbows straight and palms facing the floor. All contractions were performed bilaterally to attain symmetric loading of the spine. After maximal voluntary contractions (MVC), three sessions of contractions were performed: static arm abductions at different abduction angles, dynamic arm abductions at different velocities, and, finally, prolonged static arm abduction. At least 10 min of rest were allowed between the sessions.
MVC. Arm abduction muscle strength was measured at the position of bilateral 30° arm abduction and taken as the highest value of three to five attempts. Resting periods of 1- to 2-min duration were allowed between the MVC contractions.
Brief static arm abductions. Bilateral static arm abductions at 30, 60, and 90° (0° 
vertical arm and 90° horizontal arm) of abduction were performed for 10 s each, with and without an external weight of 1 kg in each hand. Additionally, the arms were passively positioned in the three abduction positions supported by the experimenter. The contractions and passive positions were performed in random order.
Dynamic arm abductions. Bilateral dynamic arm abductions through the range from 0 to 90°, followed by arm adductions from 90 to 0°, were performed at mean angular velocities of 9, 18, 45, and 90°/s. The 0° position and 90° position were sustained for a 10-s period between each of the dynamic contractions. The contractions were performed with and without a 1-kg weight in the hands. The dynamic contractions were guided by a metronome and performed in random order.
Prolonged static arm abduction. Prolonged bilateral 30° arm abduction was maintained for 30 min and followed by a recovery period of 20 min. During the recovery period, two 30° arm abduction tests (1 min) were performed after 10 and 20 min, respectively. During the prolonged and the test abductions, the subjects maintained position by slightly pushing their hands against a position-adjustable force bar. The external abduction force applied was 0.4 N at the level of the hands (corresponding to 0.3% MVC). During the prolonged contraction and subsequent recovery period, HR, BP, and rating of perceived exertion (RPE) were recorded.
Force and Torque Recordings
MVC was measured by using a strain-gauge-based, three-dimensional force transducer (AMTI-MC3-6-1000) mounted with a handle that the hand gripped onto. A bilateral setup was used, but only force exerted by the right hand was measured. Force data were sampled at 128 Hz, and root-mean-square (RMS) values were calculated for 0.26-s time periods. The highest 1-s value (average of 4 time periods) was considered to represent MVC. During the prolonged abduction session, left and right abduction force were measured with strain-gauge-based transducers built into two force bars that were placed to touch the dorsal side of the hands, when the subjects had attained the correct 30° abduction position. The orientation of left and right abduction forces was in the frontal plane and in the direction perpendicular to the arm. To help the subjects maintain the position, a small, constant force output of 0.4 N was requested, and visual feedback was given in front of the subjects, from both left and right transducers. To convert hand force into shoulder torque, the distance from the center of caput humeri to the handle (midhand) and to the force bar position on the hand where the external load was applied, respectively, was measured. The relative load in the abducted positions was calculated based on simple moment calculation using force and moment arm (20).
Intramuscular Recording Sites
Three local anatomic dimensions in the shoulder region were measured at a lateral and medial aspect by using ultrasound imaging (Diasonics, VST Master Series, GE-Medical, Copenhagen, Denmark) to guide the insertions of wire for EMG and Millar transducer-tipped catheter for IMP recordings (for details see Ref. 17). These recordings were limited to the supraspinatus muscle due to equipment restriction. The distance from skin surface to the supraspinatus muscle, thickness of the supraspinatus muscle, and thickness of the trapezius muscle were measured. For example, in one subject, a negative pressure was measured initially. Ultrasound imaging of the shoulder showed that the catheter was located next to the bottom of fossa supraspinata. The catheter was then moved to the central part of the muscle to obtain the actual muscle pressure. Finally, the distance from skin surface to the tip of the EMG wire and the IMP catheter tip was measured at the end of the experiment when these were withdrawn from the muscle.
IMP
Local anesthesia (Zylocain, 20 mg/ml) was given to the skin and subcutis above the right supraspinatus muscle at the site for IMP recording. A transducer-tipped pressure catheter (Millar Micro-Tip Houston, TX) was then inserted in parallel to the fiber orientation into the central part of supraspinatus muscle through a Teflon catheter (Venflon, Ø14G/2 mm OD). After insertion, the Teflon catheter was withdrawn, leaving the IMP catheter in the muscle. The IMP transducer-tipped catheter was zero adjusted (in isotonic saline) and calibrated with an electrical input and with a water column (isotonic saline) in each experiment. No systematic difference was found between the two methods. IMP was sampled at 1,024 Hz, and RMS values for 0.26-s time periods were calculated. Maximum value of IMP was calculated during MVC as the highest 1-s value (mean of 4 time periods). During the brief static contractions, the mean IMP values were calculated over 6 s (first and last 2 s were omitted of the 10-s contractions), and peak values (0.1 s) of IMP were calculated during the dynamic concentric contractions, together with IMP values over 6 s (as for the brief static contractions) during the 0 and 90° position sustained immediately before and after the dynamic concentric contractions. During the prolonged abduction, 1-min average values were calculated every minute, and in the recovery period 1-min values were calculated during the two test periods.
EMG
Intramuscular EMG was recorded from bipolar Teflon-coated steel wire electrodes with a diameter of 0.05 mm inserted into the central part of the right supraspinatus muscle through a needle. Surface EMG from the middle part of the right deltoideus muscle and the descending part of the right trapezius muscle were recorded by using bipolar surface electrodes (Medicotest, N-10-A) placed on the skin above the muscles, with a distance of 2 cm between the recording areas. Possible cross talk was demonstrated to be negligible by having the subject perform a few practical tests while recording the EMG channels. A commercially available EMG recording system was used (IC-600, Medinik, Örbyhus, Sweden). Skin surface was carefully cleaned with alcohol and shaved to decrease skin resistance. The quality of the raw EMG signals was controlled continuously on oscilloscopes and sampled online at 1,024 Hz. The same calculation periods were applied for the three channels of EMG as for the IMP, and all EMG data were presented as RMS amplitude in percent maximum EMG (EMGmax).
HR, BP, and RPE
Fingertip HR (in beats/min) and BP (in terms of mean arterial BP, mmHg) were measured continuously (Finapres, Ohmeda 2300) and displayed every minute during and after the prolonged bilateral static abduction. The values presented in the results are as measured at the fingertip, and, for correction to heart level, a mean of 34 mmHg is to be subtracted. RPE was measured according to a 10-graded Borg scale (3) every 2 min during the prolonged abduction as well as in the recovery period during the brief abductions.
Statistics
All data are presented as means ± SE, unless otherwise indicated. Repeated-measures ANOVA was performed with the dependent variables, IMP and the three EMG recordings, and the independent variables, position and load during the brief static contractions and speed and load during the dynamic contractions. In case of significance, multiple comparison was performed with the Tukey's test. If significant interaction was found, ANOVA was performed for each of the dependent variables separately (SigmaStat). Changes over time were analyzed by the Friedman test. P < 0.05 was considered statistically significant.
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RESULTS |
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TOPABSTRACTMETHODSRESULTSDISCUSSIONREFERENCES |
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The maximal abduction force measured at the hands was 54 ± 7.5 N, and the moment arm was 0.74 ± 0.02 m, resulting in maximal abduction torque in the shoulder joint of 38.9 ± 3.0 N·m, including the torque requested to counteract the weight of the arms in the 30° abducted position. During this, an IMP value of 324 ± 38 mmHg was attained in supraspinatus muscle, and the EMGmax values were 1,164 ± 249 µV for supraspinatus muscle, 628 ± 91 µV for deltoideus muscle, and 947 ± 103 µV for trapezius muscle. The 30° abducted arm position corresponded to 13.8 ± 1.1% MVC with no hand load and 14.1 ± 1.1% MVC with 0.4 N (reaction force during the prolonged contraction). The thickness of trapezius muscle was 6.3 ± 0.3 and 6.1 ± 0.3 mm for the lateral and medial aspect, and the corresponding values for supraspinatus muscle were 20.9 ± 1.7 and 11.2 ± 1.6 mm, respectively. The distance from the skin surface to the superficial fascia of supraspinatus muscle was 14.4 ± 0.9 and 13.8 ± 1.2 mm at the lateral and medial aspect, respectively. The corresponding distance to the tip of the pressure transducer was 40 ± 2 mm, and the insertion angle was 60° relative to skin surface, resulting in a vertical depth of 33 mm. Accounting for the location of the sensor placed on the side of the catheter 5 mm above the tip resulted in a pressure recording depth of 28 mm. The distance from the skin surface to the tip of the wire EMG electrode was 26 ± 1 mm and thus very close to that of the IMP recordings, with both being well below the superficial fascia of supraspinatus muscle.
Brief Static Arm Abductions
IMP in supraspinatus muscle and EMG in supraspinatus, deltoideus, and trapezius muscles increased significantly with increasing angle of abduction from 0 to 90° and with increasing external shoulder torque from passive to 0 kg and 1-kg load in the hands (Table 1). IMP tended to increase during passive arm abduction, but this was not significant. The highest IMP value of 120 mmHg was attained in the 90° abducted arm position with 1-kg load in the hand, which corresponded to 33% of the IMP during MVC. Correspondingly, supraspinatus muscle EMG amounted to 22% EMGmax in the 90° abducted arm position with 1 kg.
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Dynamic Arm Abductions
The peak IMP and EMG values from the supraspinatus muscle during the concentric dynamic contraction phases, with and without 1-kg hand load, are presented in Fig. 1, together with the corresponding data during the 90° arm abduction sustained immediately after. The values at 0° arm abduction preceding the dynamic contractions (not shown in Fig. 1) and at 90° arm abduction after the dynamic task were not significantly different from the corresponding values reported above during the brief static contractions. However, during the dynamic concentric contractions, IMP and EMG peak values were significantly higher than the values at the subsequent position at 90° arm abduction, and the values with 1-kg load were higher than those without hand load. Interestingly, EMG peak values for all muscles increased significantly with increased shortening velocity, but this was not the case for the IMP peak values. No significant differences were found in the EMG ratio between muscles (i.e supraspinatus/deltoideus, supraspinatus/trapezius, and deltoideus/trapezius) when dynamic contractions at different velocities or dynamic vs. static contraction were compared. In contrast, the peak IMP-to-EMG ratio (IMP/EMG) for supraspinatus muscle decreased with shortening velocity and was significantly lower at maximum compared with minimum velocity at no hand load. Also, the peak IMP/EMG was lower during the dynamic than the static contractions.
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Prolonged Static Arm Abduction
The average value of IMP initially in the prolonged abduction (after 1 min) amounted to 53 ± 8 mmHg and thus was similar to the 56 ± 7 mmHg in the above-reported brief static 30° arm abduction session (Table 1). During the prolonged contraction, all subjects maintained the abducted position with only minor adjustments, according to the experimenter and the force feedback from the hands. Despite this, IMP decreased in four subjects but remained at the same level in two subjects, with the overall mean value being only 30 ± 9 mmHg at 29 min of contraction. The mean values over time of these IMP data are depicted together with the corresponding data for the three EMG recordings that showed significant increases for all muscles during the prolonged abduction from initial values (Fig. 2). These initial EMG values corresponded to those reported above during the brief static 30° arm abduction session (Table 1). From the first to the last minute of abduction, the EMG values, on average, increased to 200, 220, and 217% of the initial level for supraspinatus, deltoideus, and trapezius muscles, respectively.
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The resting level of IMP measured 1 and 2 min after the prolonged abduction was similar to the resting level measured before the prolonged abduction that corresponded to those in the brief static as well as dynamic contractions. The levels of the IMP during the test contractions after 10 min, 53 ± 12 mmHg, and 20 min, 53 ± 11 mmHg, of recovery were similar to the initial (1 min) level during the prolonged abduction. For all muscles, the EMG values decreased to resting level immediately after the abduction. Of particular note, during the test abductions after 10 and 20 min, EMG values for supraspinatus and deltoideus muscles still remained significantly above the initial abduction level, whereas that for trapezius muscle had recovered within 10 min.
In the nonfatigued muscle, a linear relationship was seen between IMP and the relative EMG of supraspinatus muscle (%EMGmax) in the 30° abducted position, with and without 1-kg hand load. However, a clear progressive deviation between the IMP and EMG of supraspinatus muscle relative to the nonfatigued situation was found during the 30-min abduction period (Fig. 3). This was due to the steady EMG increase from the initial 10% EMGmax toward around twice this level (Fig. 2).
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Resting value of fingertip BP was 124.5 ± 13.1 mmHg, and this increased significantly during the prolonged contraction, from 6 mmHg above resting value at 1 min to 32 mmHg above the resting value at 29 min. Furthermore, HR increased from a resting value of 70 ± 6.3 to 74 ± 5.5 beats/min after 1 min and to 84 ± 6.1 beats/min at 29 min. RPE during the prolonged abduction increased from an initial value (1 min) of 0.5 ± 0.3 to 7.8 ± 0.5 at the end of the contraction (29 min). During the test contractions after 10 min of recovery, RPE was 2.4 ± 0.9, i.e., elevated relative to initial (1 min) value, and remained at this level also after 20 min of recovery, i.e., 2.8 ± 0.9.
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DISCUSSION |
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TOPABSTRACTMETHODSRESULTSDISCUSSIONREFERENCES |
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Association Between IMP and EMG Amplitude During Static But Not Dynamic Contractions
The present study confirmed prior investigations on the effect of abduction angle and significance of external load on the IMP in supraspinatus muscles during brief static arm abductions (13-15, 27). However, as in earlier studies on female subjects (16, 20), the absolute level of the IMP during submaximal as well as maximal contractions was lower than the level obtained in the above studies on male subjects. The somewhat high resting values were likely to be due to our setup, where some soft elastic bandage was used to compress some of the equipment to the skin.
The results from the dynamic contractions showed an increasing peak value of the IMP that was higher with increased load in the hand, but no significant increase was revealed with increased angular velocity. This was puzzling but could be due to large shape changes of the supraspinatus muscle during a 90° arm abduction. At the highest angular velocities, the instantaneous distribution of IMP may then be inhomogeneous with the time history of the IMP being dependent on the spatial location of the measuring site. In particular, IMP is a less accurate index of muscle force in the shortened muscle position (7), which is attained earlier the faster the contraction is. Thus a higher peak IMP in supraspinatus muscle during the fastest contraction may have been offset due to this occurring at a shorter muscle length than at a slower abduction velocity. This may account for the somewhat lower peak IMP at the highest contraction velocity (Fig. 1) and, thereby, a lack of significance between peak IMP and contraction velocity. From a mechanical point of view, an increase in muscle force was expected with increased movement velocity due to the need for extra force to increase the acceleration of the arm. Such corresponding increase was seen regarding the EMG for all three muscles, and it remains unclear whether EMG or IMP reflects muscle force best during dynamic contractions. By monitoring ankle joint torque and position with an isokinetic dynamometer during maximal effort plantar-flexion and dorsiflexion of the foot, IMP and EMG activity in the tibialis anterior and soleus muscles was studied (35). In this study, a dissociation was seen between IMP and ankle joint torque during eccentric muscular activity compared with static muscular activity, because IMP increased to a lesser extent during eccentric than during isometric contractions. In other words, the IMP vs. torque relationship is linear, but the slope is less for eccentric compared with isometric and concentric contractions. However, EMG vs. torque is generally lower during eccentric contractions compared with isometric contractions, indicating that the IMPEMG relationship may be similar during various contraction modes. In another study of leg muscles, IMP of soleus and tibialis anterior was analyzed during gait (2). In this investigation as in ours, IMP was continuously recorded by using Millar pressure catheters. A linear relationship existed between soleus IMP and ankle joint torque as monitored by a dynamometer during treadmill walking and running. Thus phasic elevations of IMP during exercise are probably generated due to muscle force development. Tibialis anterior IMP showed a biphasic response, with the largest peak (90 mmHg during walking and 151 mmHg during running) occurring shortly after heel strike, in concordance with the function of this muscle during gait. IMP magnitude increased with gait speed in both muscles, thus further underlining its relationship to force. Finally, at these submaximal contractions, the force vs. torque relationship was not different between contraction modes.
Dissociation of IMP and EMG Amplitude During Prolonged Static Contraction
As outlined in the Introduction, there were several reasons to expect IMP to increase in supraspinatus muscle during prolonged contractions. A previous study of prolonged shoulder abductions on muscle fluid balance indicated development of muscle edema in supraspinatus muscle that is expected to increase resting IMP (17). Individual differences may occur, depending on the individual muscle compartment compliance relative to the degree of muscle edema, but, in any case, some increase in IMP is expected if the same force is sustained. This is in contrast to the decrease in IMP seen in four of the subjects. It may be considered then that the different patterns may not reflect individual differences, but rather differences due to slightly different locations of the IMP recording site among subjects. Interestingly, in an earlier study, no such individual or local decreases in IMP were seen in the vastus lateralis muscle, although this muscle is considered to be located in a more compliant surrounding than supraspinatus muscle, and, despite an even lower contraction level of only 5%, MVC of the knee extensors was sustained (33). Furthermore, we have measured IMP throughout a wide range of recording depths, indicating that IMP in supraspinatus muscle was rather homogeneous (16). One reason for the present finding may then be changed load sharing between abductor muscles, as discussed below.
The obvious physiological advantage of a stable or even decreased IMP over time is the preservation of muscle blood flow. According to a simple model, blood flow is proportional to the pressure head that is considered to correspond to the difference between arterial BP and IMP (32). Increases in IMP increase BP (10), but, in the present study, BP did increase by 25 mmHg, despite unchanged, or even decreased, IMP. This response, in combination with a probable decrease of vascular resistance (metabolically induced), likely gradually increased blood flow relative to the initial level during the abduction. Such increased blood flow may then postpone or prevent local fatigue development. Thus, in the present study, IMP may not have been a critical factor of significance for muscle blood flow, although more advanced models on the interaction between IMP and perfusion, including spatial heterogeneity of IMP, suggest that venous pressure may exceed IMP obstructing local perfusion (37). Furthermore, despite maintained blood flow, IMP might affect performance during prolonged contraction of supraspinatus muscle of 10% MVC, because IMP exceeded 30 mmHg, a minimum pressure that has been shown to elicit necrosis if maintained for 8 h (12).
Load Sharing Between Shoulder Muscles During Abduction
The EMG of nonfatigued muscles is related to force development. However, in fatigued muscles, the EMG amplitude may increase without a corresponding increase in force output, even at low-contraction levels corresponding to 30° shoulder abduction (10% MVC) (22, 29). The lack of increased IMP in general and the occurrence of decreased IMP for some subjects suggest a decreased supraspinatus muscle force during the prolonged abduction (18). Measurement of the IMP during the brief contractions showed that the initial level of the IMP was reestablished within 10 min of recovery after the prolonged contraction. A recent study on motor control at motor unit level showed that muscles compensate for the development of fatigue by motor unit recruitment and frequency modulation (19). In particular, trapezius muscle showed only an increase in amplitude, but not a decrease in mean power frequency, as well as a fast recovery of the amplitude in concert with the present study. Thus trapezius muscle may have been less fatigued than the other shoulder muscles, and its increased amplitude has reflected an increased force development rather than fatigue development. This may indicate an increased stabilizing function of the trapezius muscle as fatigue developed in other muscles and, thereby, a change in the load sharing between shoulder muscles. Taken together, our present results suggest that the relative force distribution among shoulder muscles during the prolonged abduction probably changed over time. This means that the force distribution principles by which the shoulder function is optimized may change over time. This concept should be considered in future improvements of shoulder function models.
In conclusion, IMP and EMG amplitudes increased in proportion in supraspinatus muscle with acute increased shoulder joint abduction torque, but during dynamic contraction a dissociation of this relationship occurred because only the EMG increase was velocity dependent. Furthermore, during an 10% MVC static contraction sustained for 30 min, EMG increased to about twice its amplitude, whereas IMP remained constant or even decreased in some subjects. With recovery, a change occurred in the IMP-EMG relationship during the test contractions due to the slow recovery of the EMG amplitude. Finally, EMG amplitudes indicated the load sharing between different functional shoulder muscles not to be different during brief static and dynamic contractions, but, in combination with IMP measures, a change in load sharing among muscles was indicated during sustained fatiguing static contraction.
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FOOTNOTES |
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The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
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