Mechanomyography (MMG) records the low frequency sounds generated by: a) a gross lateral movement of the muscle that is related to the regional distribution of the contractile elements; (b) "ringing" of the muscle at its resonant frequency; and (c) dimensional changes of the active muscle fibers (5,32,33). The MMG signal provides information about various aspects of muscle function and has been used to discriminate between muscle fiber types (26,27,29,31), examine factors related to electromechanical and phonomechanical delay (35), and monitor athletic training (10). Clinically, MMG may be useful for examining muscle diseases (1,6,36) and controlling external prostheses (3). Furthermore, recent investigations (2,8,16,30) have simultaneously measured MMG and electromyography to examine the dissociation between excitation and contraction that occurs as a result of fatigue.
The amplitude of the MMG signal is affected by a number of factors including muscle tension, stiffness, mass, length (4,5,7,21,26,29,32), level of torque production (28,29,35,39,47), velocity of muscle action (18,38), and the make-up of the tissue layer between the muscle and the surface of the skin (32). Many of these factors differ between males and females which may cause a gender-related difference in the MMG responses to various types of physical activity. For example, males tend to have greater muscle mass and length and produce more torque. Furthermore, increased torque production is associated with greater muscle stiffness, which is primarily a function of the number of attached cross-bridges (4,17,20,24), and affects the amplitude of the MMG signal (4,29). In addition, Orizio (32) suggested that the make-up of the tissue layer between the muscle and surface of the skin "may act as a low pass filter for the mechanical waves traveling from the muscle to the skin surface." Therefore, it is possible that a gender difference in the thickness of the adipose tissue layer (25) may influence the MMG signal. Thus, because of differences between males and females in muscle mass, muscle length, subcutaneous adipose tissue thickness, torque production capabilities, and, potentially, muscle stiffness, there may be a gender-related difference in the MMG amplitude responses during muscular activity. With the exception of Petitjean et al. (35) (two females out of eight subjects), however, no previous studies have used female subjects or examined the possibility that there is a gender-related difference in MMG responses to dynamic muscle actions. Therefore, the purpose of this study was to determine whether there is a gender difference in the velocity-related patterns of MMG responses to maximal CON and ECC isokinetic muscle actions.
Fifteen adult males (X¯ ± SD: age = 22.5 ± 1.7 yr; height = 178.1 ± 5.8 cm; body mass = 79.2 ± 7.4 kg) and 16 adult females (age = 22.8 ± 3.4 yr; height = 168.4 ± 7.0 cm; body mass = 63.1 ± 9.4 kg) volunteered to participate in the investigation. The study was approved by the University Institutional Review Board for Human Subjects, and all subjects completed a health history questionnaire and signed a written informed consent before testing.
Isokinetic Strength Testing Procedure
The subjects were tested for maximal concentric (CON) and eccentric (ECC) isokinetic peak torque (PT) of the dominant leg (based on kicking preference) extensors at randomly ordered muscle action velocities of 30, 90, and 150°·s−1 on a calibrated Cybex 6000 dynamometer. The subjects were in a seated position with a restraining strap over the pelvis and trunk in accordance with the position described in the Cybex 6000 User's Guide (14). The input axis of the dynamometer was aligned with the axis of the knee and the nondominant leg was braced against the contralateral limb stabilization bar. Three submaximal warm-up trials preceded three maximal muscle actions at each velocity with the highest PT (nongravity corrected) selected as the representative score. Three minutes and 5 min of rest were allowed between testing at each velocity and between CON and ECC muscle actions, respectively.
The MMG was detected by a piezoelectric crystal contact sensor (Hewlett-Packard 21050A, bandwidth 0.02-2000 Hz) that was placed on the vastus lateralis muscle between the head of the greater trochanter and lateral condyle of the femur. A stabilizing ring and double-sided adhesive tape were used to assure consistent contact pressure of the sensor (9).
Of the three maximal muscle actions at each velocity, the trial that resulted in the highest PT for CON and ECC isokinetic muscle actions was selected for MMG analysis. The MMG signal was sampled at 1000 points·s−1 and was not filtered to analyze the very low frequency sounds that are typical of MMG (9). The MMG amplitude (mV) value for the isokinetic muscle actions was calculated for a time period that corresponded to a 90° range of motion (i.e., at 30°·s−1 the amplitude for 3.0 s of the MMG signal was calculated, while at 150°·s−1 the amplitude for 0.6 s was calculated) beginning with the onset of the MMG signal (MP100, Biopac Systems Inc., Santa Barbara, CA.). This allowed for comparisons among the muscle action velocities which were based on a standardized range of motion of 90°.
Separate two-way mixed factorial ANOVAs (gender × velocity) were used to analyze the CON PT, ECC PT, CON MMG amplitude, and ECC MMG amplitude data. Significant interactions were followed with one-way repeated measures ANOVAs and Tukey post-hoc comparisons for each gender. Independent t-tests for PT and MMG amplitude were used to determine differences between the males and females. Dependent t-tests were used to determine differences between the ECC and CON muscle actions. An alpha of 0.05 was considered significant for all analyses.
Peak Torque Analysis
Figure 1 presents the relationships for CON and ECC PT (Nm) versus muscle action velocity (degrees per second) for the males and females. The two-way mixed factorial ANOVA for CON muscle actions resulted in a significant gender by velocity interaction. Separate one-way repeated measures ANOVAs with Tukey post-hoc comparisons indicated significant, but nonparallel, decreases in PT across velocities for the males and females. In addition, independent t-tests revealed that the males displayed significantly greater PT than the females at each velocity. For ECC muscle actions there was no significant interaction or main effect for velocity; however, there was a significant main effect for gender with the males exhibiting greater ECC PT than the females. We have previously reported intraclass reliability correlations for CON (18) and ECC (38) isokinetic PT, which ranged from R = 0.84 to 0.97 and R = 0.88 to 0.97, respectively, with no significant (P > 0.05) differences between the mean values for test versus retest at any muscle action velocity.
MMG Analysis. Figure 2 displays the relationship for CON and ECC MMG amplitude (mV) versus muscle action velocity (degrees per second) for the males and females. The two-way mixed factorial ANOVA for CON MMG amplitude resulted in a significant gender by velocity interaction. One-way repeated measures ANOVAs with Tukey post-hoc comparisons indicated that the amplitude values at 90 and 150°·s−1 were significantly greater than at 30°·s−1 for both the males and females. Independent t-tests revealed that the CON MMG amplitude values were significantly greater for the males than females at each muscle action velocity. For ECC muscle actions, the interaction was nonsignificant. There were significant main effects for velocity (collapsed across gender) and gender (collapsed across velocity). For velocity, the ECC MMG amplitude value at 150°·s−1 was significantly greater than at 30 and 90°·s−1. In addition, the males had greater MMG amplitude values than the females. In addition, dependent t-tests revealed that CON MMG amplitude was significantly greater than ECC MMG for the males at 90°·s−1 and the females at 30 and 90°·s−1. We have previously reported intraclass reliability correlations for CON (18) and ECC (38) MMG amplitude, which ranged from R = 0.90 to 0.99 and R = 0.97 to 0.98, respectively, with no significant (P > 0.05) differences between the mean values for test versus retest at any muscle action velocity.
The results of the present investigation indicated that CON isokinetic PT decreased as the velocity of muscle action increased in the males and females. These findings agreed with a number of studies that have described the CON isokinetic torque-velocity relationship in males (11,12,22,23,37,41,42-44,46) and females (11,13,22,23,37,42,45). In addition, the significant gender by velocity interaction for CON PT indicated nonparallel decreases in torque responses between the males and females. These findings were consistent with previous investigations (12,22,42) that have reported a greater percent decline in CON PT across velocity in females than males, possibly because of a gender difference in central nervous system inhibition. For the males and females, ECC isokinetic PT remained constant as the velocity of muscle action increased. These results agreed with previous studies (12,23,37,45,46) that have reported that ECC PT is independent of velocity for both genders.
MMG Amplitude and Concentric Muscle Actions
The results of the present investigation for CON MMG amplitude (Fig. 2) indicated: a) positive relationships between MMG amplitude and velocity of muscle action for both the males and females, b) a significantly greater rate of increase in MMG amplitude across velocities for the males than females, and c) significantly greater MMG amplitude values for the males than females at each muscle action velocity.
Increase in MMG Amplitude with Velocity
The positive relationships between MMG amplitude and velocity found in the present study for CON muscle actions were consistent with the findings from our recent study (18) of eight males measured isokinetically at velocities ranging from 60 to 360°·s−1. The present findings, in conjunction with our previous results (18), indicate that for both males and females there was a velocity-related dissociation between MMG amplitude and PT during maximal CON isokinetic muscle actions. That is, as the velocity of CON muscle action increased, MMG amplitude increased while PT decreased. The reason for the increase in MMG amplitude is unknown but may have resulted from velocity-related (18,38): a) decreases in muscle stiffness, b) increases in the rate of actin-myosin cycling, or c) increases in the turbulence of the intra- and/or extracellular mediums.
Muscle stiffness. The MMG signal is, in part, a function of lateral oscillations of muscle fibers that are generated at the resonant frequency of the muscle. Barry and Cole (5) have reported that during an isometric twitch of the frog gastrocnemius muscle, muscle stiffness may dominate changes in the resonant frequency and, therefore, the MMG signal can be used "... as a monitor of muscle stiffness." Other investigations (4,29) support the suggestion of Barry and Cole (5) in that a high level of muscle stiffness suppressed MMG amplitude during maximal or near maximal isometric contractions. Muscle stiffness is primarily a function of the number of loaded cross-bridges (4,17,19,20,24,28). It has been suggested (19,28) that at slow velocities all fiber types contribute to torque production; however, as the velocity of muscle action increases PT decreases because slow muscle fibers become unloaded. Therefore, it is possible that at slow velocities fast and slow fibers contributed to a high level of stiffness which interfered with muscle fiber oscillations and the amplitude of the MMG signal. As the velocity of muscle action increased, however, fewer fibers were loaded, resulting in reduced muscle stiffness and PT which allowed increased fiber oscillations and greater MMG amplitude.
Cross-bridge dynamics. Another factor that may have influenced the degree of muscle fiber oscillations is actin-myosin cross-bridge activity. Oster and Jaffe (34) reported that in isolated muscle preparations oscillations may be generated from the "making and breaking of cross-bridges," and "... a lack of synchrony in the oscillation of the sarcomeres" may be the origin of the MMG signal. Therefore, it is possible that as the velocity of muscle action increased the rate of cross-bridge cycling increased (34) which in turn caused greater vibratory motions of the sarcomeres resulting in increased MMG amplitude. This hypothesis, however, is not supported by recent findings (40) using isometric muscle actions that suggest that the pattern of MMG activity "reflects oscillations of muscle fibers rather than intrinsic mechanisms of muscle actions."
Turbulence of cellular mediums. A third factor that may have affected the MMG amplitude with increasing velocity for the CON muscle actions is the hydrodynamics within the muscle. Previous investigations have suggested that the fluid surrounding muscle fibers "... radiates a sort of slosh" which generates the sounds produced during movement (5), and the MMG signal "... must reflect the intrinsic properties of the sarcomere, such as contraction speed" (26). Therefore, as the velocity of muscle action increased there may have been greater intracellular and/or extracellular fluid turbulence that could account for the increased MMG amplitude. In addition, the velocity-stiffness relationship discussed previously may have played a role in the degree of fluid disturbance. The higher muscle action velocity may have resulted in decreased muscle stiffness, allowing the fibers to oscillate more freely, which could have resulted in a greater degree of fluid turbulence and, therefore, greater MMG amplitude.
Gender Difference in MMG Responses
Gender difference in the patterns of increase in MMG. The reason for the gender difference in the patterns of increase in CON MMG amplitude with velocity (Fig. 2) is unknown but may have been indirectly associated with velocity-related gender differences in the CON isokinetic PT patterns. In the present study, as well as others (12,22,42), it has been reported that females exhibit a greater percent decline in CON PT across velocity than males. This difference in the present investigation was represented by 33.3% and 28.5% declines in CON PT with increasing velocity in the females and males, respectively. Previous investigations (15,35) have reported that the amplitude of the MMG signal is sensitive to torque changes during dynamic muscle actions. Therefore, the greater percent decline across velocity in CON PT in females than in males may have resulted in an attenuation in MMG amplitude across velocity.
Greater MMG amplitude in males than females. The present findings indicated that males exhibited greater MMG amplitude at all CON muscle action velocities than the females (Fig. 2). The amplitude of the MMG signal is affected by a number of factors including muscle mass (4,5,7,21,26,29,32) and the tissue layer between the muscle and the surface of the skin (32). Although not measured in this investigation, the tendency of males to have greater muscle mass than females may have resulted in greater MMG amplitude values. In addition, the tendency of females to have a thicker adipose tissue layer over the vastus lateralis than males (25) may have attenuated the MMG signal and resulted in lower MMG amplitude values (32).
MMG Amplitude and Eccentric Muscle Actions
The results of the present investigation for ECC MMG amplitude (Fig. 2) revealed: a) positive relationships between MMG amplitude and velocity of muscle action for both the males and females, b) no gender difference in the patterns of ECC MMG responses with increasing velocity of muscle action, and c) significantly greater MMG amplitude values for males than for females at each muscle action velocity.
Increase in MMG Amplitude with Velocity
The positive relationships between MMG amplitude and velocity found in the present study for ECC muscle actions were consistent with the findings from our recent study (38) of eight males measured isokinetically at velocities ranging from 60 to 180°·s−1. The factors underlying the increases may be similar to those that affected the MMG amplitude during CON muscle actions. It is possible that an increase across velocity in the rate of making and pulling apart of the cross-bridges during the ECC muscle actions resulted in increased MMG amplitude. Furthermore, the velocity of limb movement may have affected the fluid hydrodynamics during the ECC muscle actions. As the velocity of muscle action increased, there may have been greater intracellular and/or extracellular fluid turbulence which resulted in increased MMG amplitude.
Gender Difference in MMG Responses
Gender difference in the patterns of increase in MMG. Unlike the CON muscle actions, there was no velocity-related gender difference in the patterns of increase in MMG during the ECC muscle actions. These results indirectly support our hypothesis that a gender difference in the patterns of CON MMG responses may have resulted from differences in the percent decline in CON isokinetic PT between males and females with increasing velocity. In the present study, ECC PT remained constant with increasing velocity for both the males and females, and there was no gender difference in the increase in MMG amplitude across velocities.
Greater MMG amplitude in males than females. The physiological mechanisms responsible for the greater MMG amplitude during ECC muscle actions in males than in females may be the same as those for CON muscle actions. Greater muscle mass in males and a thicker adipose tissue layer in females may have affected the MMG signal.
MMG Amplitude for Concentric Versus Eccentric Muscle Actions
The present findings indicated that MMG amplitude was consistently lower for ECC than CON isokinetic muscle actions at all test velocities for both the males and females. These differences were statistically significant at 90°·s−1 for males and at 30 and 90°·s−1 for females. These findings may have been a result of muscle action-related differences (ECC vs CON) in muscle stiffness and torque production. The higher ECC PT than CON PT may have resulted in greater muscle stiffness during the ECC muscle actions which interfered with fiber oscillations and resulted in lower MMG amplitude.
In conclusion, MMG amplitude increased with velocity in both the males and females for CON and ECC isokinetic muscle actions. There was a gender difference in the velocity-related patterns of MMG responses to maximal CON isokinetic muscle actions that may have been a result of velocity-related gender differences in CON PT patterns. There was no gender difference, however, in the patterns of ECC MMG responses. In addition, males displayed greater CON and ECC MMG amplitudes at all muscle action velocities than females which may have been related to gender differences in muscle mass and/or thickness of the adipose tissue layer. Furthermore, the MMG amplitude was lower for ECC than CON isokinetic muscle actions at all test velocities.
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