Electromyographic and mechanomyographic estimation of motor unit activation strategy in voluntary force production.

Electromyographic and mechanomyographic estimation of motor unit activation strategy in voluntary force production. In order to determine whether electromyogram (EMG) and mechanomyogram (MMG) are suitable for the noninvasive estimation of the motor unit (MU) activation strategy, the EMG/force and MMG/force relationships were examined simultaneously during isometric ramp contractions in biceps brachii muscle. The highest mean power frequency (MPF) of the EMG, which reflects the full MU recruitment, was determined at 51% MVC. Two obvious inflection points were identified on the MMG-amplitude/force relationship that showed an initial slow increase followed by a rapid increase and a progressive decrease at higher force levels. Our results suggest that the MMG amplitude allows the estimation of the beginning of recruitment of MUs that innervate the first-twitch fibers in addition to identification of the full MU recruitment. The rate coding strategy was qualitatively reflected by the MMG-MPF/force relationship. We conclude that the MU activation strategy is estimated in more detail by the MMG than by the EMG.

Reliability of the mechanomyogram detected with an accelerometer during voluntary contractions.

The accelerometer is used for mechanomyogram (MMG) recordings of muscle contractions. Although the mechanical characteristics of other MMG transducers have been determined with reference to the accelerometer, mechanical aspects of the accelerometer itself, including the weight of the transducer, have not been verified. This study was designed to reinvestigate the mechanical variable of the MMG signal detected with an accelerometer, with reference to a laser distance sensor (LDS), and then to clarify the influence of the accelerometer weight on the MMG recording during muscle contractions. The study was performed during mechanical sinusoidal vibrations and during voluntary contractions of the quadriceps muscle. Maximum differences in the amplitude spectral density functions between the LDS signal and the double integral of the accelerometer signal were approximately 4 microm. The results verified that the MMG signal from the accelerometer accurately reflected the acceleration of body surface vibration. However, the MMG signal was gradually distorted when weight was added to the accelerometer: the addition of 4.0 g (total 6.0 g, including 2.0 g of accelerometer) substantially attenuated the MMG signal. The results suggest that the appropriate weight for the accelerometer should be less than 5.0 g for measurements of the quadriceps muscle and indicate that the transducer weight must be taken into account for accurate measurement of muscles of different sizes.

Mechanomyogram and force relationship during voluntary isometric ramp contractions of the biceps brachii muscle.

The aim of the present study was to examine the non-stationary mechanomyogram (MMG) during voluntary isometric ramp contractions of the biceps brachii muscles using the short-time Fourier transform, and to obtain more detailed information on the motor unit (MU) activation strategy underlying in the continuous MMG/force relationship. The subjects were asked to exert ramp contractions from 5% to 80% of the maximal voluntary contraction (MVC) at a constant rate of 10% MVC/s. The root mean squared (RMS) amplitude of the MMG began to increase slowly at low levels of force, then there was a slight reduction between 12% and 20% MVC. After that, a progressive increase was followed by a decrease beyond 60% MVC. As to the mean power frequency (MPF), a relatively rapid increase up to 30% MVC was followed by a period of slow increment between 30% and 50% MVC. Then temporary reduction at around 50% MVC and a further rapid increase above 60% MVC was observed. The interaction between amplitude and MPF of the MMG in relation to the MU activation strategy is discussed for five force regions defined on the basis of their inflection points in the RMS-amplitude/force and MPF/force relationships. It was found that the MMG during ramp contractions enables deeper insights into the MU activation strategy than those determined during traditional separate contractions. In addition, this contraction protocol is useful not only to ensure higher force resolution in the MMG/force relationship, but also to markedly shorten the time taken for data acquisition and to reduce the risk of fatigue.

Mechanical behaviour of condenser microphone in mechanomyography.

Condenser microphones (MIC) have been widely used in mechanomyography, together with accelerometers and piezoelectric contact sensors. The aim of the present investigation was to clarify the mechanical variable (acceleration, velocity or displacement) indicated by the signal from a MIC transducer using a mechanical sinusoidal vibration system. In addition, the mechanomyogram (MMG) was recorded simultaneously with a MIC transducer and accelerometer (ACC) during voluntary contractions to confirm the mechanical variable reflected by the actual MMG and to examine the influence of motion artifact on the MMG. To measure the displacement-frequency response, mechanical sinusoidal vibrations of 3 to 300 Hz were applied to the MIC transducer with different sizes of air chambers (5, 10, 15 and 20 mm in diameter and 15, 20 or 25 mm long). The MIC transducer showed a linear relationship between the output amplitude and the vibration displacement, however, its frequency response declined with decreasing diameter and decreasing length of the air chamber. In fact, the cut-off frequency (-3dB) of the MIC transducer with the 5-mm-diameter chamber was 10, 8 and 4 Hz for the length 15, 20 and 25 mm, respectively. The air chamber with at least a diameter of 10 mm and a length of 15 mm is recommended for the MIC transducer. The sensitivity of this MIC transducer arrangement was 92 mV microm(-1) when excited at 100 Hz. During voluntary contraction, the amplitude spectral density function of the MMG from the MIC transducer resembled that of the double integral of the ACC transducer signal. The angle of the MIC transducer was delayed by 180 degrees in relation to the ACC transducer signal. The sensitivity of the MIC transducer was reduced to one-third because of the peculiar volume change of air chamber when the MMG was detected on the surface of the skin. In addition, the MIC transducer was contaminated by a smaller motion artifact than that from the ACC transducer. The maximal peak amplitude of the MIC and ACC transducer signal with the motion artifact was 7.7 and 12.3 times as much as the RMS amplitude of each signal without the motion artifact, respectively. These findings suggest that the MIC transducer acts as a displacement meter in the MMG. The MIC transducer seems to be a possible candidate for recording the MMG during dynamic muscle contractions as well as during sustained contractions.

Mechanomyographic investigation of muscle contractile properties in preadolescent boys.

The aim of the present study was to elucidate the muscle contractile properties of preadolescent boys using the mechanomyogram (MMG) and electromyogram (EMG). In 8 preadolescent boys and 10 male young adults, the EMG and MMG were recorded from quadriceps muscles during isometric knee extension contractions at levels of 10 to 80% MVC. The relationship of EMG and MMG to absolute force was approximated by a regression line (r > 0.8). The regression line of the EMG in preadolescent was located above that of the adults, whereas the MMG in both subject groups was fitted by virtually the same regression line. Thus, the MMG seems to be indicative of absolute force in preadolescents as well as young adults. Both groups showed a non-linear increase in the EMG and MMG with relative force (%MVC). The EMG and %MVC relationship was not significantly different between the two groups, whereas the MMG in preadolescents was smaller than that of adults over the range of force studied. The MMG increment with %MVC was more remarkable above 30 or 40% MVC in both groups; however, the rate of the increment in preadolescents was smaller than that in adults. The MMG and %MVC relationship suggests that the muscle contractile properties in preadolescent boys are characterized by immaturity of the fast twitch fibers.

Strain on the gastrocnemii and hamstrings affecting standing balance on an inclined plane in spastic cerebral palsy. A study using a geometric model.

The purpose of this study was to use an non-invasive method to determine whether strain on the gastrocnemii and hamstrings influences postural balance in spastic cerebral palsy (CP). Changes in alignment during standing posture with subjects positioned on a platform that was gradually inclined were measured in 10 normal children and 11 children with CP. The changes in postural alignment were plotted and geometric models used to determine the lines where the gastrocnemii and hamstrings were maximally stretched. In this way the relationship between postural alignment and the amount of strain on the gastrocnemii and hamstrings was investigated. On the inclined platform, which caused ankle joints to become dorsiflexed as the inclination angle increased, the gastrocnemii began to be strained and the hip joints began to be flexed (trunk bent forward) at the same time. In the children with CP, the gastrocnemii were more strained by smaller degrees of inclination. Furthermore, there was one child with CP whose hamstrings were also strained on the inclined platform. We confirmed that postural balance was affected by strain on the gastrocnemii and hamstrings.

Technical aspects of mechnomyography recording with piezoelectric contact sensor.

The piezoelectric contact sensor has been widely utilised in mechanomyography (MMG). The authors aim to clarify the mechanical variables (i.e. acceleration, velocity or displacement) reflected by the MMG signal detected with a piezoelectric contact sensor (PEC), and compare the results with those obtained simultaneously by an accelerometer (ACC). To measure the acceleration-frequency response, a mechanical sinusoidal excitation of 5 to 300 Hz at a constant magnitude of 0.01 G was applied to the two transducers. The acceleration-frequency response of the ACC transducer was confirmed to be almost flat. The PEC without any restriction of the transducer housing (including the combined seismic mass) demonstrated a similar response to the ACC transducer. The PEC transducer output with restricted housing decreased with increasing sinusoidal frequency and an attenuation slope of -40 dB/decade and phase angle of -180 degrees. The voluntary MMG signal during isometric knee extension was recorded simultaneously with the two transducers. The amplitude spectral density distribution of the MMG from the PEC transducer was narrow and the mean frequency was approximately one-half that obtained from the ACC transducer. The amplitude spectral density distribution with the PEC transducer resembled that of the double integral over time of the ACC transducer signal. The phase angle of the PEC transducer signal was different from that of the ACC transducer signal by approximately -180 degrees. These results suggest that the PEC transducer acts as a displacement meter of muscle vibration. In addition, differences in the MMG frequency components relating to the transducer type must be taken into consideration when investigating the mechanical activity of muscle.

Acoustic and electrical activities during voluntary isometric contraction of biceps brachii muscles in patients with spastic cerebral palsy.

This study was designed to compare electromyogram (EMG) and acoustic myogram (AMG) recordings of biceps brachii muscles in patients with spastic cerebral palsy (CP). The maximal voluntary contraction (MVC) in the CP group was approximately one half of that of the normal group even after being normalized by the muscle cross-sectional area (CSA) (18.6 - 5.9 kNm/m2 in CP, 37.3 +/- 2.9 kNm/m2 in normal). Both CP and normal groups demonstrated a progressive increase in the root mean squared values per unit muscle CSA in the EMG (RMSEMG/CSA) as well as in the AMG (RMSAMG/CSA) with increasing force up to 50% MVC. The increasing magnitude of the RMSEMG/CSA with force was not significantly different between two subject groups. However, all the levels of force resulted in significantly smaller RMSAMG/CSA in the CP group compared to the normal group. The ratios of RMSAMG to RMSEMG in the CP group (0.75 +/- 0.03 m/s2/mV) were significantly smaller than those in the normal group (1.37 +/- 0.07 m/s2/mV) at force levels above 30% MVC. These results suggest that motor disabilities in CP patients are caused not only by primary neural impairment but also by secondary deterioration in muscular contractile properties, probably resulting from muscle fiber atrophy. This appears to be more selective in fast twitch fibers.

The effects of mechanical compression and hypoxia on nerve root and dorsal root ganglia. An analysis of ectopic firing using an in vitro model.

STUDY DESIGN: This study analyzed in vitro experiments of ectopic firing evoked by mechanical compression or hypoxia of canine lumbar dorsal roots with dorsal root ganglia using an in vitro model. OBJECTIVES: The results were correlated to understand the pathophysiology of radiculopathy, which manifests abnormal sensation and pain. SUMMARY OF BACKGROUND DATA: It has been speculated that blood flow in the nerve root and mechanical compression play major roles in the production of radiculopathy symptoms. However, no precise experimental studies have been conducted on the relationship between these factors and the development of ectopic firing. METHODS: Canine lumbar dorsal roots with dorsal root ganglia were immersed in an oxygenated artificial cerebrospinal fluid, and activity of the nerve root was recorded using bipolar platinum electrodes. Using this model, the effects of quantitative mechanical compression and hypoxia on the ectopic firing were analyzed. RESULTS: When compression was applied, mechanical thresholds for eliciting firing were much lower in dorsal root ganglia than in dorsal roots, and the firing lasted for a longer period in dorsal root ganglia. Under hypoxia, dorsal root ganglia showed firing, and their thresholds from mechanical stimuli decreased significantly. In dorsal roots, impulse propagation was not affected, whereas firing was seen under the hypoxic condition. CONCLUSION: Dorsal root ganglia are highly sensitive to mechanical compression and hypoxia and closely related to abnormal sensations and pain in radiculopathy.

The physiology of mechanoreceptors in the anterior cruciate ligament. An experimental study in decerebrate-spinalised animals.

The physiological role of mechanoreceptors in the anterior cruciate ligament (ACL) was studied in unanaesthetised decerebrate-spinalised cats and dogs. Tonic activity in the quadriceps and the hamstring increased in response to physiological loading of the ACL. Evoked potentials in the posterior articular nerve (PAN) were elicited by electrical stimulation of the surface of the ligament. ACL loading also induced significant discharges from the PAN. The results suggest that ACL loading has an excitatory effect on the thigh muscles through a multimotor neurone output, and that the PAN is one of the afferent routes from the mechanoreceptors of the ACL. The ACL-muscle reflex may therefore play a physiological role in maintaining knee kinematics.

An investigation of the accuracy in measuring the body center of pressure in a standing posture with a force plate.

It has been reported that accuracy in the measured point of force application depends on the position and magnitude of load. The aim of this study is to investigate the error in measuring the body center of pressure in a standing posture with a force plate and to improve the accuracy by proposing a correction algorithm. The measurement of the point of force application with a calibrated load demonstrated its co-ordinates, which were scaled down towards the zero co-ordinate and moved parallel relative to the true co-ordinates. This trend was exaggerated with any decrease in magnitude of the load. The root mean square (r.m.s.) and maximum of errors at light load of 10 kg were 10 and 18 mm in the X axis, and 6 and 12 mm in the Y axis, respectively. The error seems to result from such causes as (i) non-linearity of the load cell, (ii) deformation of the top plate due to load application and (iii) differences in characteristics among individual load cells, including amplifiers. A mathematical representation of the measured point of force application accounting for these causes of error has been made and an algorithm for estimating the true point at any magnitude of load with only one correction equation is proposed. Actual correction of the measured point demonstrated an expected improvement in the r.m.s. error to less than 1 mm at any magnitude of load greater than 10 kg and the validity of this algorithm was confirmed.

Analysis of spinal cord evoked potential and locomotor function during acute spinal cord compression in cats.

The aim of this study was to investigate whether or not conductive spinal cord evoked potentials and spinal cord function change correspondingly with each other. The relationship between conductive spinal cord evoked potentials and locomotor function during acute spinal cord compression in animals was investigated. In decerebrate cats, controlled locomotion can be induced by electrical stimuli in the mesencephalic locomotor region. Conductive spinal cord evoked potentials were recorded at the L3 level of the spinal cord and stimuli were given at the T4 segment. The locomotor function was evaluated through electromyograms of the hind limbs. By compressing the spinal cord at L1, both the conductive spinal cord evoked potentials and the locomotor function gradually decreased. When the first negative potential amplitude of conductive spinal cord evoked potentials was decreased to half the level found in normal cats, locomotor function was injured irreversibly. These results showed that changes in the conductive spinal cord evoked potentials were related to changes in locomotor function. The 50% level of the first negative potential amplitude was considered to be the critical level at which irreversible spinal cord paralysis occurred in the cats.

[The influence of soft tissue contractures on the walking ability of patients with spastic cerebral palsy].

The influence of soft tissue contractures on walking ability was investigated in 73 patients with spastic cerebral palsy. The relationship between the degree of soft tissue contractures and ambulation level was analyzed using Hayashi's quantification theory type II. The walking energy expenditure and electromyography magnitude (E.M.G. magnitude) were also measured in 11 patients who could walk on treadmill. Hayashi's quantification theory type II showed a significant correlation between the degree of soft tissue contractures and ambulation level. Both energy expenditure and E.M.G. magnitude were found to be significantly higher than in normal subjects. The levels increased in relation to walking speed. A significant correlation between walking energy expenditure and the degree of soft tissue contractures was found using multiple regression analysis. From these results, it was concluded that the soft tissue contractures had a strong influence on ambulation level and walking energy expenditure in spastic cerebral palsy.