Non-invasive assessment of single motor unit mechanomyographic response and twitch force by spike-triggered averaging.
ABSTRACT A method for non-invasive assessment of single motor unit (MU) properties from electromyographic (EMG), mechanomyographic (MMG) and force signals is proposed. The method is based on the detection and classification of single MU action potentials from interference multichannel surface EMG signals and on the spike-triggered average of the MMG (detected by an accelerometer) and force signals. The first dorsal interosseous (FDI) and abductor digiti minimi (ADM) muscles were investigated at contraction levels of 2% and 5% of the maximum voluntary contraction (MVC) force. A third contraction was performed by selective activation of a single MU with surface MU action potential visual feedback provided to the subject. At 5% MVC, the mean (+/-standard error) single MU MMG peak-to-peak value was 11.0+/-1.8 mm s(-2) (N= 17) and 32.3+/-6.5 mm s(-2) (N=20) for the FDI and ADM muscles, respectively. The peak of the twitch force was, at the same contraction level, 7.41+/-1.34 mN and 14.42+/-2.92 mN, for the FDI and ADM muscles, respectively. The peak-to-peak value of the MMG was significantly different for the same MU at different contraction levels, indicating a non-linear summation of the single MU contributions. For the FDI muscle, the MMG peak-to-peak value of individual MUs was 21.5+/-7.8 mm s(-2), when such MUs were activated with visual feedback provided to the subject, whereas, for the same MUs, it was 11.8+/-3.8 mm s(-2), when the subject maintained a constant force level of 2% MVC. The method proposed allows the non-invasive assessment of single MU membrane and contractile properties during voluntary contractions.
- [show abstract] [hide abstract]
ABSTRACT: Work-related shoulder-neck pain is a major health risk in computer operators. To understand the physiological mechanisms behind the development of these disorders, EMG recordings of some minutes up to several hours must be accurately decomposed. For this reason we developed EMG-LODEC, an automatic decomposition software program, especially designed for multi-channel long-term recordings of signals detected during slight muscle movements. The subjects executed a 30-min computer task to simulate real work conditions while working at an ergonomically designed workstation. Six-channel intramuscular EMG signals were recorded from two positions of the upper trapezius muscle. The EMG signals were decomposed into individual motor unit action potential trains using EMG-LODEC. The study design enabled us first to study the dependence of intramuscular analysis on the insertion points and second to test the accuracy of the decomposition technique under laboratory conditions during a real experiment. The two positions yielded 887 motor units--452 located in position 1 and 435 in position 2. Although the numbers of detected action potentials were strongly correlated between the two insertion positions, different motor units were mostly recorded. In particular, the detection of continuously active motor units is specific for the selected insertion points and may not be representative of a muscle, not even for parts with common functions. The approach for the quantitative evaluation of the decomposition technique was to independently decompose two signals that were simultaneously detected by separate sets of wire electrodes placed close to each other in the muscle. Common trains discovered in each signal were compared for consistency. A cross-correlation analysis was performed to find corresponding motor unit pairs that were concurrently active. Concurrently active motor units were found in six subjects. For these motor units the extent of simultaneous occurrence of motor unit action potentials between the two positions ranged from 23% to 78% depending on the distinction of the single motor units and the number of superimposed motor unit action potentials. High concordance was seen in 3 out of the 15 motor unit pairs. Based on the results, EMG-LODEC is capable of providing reliable decompositions with satisfying accuracy and reasonable processing time. EMG-LODEC is suitable for the study of motor unit discharge patterns and recruitment order in subjects with and without musculoskeletal pain during long-term measurements to study work-related musculoskeletal disorders.Arbeitsphysiologie 09/2003; 89(6):526-35. · 2.66 Impact Factor
- [show abstract] [hide abstract]
ABSTRACT: The primary function of skeletal muscle is to produce force for postural control and movement. Although the contractile properties of the whole muscle are useful functional indicators, they do not accurately reflect the heterogeneity of the constituent motor units (MUs) and their changes in health and disease. However, data on the contractile properties of human MUs, in comparison to other animal species, are relatively sparse. This, in part, is due to greater methodological challenges of in vivo studies of MUs in the human. The purpose of this review is to critically appraise the methods used in humans; to describe the normative data from different muscle groups; to discuss differences between data from healthy humans and other animal species; and, last, to characterize changes of the MU contractile properties in aging, disease, and in response to intervention. Because the spike-triggered averaging technique can only be used to study the twitch properties, other methods were subsequently developed to measure a wider range of contractile properties. Although there is general agreement between human data and those from other animal species, major differences do exist. Potential reasons for these discrepancies include true biological differences, but differences in the techniques used may also be responsible. Although limited, measurement of MU contractile properties in humans has provided insight into the changes associated with aging and motoneuronal diseases and provides a means of gauging their adaptive capacity for training and immobilization.Muscle & Nerve 10/2001; 24(9):1113-33. · 2.31 Impact Factor
- [show abstract] [hide abstract]
ABSTRACT: Soundmyogram (SMG) and electromyogram signals were recorded simultaneously from the relatively fast medial gastrocnemius (MG) and slow soleus (SOL) during voluntary and electrically induced contractions. Using a spike-triggered averaging technique, the averaged elementary sound and corresponding MU spikes were also obtained from about 35 different MUs identified. The rms-SMG of MG increased as a function of force (P < 0.01). On the contrary, these values for SOL increased up to 60% MVC (P < 0.01), but decreased at 80% MVC. The relationship between the peak to peak amplitude of SMG and MU spike indicated significant positive correlations (r = 0.631 to approximately 0.657, P < 0.01). During electrical stimulation at 5 Hz, the SMG power spectral peak frequency (PF) was matched with stimulation frequency in both muscles. At higher stimulation frequencies, e.g., > 15 Hz, only in the MG was SMG-PF synchronized with stimulation frequency; the slow SOL did not show such synchronization. Our data suggest that the SMG frequency components might reflect active motor unit firing rates, and that the SMG amplitude depends upon mechanical properties of contraction, muscle fiber composition, and firing rate during voluntary and electrically induced contractions.Journal of Electromyography and Kinesiology 06/1999; 9(3):209-17. · 1.64 Impact Factor