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Dario Farina - One of the best experts on this subject based on the ideXlab platform.

  • sympathetic induced changes in discharge rate and spike triggered Average twitch torque of low threshold motor units in humans
    The Journal of Physiology, 2008
    Co-Authors: Silvestro Roatta, Lars Arendtnielsen, Dario Farina
    Abstract:

    Animal and in vitro studies have shown that the sympathetic nervous system modulates the contractility of skeletal muscle fibres, which may require adjustments in the motor drive to the muscle in voluntary contractions. In this study, these mechanisms were investigated in the tibialis anterior muscle of humans during sympathetic activation induced by the cold pressor test (CPT; left hand immersed in water at 4°C). In the first experiment, 11 healthy men performed 20 s isometric contractions at 10% of the maximal torque, before, during and after the CPT. In the second experiment, 12 healthy men activated a target motor unit at the minimum stable discharge rate for 5 min in the same conditions as in experiment 1. Intramuscular electromyographic (EMG) signals and torque were recorded and used to assess the motor unit discharge characteristics (experiment 1) and Spike-Triggered Average twitch torque (experiment 2). CPT increased the diastolic blood pressure and heart rate by (mean ±s.d.) 18 ± 9 mmHg and 4.7 ± 6.5 beats min−1 (P < 0.01), respectively. In experiment 1, motor unit discharge rate increased from 10.4 ± 1.0 pulses s−1 before to 11.1 ± 1.4 pulses s−1 (P < 0.05) during the CPT. In experiment 2, the twitch half-relaxation time decreased by 15.8 ± 9.3% (P < 0.05) during the CPT with respect to baseline. These results provide the first evidence of an adrenergic modulation of contractility of muscle fibres in individual motor units in humans, under physiological sympathetic activation.

  • amplitude cancellation of motor unit action potentials in the surface electromyogram can be estimated with spike triggered averaging
    Journal of Neurophysiology, 2008
    Co-Authors: Dario Farina, Corrado Cescon, Francesco Negro, Roger M. Enoka
    Abstract:

    The study presents analytical, simulation, and experimental analyses of amplitude cancellation of motor-unit action potentials (APs) in the interference electromyogram (EMG) and its relation to the size of the Spike-Triggered Average (STA) EMG. The amount of cancellation of motor-unit APs decreases monotonically as a function of the ratio between the root mean square (RMS) of the motor-unit AP and the RMS of the interference EMG signal. The theoretical derivation of this association indicates a method to measure cancellation in individual motor units by STA of the interference and squared EMGs. The theoretical relation was examined in both simulated EMG signals generated by populations of 200 motor units and experimental recordings of 492 and 184 motor-unit APs in the vastus medialis and abductor digiti minimi muscles, respectively. Although the theoretical relation predicted (R2 = 0.95; P < 0.001) the amount of cancellation in the simulated EMGs, the presence of motor-unit synchronization decreased the strength of the association for small APs. The decrease in size of the STA obtained from the squared EMG relative to that extracted from the interference EMG was predicted by the experimental measure of cancellation (R2 = 0.65; P < 0.001, for vastus medialis; R2 = 0.26; P < 0.05, for abductor digiti minimi). The results indicate that cancellation of APs in the interference EMG can be analytically predicted and experimentally measured with STA from the discharge times of the motor units into the surface EMG.

  • the pain induced decrease in low threshold motor unit discharge rate is not associated with the amount of increase in spike triggered Average torque
    Clinical Neurophysiology, 2008
    Co-Authors: Dario Farina, Lars Arendtnielsen, Silvestro Roatta, Thomas Gravennielsen
    Abstract:

    Abstract Objective Activation of nociceptive afferents decreases motor unit discharge rates in static contractions. There is also evidence that during experimental muscle pain the motor unit twitch force increases, which has been hypothesized to compensate for the decrease in discharge rate to maintain constant force. This study examined whether there is an association between the magnitude of change in motor unit discharge rate and the amount of increase in the Spike-Triggered Average torque during experimental muscle pain. Methods Sixteen subjects performed three constant-torque isometric ankle dorsi-flexions at 10% of the maximal force (MVC) alternated with two contractions at constant discharge rate of a target motor unit, before and following injection of 0.5ml of hypertonic (painful) or isotonic (control) saline into the tibialis anterior muscle. Results The discharge rate of the target unit at 10% MVC decreased following injection of hypertonic saline ( P P R =0.08). Propagation velocity and action potential peak-to-peak amplitude did not change with pain. Conclusions The pain-induced modifications in the estimated motor unit twitch torque (1) were not caused by changes in muscle fiber action potential, and (2) were not associated with the decrease in discharge rate. Significance Maintenance of constant force during static painful contractions is not explained by a matching between changes in contractile and control motor unit properties.

  • Cancellation of surface action potential amplitude in motor units of the vastus medialis muscle
    Neuroscience, 2007
    Co-Authors: Corrado Cescon, Francesco Negro, Roger M. Enoka, Dario Farina
    Abstract:

    The surface EMG underestimates the amount of motor unit activity due to the loss of information that occurs when positive and negative phases of action potentials cancel one another and reduce the amplitude of the signal (cancellation) It has been shown in simulation [1] that cancellation of action potentials of individual motor units is linearly correlated to the decrease in size of the Spike-Triggered Average of the rectified or squared EMG Thus, Spike-Triggered averaging [2,3] of the surface EMG can be in principle used to estimate amplitude cancellation but this has not been supported theoretically or verified experimentally Theoretical analysis of cancellation of single motor unit action potentials in relation to Spike-Triggered averaging to explain the results reported previously in simulation [1] Validation of the theoretical predictions with experimental recordings of motor units in the vastus medialis muscle The amount of cancellation of action potentials of individual motor units has been theoretically proven to depend only on the ratio between the RMS of the motor unit action potential and the RMS of the interference signal. According to this theoretical derivation, small surface action potentials are cancelled more than large action potentials. The ratio between the Spike-Triggered Average of the interference and the squared surface EMG is theoretically equivalent to the degree of cancellation of the action potential, which explains the results reported in [1]. The theoretical predictions have been experimentally validated for the vastus medialis muscle. Cancellation of action potential amplitude of single motor units can thus be theoretically predicted and experimentally quantified.

  • sympathetic modulation by cold pressor test alters the spike triggered Average torque and discharge rate of low threshold motor units
    2007
    Co-Authors: Silvestro Roatta, Lars Arendtnielsen, Corrado Cescon, Dario Farina
    Abstract:

    The sympathetic nervous system mediates functions that support motor activity and may affect the contractility of skeletal muscle as indicated by animal and in vitro studies (1,2). Evidence of this modulatory action in humans is still missing. This study aimed at investigating changes in motor unit twitch torque and discharge rate during sympathetic activation induced by the cold pressor test (CPT) in healthy male subjects. Two groups of subjects were studied according to two experimental protocols. In both studies subjects in sitting position were asked to perform ankle dorsi-flexions before, during, 1min after and 9 min after CPT (left hand immersed in water at 4°C for 5 min) (Fig. 1). The following signals were recorded: intramuscular EMG with wire electrodes from the tibialis anterior muscle, torque produced at the ankle joint, heart rate (HR), arterial blood pressure (ABP), and subjective pain rating. STUDY I: constant discharge rate. 12 subjects (age, mean ± SD, 28 ± 4.6 yr) were asked to maintain the discharge rate of a given motor unit (target motor unit) as low as possible (~8 pps), being provided with visual and auditory feedback of the intramuscular EMG signal. The twitch torque of the target motor unit was extracted from the torque signal by Spike-Triggered averaging. STUDY II: constant force. 11 subjects (age, 26.5 ± 4.5 yr) were asked to maintain an isometric contraction at 10% of the maximum voluntary contraction with force feedback. Individual motor units activated during the contractions were off-line identified and their discharge rate estimated. Statistics: Kruskal-Wallis test, Wilcoxon matched Paired test. Histograms: mean+SE 1) During CPT the motor unit twitch torque is shortened (study I); the resulting decreased degree of fusion of the twitches explains the observed increase in the estimated twitch amplitude. These effects are likely mediated by a direct action of adrenaline on skeletal muscle fibers. 2) The force produced by low-threshold motor units at a fixed discharge rate presumably decreases during CPT due to twitch shortening. 3) Motor unit discharge rate increases during CPT (study II), probably as a compensatory mechanism to maintain a constant force level. The study shows for the first time in humans that sympathetic activation substantially affects the contractility of skeletal muscle fibers and the discharge rate of motor units

Roger M. Enoka - One of the best experts on this subject based on the ideXlab platform.

  • amplitude cancellation of motor unit action potentials in the surface electromyogram can be estimated with spike triggered averaging
    Journal of Neurophysiology, 2008
    Co-Authors: Dario Farina, Corrado Cescon, Francesco Negro, Roger M. Enoka
    Abstract:

    The study presents analytical, simulation, and experimental analyses of amplitude cancellation of motor-unit action potentials (APs) in the interference electromyogram (EMG) and its relation to the size of the Spike-Triggered Average (STA) EMG. The amount of cancellation of motor-unit APs decreases monotonically as a function of the ratio between the root mean square (RMS) of the motor-unit AP and the RMS of the interference EMG signal. The theoretical derivation of this association indicates a method to measure cancellation in individual motor units by STA of the interference and squared EMGs. The theoretical relation was examined in both simulated EMG signals generated by populations of 200 motor units and experimental recordings of 492 and 184 motor-unit APs in the vastus medialis and abductor digiti minimi muscles, respectively. Although the theoretical relation predicted (R2 = 0.95; P < 0.001) the amount of cancellation in the simulated EMGs, the presence of motor-unit synchronization decreased the strength of the association for small APs. The decrease in size of the STA obtained from the squared EMG relative to that extracted from the interference EMG was predicted by the experimental measure of cancellation (R2 = 0.65; P < 0.001, for vastus medialis; R2 = 0.26; P < 0.05, for abductor digiti minimi). The results indicate that cancellation of APs in the interference EMG can be analytically predicted and experimentally measured with STA from the discharge times of the motor units into the surface EMG.

  • Cancellation of surface action potential amplitude in motor units of the vastus medialis muscle
    Neuroscience, 2007
    Co-Authors: Corrado Cescon, Francesco Negro, Roger M. Enoka, Dario Farina
    Abstract:

    The surface EMG underestimates the amount of motor unit activity due to the loss of information that occurs when positive and negative phases of action potentials cancel one another and reduce the amplitude of the signal (cancellation) It has been shown in simulation [1] that cancellation of action potentials of individual motor units is linearly correlated to the decrease in size of the Spike-Triggered Average of the rectified or squared EMG Thus, Spike-Triggered averaging [2,3] of the surface EMG can be in principle used to estimate amplitude cancellation but this has not been supported theoretically or verified experimentally Theoretical analysis of cancellation of single motor unit action potentials in relation to Spike-Triggered averaging to explain the results reported previously in simulation [1] Validation of the theoretical predictions with experimental recordings of motor units in the vastus medialis muscle The amount of cancellation of action potentials of individual motor units has been theoretically proven to depend only on the ratio between the RMS of the motor unit action potential and the RMS of the interference signal. According to this theoretical derivation, small surface action potentials are cancelled more than large action potentials. The ratio between the Spike-Triggered Average of the interference and the squared surface EMG is theoretically equivalent to the degree of cancellation of the action potential, which explains the results reported in [1]. The theoretical predictions have been experimentally validated for the vastus medialis muscle. Cancellation of action potential amplitude of single motor units can thus be theoretically predicted and experimentally quantified.

  • reduced control of motor output in a human hand muscle of elderly subjects during submaximal contractions
    Journal of Neurophysiology, 1993
    Co-Authors: M E Galganski, Andrew J Fuglevand, Roger M. Enoka
    Abstract:

    1. The effect of age on the motor output of the first dorsal interosseous muscle of 22 (6 female, 16 male) human subjects was investigated. The purpose of the study was to determine the effect of age on the control of muscle force and the associated changes in the discharge behavior and mechanical properties of single motor units. 2. Each subject performed three tasks requiring isometric abduction of the left index finger: a maximum voluntary contraction (MVC), a constant-force task, and a threshold task. The ability to control force was assessed during the constant-force task by quantifying the variation in isometric force about four submaximal target forces (5, 20, 35, and 50% MVC). The threshold task involved sustaining the discharge of the isolated motor unit at a low, steady rate for approximately 3 min. 3. The discharge behavior and the mechanical properties of single motor units were determined during the threshold task by measuring the interimpulse intervals and the peak amplitude and time to peak of the Spike-Triggered Average force. 4. The data indicated that age had an effect on the variation of force about submaximal target forces (range: 5-50% MVC), and that these force variations, when calculated relative to the target force, were greater at lower force levels in the elderly subjects.(ABSTRACT TRUNCATED AT 250 WORDS)

Thomas Gravennielsen - One of the best experts on this subject based on the ideXlab platform.

  • the pain induced decrease in low threshold motor unit discharge rate is not associated with the amount of increase in spike triggered Average torque
    Clinical Neurophysiology, 2008
    Co-Authors: Dario Farina, Lars Arendtnielsen, Silvestro Roatta, Thomas Gravennielsen
    Abstract:

    Abstract Objective Activation of nociceptive afferents decreases motor unit discharge rates in static contractions. There is also evidence that during experimental muscle pain the motor unit twitch force increases, which has been hypothesized to compensate for the decrease in discharge rate to maintain constant force. This study examined whether there is an association between the magnitude of change in motor unit discharge rate and the amount of increase in the Spike-Triggered Average torque during experimental muscle pain. Methods Sixteen subjects performed three constant-torque isometric ankle dorsi-flexions at 10% of the maximal force (MVC) alternated with two contractions at constant discharge rate of a target motor unit, before and following injection of 0.5ml of hypertonic (painful) or isotonic (control) saline into the tibialis anterior muscle. Results The discharge rate of the target unit at 10% MVC decreased following injection of hypertonic saline ( P P R =0.08). Propagation velocity and action potential peak-to-peak amplitude did not change with pain. Conclusions The pain-induced modifications in the estimated motor unit twitch torque (1) were not caused by changes in muscle fiber action potential, and (2) were not associated with the decrease in discharge rate. Significance Maintenance of constant force during static painful contractions is not explained by a matching between changes in contractile and control motor unit properties.

  • spike triggered Average torque and muscle fiber conduction velocity of low threshold motor units following submaximal endurance contractions
    Journal of Applied Physiology, 2005
    Co-Authors: Dario Farina, Lars Arendtnielsen, Thomas Gravennielsen
    Abstract:

    The motor unit twitch torque is modified by sustained contraction, but the association to changes in muscle fiber electrophysiological properties is not fully known. Thus twitch torque, muscle fiber conduction velocity, and action potential properties of single motor units were assessed in 11 subjects following an isometric submaximal contraction of the tibialis anterior muscle until endurance. The volunteers activated a target motor unit at the minimum discharge rate in eight 3-min-long contractions, three before and five after an isometric contraction at 40% of the maximal torque, sustained until endurance. Multichannel surface electromyogram signals and joint torque were Averaged with the target motor unit potential as trigger. Discharge rate (mean +/- SE, 6.6 +/- 0.2 pulses/s) and interpulse interval variability (33.3 +/- 7.0%) were not different in the eight contractions. Peak twitch torque and recruitment threshold increased significantly (93 +/- 29 and 12 +/- 5%, P <0.05) in the contraction immediately after the endurance task with respect to the preendurance values (0.94 +/- 0.26 mN.m and 3.7 +/- 0.5% of the maximal torque), whereas time to peak of the twitch torque did not change (74.4 +/- 10.1 ms). Muscle fiber conduction velocity decreased and action potential duration increased in the contraction after the endurance (6.3 +/- 1.8 and 9.8 +/- 1.8%, respectively, P <0.05; preendurance values, 3.9 +/- 0.2 m/s and 11.1 +/- 0.8 ms), whereas the surface potential peak-to-peak amplitude did not change (27.1 +/- 3.1 microV). There was no significant correlation between the relative changes in muscle fiber conduction velocity or surface potential duration and in peak twitch torque (R2= 0.04 and 0.10, respectively). In conclusion, modifications in peak twitch torque of low-threshold motor units with sustained contraction are mainly determined by mechanisms not related to changes in action potential shape and in its propagation velocity.

Corrado Cescon - One of the best experts on this subject based on the ideXlab platform.

  • amplitude cancellation of motor unit action potentials in the surface electromyogram can be estimated with spike triggered averaging
    Journal of Neurophysiology, 2008
    Co-Authors: Dario Farina, Corrado Cescon, Francesco Negro, Roger M. Enoka
    Abstract:

    The study presents analytical, simulation, and experimental analyses of amplitude cancellation of motor-unit action potentials (APs) in the interference electromyogram (EMG) and its relation to the size of the Spike-Triggered Average (STA) EMG. The amount of cancellation of motor-unit APs decreases monotonically as a function of the ratio between the root mean square (RMS) of the motor-unit AP and the RMS of the interference EMG signal. The theoretical derivation of this association indicates a method to measure cancellation in individual motor units by STA of the interference and squared EMGs. The theoretical relation was examined in both simulated EMG signals generated by populations of 200 motor units and experimental recordings of 492 and 184 motor-unit APs in the vastus medialis and abductor digiti minimi muscles, respectively. Although the theoretical relation predicted (R2 = 0.95; P < 0.001) the amount of cancellation in the simulated EMGs, the presence of motor-unit synchronization decreased the strength of the association for small APs. The decrease in size of the STA obtained from the squared EMG relative to that extracted from the interference EMG was predicted by the experimental measure of cancellation (R2 = 0.65; P < 0.001, for vastus medialis; R2 = 0.26; P < 0.05, for abductor digiti minimi). The results indicate that cancellation of APs in the interference EMG can be analytically predicted and experimentally measured with STA from the discharge times of the motor units into the surface EMG.

  • Cancellation of surface action potential amplitude in motor units of the vastus medialis muscle
    Neuroscience, 2007
    Co-Authors: Corrado Cescon, Francesco Negro, Roger M. Enoka, Dario Farina
    Abstract:

    The surface EMG underestimates the amount of motor unit activity due to the loss of information that occurs when positive and negative phases of action potentials cancel one another and reduce the amplitude of the signal (cancellation) It has been shown in simulation [1] that cancellation of action potentials of individual motor units is linearly correlated to the decrease in size of the Spike-Triggered Average of the rectified or squared EMG Thus, Spike-Triggered averaging [2,3] of the surface EMG can be in principle used to estimate amplitude cancellation but this has not been supported theoretically or verified experimentally Theoretical analysis of cancellation of single motor unit action potentials in relation to Spike-Triggered averaging to explain the results reported previously in simulation [1] Validation of the theoretical predictions with experimental recordings of motor units in the vastus medialis muscle The amount of cancellation of action potentials of individual motor units has been theoretically proven to depend only on the ratio between the RMS of the motor unit action potential and the RMS of the interference signal. According to this theoretical derivation, small surface action potentials are cancelled more than large action potentials. The ratio between the Spike-Triggered Average of the interference and the squared surface EMG is theoretically equivalent to the degree of cancellation of the action potential, which explains the results reported in [1]. The theoretical predictions have been experimentally validated for the vastus medialis muscle. Cancellation of action potential amplitude of single motor units can thus be theoretically predicted and experimentally quantified.

  • sympathetic modulation by cold pressor test alters the spike triggered Average torque and discharge rate of low threshold motor units
    2007
    Co-Authors: Silvestro Roatta, Lars Arendtnielsen, Corrado Cescon, Dario Farina
    Abstract:

    The sympathetic nervous system mediates functions that support motor activity and may affect the contractility of skeletal muscle as indicated by animal and in vitro studies (1,2). Evidence of this modulatory action in humans is still missing. This study aimed at investigating changes in motor unit twitch torque and discharge rate during sympathetic activation induced by the cold pressor test (CPT) in healthy male subjects. Two groups of subjects were studied according to two experimental protocols. In both studies subjects in sitting position were asked to perform ankle dorsi-flexions before, during, 1min after and 9 min after CPT (left hand immersed in water at 4°C for 5 min) (Fig. 1). The following signals were recorded: intramuscular EMG with wire electrodes from the tibialis anterior muscle, torque produced at the ankle joint, heart rate (HR), arterial blood pressure (ABP), and subjective pain rating. STUDY I: constant discharge rate. 12 subjects (age, mean ± SD, 28 ± 4.6 yr) were asked to maintain the discharge rate of a given motor unit (target motor unit) as low as possible (~8 pps), being provided with visual and auditory feedback of the intramuscular EMG signal. The twitch torque of the target motor unit was extracted from the torque signal by Spike-Triggered averaging. STUDY II: constant force. 11 subjects (age, 26.5 ± 4.5 yr) were asked to maintain an isometric contraction at 10% of the maximum voluntary contraction with force feedback. Individual motor units activated during the contractions were off-line identified and their discharge rate estimated. Statistics: Kruskal-Wallis test, Wilcoxon matched Paired test. Histograms: mean+SE 1) During CPT the motor unit twitch torque is shortened (study I); the resulting decreased degree of fusion of the twitches explains the observed increase in the estimated twitch amplitude. These effects are likely mediated by a direct action of adrenaline on skeletal muscle fibers. 2) The force produced by low-threshold motor units at a fixed discharge rate presumably decreases during CPT due to twitch shortening. 3) Motor unit discharge rate increases during CPT (study II), probably as a compensatory mechanism to maintain a constant force level. The study shows for the first time in humans that sympathetic activation substantially affects the contractility of skeletal muscle fibers and the discharge rate of motor units

Lars Arendtnielsen - One of the best experts on this subject based on the ideXlab platform.

  • sympathetic induced changes in discharge rate and spike triggered Average twitch torque of low threshold motor units in humans
    The Journal of Physiology, 2008
    Co-Authors: Silvestro Roatta, Lars Arendtnielsen, Dario Farina
    Abstract:

    Animal and in vitro studies have shown that the sympathetic nervous system modulates the contractility of skeletal muscle fibres, which may require adjustments in the motor drive to the muscle in voluntary contractions. In this study, these mechanisms were investigated in the tibialis anterior muscle of humans during sympathetic activation induced by the cold pressor test (CPT; left hand immersed in water at 4°C). In the first experiment, 11 healthy men performed 20 s isometric contractions at 10% of the maximal torque, before, during and after the CPT. In the second experiment, 12 healthy men activated a target motor unit at the minimum stable discharge rate for 5 min in the same conditions as in experiment 1. Intramuscular electromyographic (EMG) signals and torque were recorded and used to assess the motor unit discharge characteristics (experiment 1) and Spike-Triggered Average twitch torque (experiment 2). CPT increased the diastolic blood pressure and heart rate by (mean ±s.d.) 18 ± 9 mmHg and 4.7 ± 6.5 beats min−1 (P < 0.01), respectively. In experiment 1, motor unit discharge rate increased from 10.4 ± 1.0 pulses s−1 before to 11.1 ± 1.4 pulses s−1 (P < 0.05) during the CPT. In experiment 2, the twitch half-relaxation time decreased by 15.8 ± 9.3% (P < 0.05) during the CPT with respect to baseline. These results provide the first evidence of an adrenergic modulation of contractility of muscle fibres in individual motor units in humans, under physiological sympathetic activation.

  • the pain induced decrease in low threshold motor unit discharge rate is not associated with the amount of increase in spike triggered Average torque
    Clinical Neurophysiology, 2008
    Co-Authors: Dario Farina, Lars Arendtnielsen, Silvestro Roatta, Thomas Gravennielsen
    Abstract:

    Abstract Objective Activation of nociceptive afferents decreases motor unit discharge rates in static contractions. There is also evidence that during experimental muscle pain the motor unit twitch force increases, which has been hypothesized to compensate for the decrease in discharge rate to maintain constant force. This study examined whether there is an association between the magnitude of change in motor unit discharge rate and the amount of increase in the Spike-Triggered Average torque during experimental muscle pain. Methods Sixteen subjects performed three constant-torque isometric ankle dorsi-flexions at 10% of the maximal force (MVC) alternated with two contractions at constant discharge rate of a target motor unit, before and following injection of 0.5ml of hypertonic (painful) or isotonic (control) saline into the tibialis anterior muscle. Results The discharge rate of the target unit at 10% MVC decreased following injection of hypertonic saline ( P P R =0.08). Propagation velocity and action potential peak-to-peak amplitude did not change with pain. Conclusions The pain-induced modifications in the estimated motor unit twitch torque (1) were not caused by changes in muscle fiber action potential, and (2) were not associated with the decrease in discharge rate. Significance Maintenance of constant force during static painful contractions is not explained by a matching between changes in contractile and control motor unit properties.

  • sympathetic modulation by cold pressor test alters the spike triggered Average torque and discharge rate of low threshold motor units
    2007
    Co-Authors: Silvestro Roatta, Lars Arendtnielsen, Corrado Cescon, Dario Farina
    Abstract:

    The sympathetic nervous system mediates functions that support motor activity and may affect the contractility of skeletal muscle as indicated by animal and in vitro studies (1,2). Evidence of this modulatory action in humans is still missing. This study aimed at investigating changes in motor unit twitch torque and discharge rate during sympathetic activation induced by the cold pressor test (CPT) in healthy male subjects. Two groups of subjects were studied according to two experimental protocols. In both studies subjects in sitting position were asked to perform ankle dorsi-flexions before, during, 1min after and 9 min after CPT (left hand immersed in water at 4°C for 5 min) (Fig. 1). The following signals were recorded: intramuscular EMG with wire electrodes from the tibialis anterior muscle, torque produced at the ankle joint, heart rate (HR), arterial blood pressure (ABP), and subjective pain rating. STUDY I: constant discharge rate. 12 subjects (age, mean ± SD, 28 ± 4.6 yr) were asked to maintain the discharge rate of a given motor unit (target motor unit) as low as possible (~8 pps), being provided with visual and auditory feedback of the intramuscular EMG signal. The twitch torque of the target motor unit was extracted from the torque signal by Spike-Triggered averaging. STUDY II: constant force. 11 subjects (age, 26.5 ± 4.5 yr) were asked to maintain an isometric contraction at 10% of the maximum voluntary contraction with force feedback. Individual motor units activated during the contractions were off-line identified and their discharge rate estimated. Statistics: Kruskal-Wallis test, Wilcoxon matched Paired test. Histograms: mean+SE 1) During CPT the motor unit twitch torque is shortened (study I); the resulting decreased degree of fusion of the twitches explains the observed increase in the estimated twitch amplitude. These effects are likely mediated by a direct action of adrenaline on skeletal muscle fibers. 2) The force produced by low-threshold motor units at a fixed discharge rate presumably decreases during CPT due to twitch shortening. 3) Motor unit discharge rate increases during CPT (study II), probably as a compensatory mechanism to maintain a constant force level. The study shows for the first time in humans that sympathetic activation substantially affects the contractility of skeletal muscle fibers and the discharge rate of motor units

  • spike triggered Average torque and muscle fiber conduction velocity of low threshold motor units following submaximal endurance contractions
    Journal of Applied Physiology, 2005
    Co-Authors: Dario Farina, Lars Arendtnielsen, Thomas Gravennielsen
    Abstract:

    The motor unit twitch torque is modified by sustained contraction, but the association to changes in muscle fiber electrophysiological properties is not fully known. Thus twitch torque, muscle fiber conduction velocity, and action potential properties of single motor units were assessed in 11 subjects following an isometric submaximal contraction of the tibialis anterior muscle until endurance. The volunteers activated a target motor unit at the minimum discharge rate in eight 3-min-long contractions, three before and five after an isometric contraction at 40% of the maximal torque, sustained until endurance. Multichannel surface electromyogram signals and joint torque were Averaged with the target motor unit potential as trigger. Discharge rate (mean +/- SE, 6.6 +/- 0.2 pulses/s) and interpulse interval variability (33.3 +/- 7.0%) were not different in the eight contractions. Peak twitch torque and recruitment threshold increased significantly (93 +/- 29 and 12 +/- 5%, P <0.05) in the contraction immediately after the endurance task with respect to the preendurance values (0.94 +/- 0.26 mN.m and 3.7 +/- 0.5% of the maximal torque), whereas time to peak of the twitch torque did not change (74.4 +/- 10.1 ms). Muscle fiber conduction velocity decreased and action potential duration increased in the contraction after the endurance (6.3 +/- 1.8 and 9.8 +/- 1.8%, respectively, P <0.05; preendurance values, 3.9 +/- 0.2 m/s and 11.1 +/- 0.8 ms), whereas the surface potential peak-to-peak amplitude did not change (27.1 +/- 3.1 microV). There was no significant correlation between the relative changes in muscle fiber conduction velocity or surface potential duration and in peak twitch torque (R2= 0.04 and 0.10, respectively). In conclusion, modifications in peak twitch torque of low-threshold motor units with sustained contraction are mainly determined by mechanisms not related to changes in action potential shape and in its propagation velocity.