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Firing rates of motor units in human vastus lateralis muscle during fatiguing isometric contractions.
J Appl Physiol (1985). 2005 Jul; 99(1):268-80.JA

Abstract

We investigated the firing rate of motor units in the vastus lateralis muscle in five healthy young men (mean = 21.4 yr, SD = 0.9) during a sequence of isometric constant-torque contractions repeated to exhaustion. The contractions were sustained at 20% of the maximal voluntary level, measured at the beginning of the test sequence. Electromyographic (EMG) signals were recorded via quadrifilar fine-wire electrodes and subsequently decomposed into their constituent motor unit action potentials to obtain the motor unit firing times. In addition, we measured the whole muscle mechanical properties during the fatigue task using electrical stimulation. The firing rate of motor units first decreased within the first 10-20% of the endurance time of the contractions and then increased. The firing rate increase was accompanied by recruitment of additional motor units as the force output remained constant. The elicited twitch and tetanic torque responses first increased and then decreased. The two processes modulated in a complementary fashion at the same time. Our data suggest that, when the vastus lateralis muscle is activated to maintain a constant torque output, its motoneuron pool receives a net excitatory drive that first decreases to compensate for the short-lived potentiation of the muscle force twitch and then increases to compensate for the diminution of the force twitch. The underlying inverse relationship between the firing rate and the recruitment threshold that has been reported for nonfatigued contractions is maintained. We, therefore, conclude that the central nervous system control of vastus lateralis motor units remains invariant during fatigue in submaximal isometric isotonic contractions.

Authors+Show Affiliations

NeuroMuscular Research Center, Boston University, 19 Deerfield St., Boston, MA 02215, USA. aadam@bu.eduNo affiliation info available

Pub Type(s)

Clinical Trial
Journal Article
Research Support, N.I.H., Extramural
Research Support, U.S. Gov't, Non-P.H.S.
Research Support, U.S. Gov't, P.H.S.

Language

eng

PubMed ID

16036904

Citation

Adam, Alexander, and Carlo J. De Luca. "Firing Rates of Motor Units in Human Vastus Lateralis Muscle During Fatiguing Isometric Contractions." Journal of Applied Physiology (Bethesda, Md. : 1985), vol. 99, no. 1, 2005, pp. 268-80.
Adam A, De Luca CJ. Firing rates of motor units in human vastus lateralis muscle during fatiguing isometric contractions. J Appl Physiol. 2005;99(1):268-80.
Adam, A., & De Luca, C. J. (2005). Firing rates of motor units in human vastus lateralis muscle during fatiguing isometric contractions. Journal of Applied Physiology (Bethesda, Md. : 1985), 99(1), 268-80.
Adam A, De Luca CJ. Firing Rates of Motor Units in Human Vastus Lateralis Muscle During Fatiguing Isometric Contractions. J Appl Physiol. 2005;99(1):268-80. PubMed PMID: 16036904.
* Article titles in AMA citation format should be in sentence-case
TY - JOUR T1 - Firing rates of motor units in human vastus lateralis muscle during fatiguing isometric contractions. AU - Adam,Alexander, AU - De Luca,Carlo J, PY - 2005/7/23/pubmed PY - 2005/9/20/medline PY - 2005/7/23/entrez KW - NASA Discipline Musculoskeletal KW - Non-NASA Center SP - 268 EP - 80 JF - Journal of applied physiology (Bethesda, Md. : 1985) JO - J. Appl. Physiol. VL - 99 IS - 1 N2 - We investigated the firing rate of motor units in the vastus lateralis muscle in five healthy young men (mean = 21.4 yr, SD = 0.9) during a sequence of isometric constant-torque contractions repeated to exhaustion. The contractions were sustained at 20% of the maximal voluntary level, measured at the beginning of the test sequence. Electromyographic (EMG) signals were recorded via quadrifilar fine-wire electrodes and subsequently decomposed into their constituent motor unit action potentials to obtain the motor unit firing times. In addition, we measured the whole muscle mechanical properties during the fatigue task using electrical stimulation. The firing rate of motor units first decreased within the first 10-20% of the endurance time of the contractions and then increased. The firing rate increase was accompanied by recruitment of additional motor units as the force output remained constant. The elicited twitch and tetanic torque responses first increased and then decreased. The two processes modulated in a complementary fashion at the same time. Our data suggest that, when the vastus lateralis muscle is activated to maintain a constant torque output, its motoneuron pool receives a net excitatory drive that first decreases to compensate for the short-lived potentiation of the muscle force twitch and then increases to compensate for the diminution of the force twitch. The underlying inverse relationship between the firing rate and the recruitment threshold that has been reported for nonfatigued contractions is maintained. We, therefore, conclude that the central nervous system control of vastus lateralis motor units remains invariant during fatigue in submaximal isometric isotonic contractions. SN - 8750-7587 UR - https://www.unboundmedicine.com/medline/citation/16036904/Firing_rates_of_motor_units_in_human_vastus_lateralis_muscle_during_fatiguing_isometric_contractions_ L2 - http://www.physiology.org/doi/full/10.1152/japplphysiol.01344.2004?url_ver=Z39.88-2003&rfr_id=ori:rid:crossref.org&rfr_dat=cr_pub=pubmed DB - PRIME DP - Unbound Medicine ER -