Fatigue brought about by intense muscular contraction typically is accompanied by a reduction in motor-unit firing rate. The decrease in motor-unit output with fatigue appears to be caused by two interacting processes: 1) a decline in the net excitatory drive to motoneurons and 2) adaptation in the responsiveness of motoneurons to synaptic input. Whether a reduction in motor-unit firing rate in itself contributes to force loss associated with fatigue, however, is an unresolved question. The neuromuscular wisdom hypothesis suggests that decreases in firing rate help to maintain force by optimizing the input to motor units as their contractile properties change. On the other hand, recent work indicates that mechanical function of some motor units is altered during prolonged activity such that diminished firing rate would augment force loss and, thereby, contribute to fatigue. Neural adaptations, therefore, may serve to limit the extent of muscular activity. NEUROSCIENTIST 2:203-206, 1996
McComas AJ, Miller RG, Gandevia SCFatigue brought on by malfunction of the central and peripheral nervous systems . In: Gandevia SC, Enoka RM, McComas AJ, Stuart DG, Thomas CK, editors. Fatigue : neural and muscular mechanisms. New York: Plenum Press1995;495-512.
2.
Fitts RHCellular mechanisms of muscle fatigue. Physiol Rev1994;74:49-94.
3.
Bigland-Ritchie B., Johansson R., Lippold Ocj, Smith S., Woods, JJChanges in motoneurone firing rates during sustained maximal voluntary contractions. J Physiol (Lond) 1983;340:335-346.
4.
Windhorst U., Boorman G.Overview: potential role of segmental motor circuitry in muscle fatigue . In: Gandevia SC, Enoka RM, McComas AJ, Stuart DG, Thomas CK, editors. Fatigue: neural and muscular mechanisms. New York: Plenum Press1995;241-258.
5.
Burke REMotor units: anatomy, physiology, and functional organization. In: Brooks VB, editor. Handbook of physiology. The nervous system [Sect. 1]. Vol. 2, Pt. 1. Bethesda, MD: American Physiological Society1981;345-422.
6.
Henneman E., Mendell LMFunctional organization of motoneuron pool and its m-puts. In: Brooks VB, editor. Handbook of physiology. The nervous system [Sect. 1] Vol. 2, Pt. 1. Bethesda, MD: American Physiological Society1981;423-507.
7.
Enoka RMMorphological features and activation patterns of motor units. J Clin Neurophysiol1995;12:538-559.
8.
Kernell D., Eerbeek O., Verhey BARelation between isometric force and stimulation rate in cat's hindlimb motor units of different twitch contraction time. Exp Brain Res1983;50:220-227.
9.
Bigland-Ritchie B., Johansson R., Lippold Ocj, Woods JJContractile speed and EMG changes during fatigue of sustained maximal voluntary contractions . J Neurophysiol1983;50:313-324
10.
Fuglevand AJ , Zackowski KM, Huey KA, Enoka RMImpairment of neuromuscular propagation during human fatiguing contractions at submaximal forces. J Physiol (Lond) 1993;460:549-572.
11.
Fuglevand AJThe role of the sarcolemma action potential in fatigue. In: Gandevia SC, Enoka RM, McComas AJ, Stuart DG, Thomas CK, editors. Fatigue: neural and muscular mechanismsNew York: Plenum Press1995;101-108.
12.
Gnmby L., Hannerz J., Hedman B.The fatigue and voluntary discharge properties of single motor units in man . J Physiol (Lond) 1981;316:545-554.
13.
Kernell D.Neuromuscular frequency-coding and fatigue. In: Gandevia SC, Enoka RM, McComas AJ, Stuart DG, Thomas CK, editors. Fatigue: neural and muscular mechanisms. New York: Plenum Press1995;135-145.
14.
Brasil-Neto JP, Pascual-Leone A., Valls-Solé J., Cammarota A., Cohen LG, Hallet M.Postexercise depression of motor evoked potentials: a measure of central nervous system fatigue. Exp Brain Res1993;93:181-184.
15.
McKay WB, Tuel SM, Sherwood AM, Stokic DS, Dimitrijevic MRFocal depression of cortical excitability induced by fatiguing muscle contraction: a transcranial magnetic stimulation study. Exp Brain Res1995;105.276-282.
16.
Zanette G., Bonato C., Polo A., Tinazzi M., Manganotti P., Fiaschi A.Long-lasting depression of motor-evoked potentials to transcranial magnetic stimulation followmg exercise. Exp Brain Res1995;107:80-86.
17.
Macefield G. , Hagbarth K-E., Gorman R., Gandevia SC, Burke D.Decline in spindle support to α-motoneurones during sus-tamed voluntary contractions. J Physiol (Lond) 1991;440:497-512.
18.
Jami L., Zytnicki D., Horcholle-Bossavit G.Tendon organ discharges and their central effects during muscle contraction . In: Ferrell WR, Proske U, editors. Neural control of movement. New York: Plenum Press1995;43-49.
19.
Woods JJ, Furbush F., Bigland-Ritchie B.Evidence for a fatigue-induced reflex inhibition of motoneuron firing rates . J Neurophysiol1987;58:125-137.
20.
Garland SJ, Kaufman MPRole of muscle afferents in the inhibition of motoneurons during fatigue . In: Gandevia SC, Enoka RM, McComas AJ, Stuart DG, Thomas CK, editors. Fatigue: neural and muscular mechanisms. New York: Plenum Press1995;271-278.
21.
Smoway LI, Hill JM, Pickar JG, Kaufman MPEffects of contraction and lactic acid on the discharge of group III muscle afferents in cats. J Neurophysiol1993 ;69: 1053-1059.
22.
Rybicki KJ, Waldrop TG, Kaufman MPIncreasing gracilis interstitial potassium concentrations stimulates group III and IV afferents. J Appl Physiol1985;58:936-941
23.
Schwindt PC, Cnll WEMembrane properties of cat spinal motoneurons. In: RA Pandolf, editor. Handbook of the spinal cord. Anatomy and physiology. New York: Dekker1984;199-242.
24.
Kernell D., Monster AWTime course and properties of late adaptation in spinal motoneurones of the cat. Exp Brain Res1982;46:191-196
25.
Spielman JM , Laouns Y., Nordstrom MA, Robmson GA, Reinking RM, Stuart DGAdaptation of cat motoneurons to sus-tamed and intermittent extracellular activation. J Physiol (Lond 1993;464:75-120.
26.
Sawczuk A., Powers RK, Binder MDSpike frequency adaptation studied in hypoglossal motoneurons of the rat. J Neurophysiol1995;73:1799-1810.
27.
Kernell D., Monster AWMotoneurone properties and motor fatigue: an intracellular study of gastrocnemius motoneurones of the cat. Exp Brain Res1982 ;46: 197-204.
28.
Sawczuk A., Powers RK, Binder MDIntnnsic properties of motoneurons: implications for muscle fatigue . In: Gandevia SC, Enoka RM, McComas AJ, Stuart DG, Thomas CK, editors. Fatigue: neural and muscular mechanisms. New York: Plenum Press1995;123-134.
29.
Marsden CD, Meadows JC, Merton PA "Muscular wisdom" that mimmizes fatigue during prolonged effort in man: peak rates of motoneurone discharge and slowing of discharge during fatigue . In. JE Desmedt, editor. Motor control mechanisms in health and disease. New York : Raven Press1983;169-211.
30.
Binder-MacLeod SA, McDermond LRChanges in the force-frequency relationship of the human quadriceps femons muscle following electrically and voluntanly induced fatigue. Phys Ther1992;72: 95-104.
31.
Thomas CK, Bigland-Ritchie B., Johansson RSForce-frequency relationships of human thenar motor units. J Neurophysiol1991;65:1509-1516.
32.
Fuglevand AJ , Macefield VG, Bigland-Ritchie B.Force-frequency and fatigue properties in human long-finger flexor motor units. Soc Neurosci Abstr1995;21: 1435.
33.
Gandevia SC, Allen GM, McKenzie DKCentral fatigue: critical issues, quantification and practical implications . In: Gandevia SC, Enoka RM, McComas AJ, Stuart DG, Thomas CK, editors. Fatigue: neural and muscular mechanisms. New York: Plenum Press1995;281-294.
34.
Edwards Rht.Biochemical bases of fatigue in exercise performance: catastrophe theory of muscular fatigue. In: Knuttgen HG, Vogel JA, Poortmans J, editors. Biochemistry of exercise . Champaign: Human Kinetics1983;3-28.
