Changes in muscle activation can prolong the endurance time of a submaximal isometric contraction in humans.

Fourteen young subjects (7 men and 7 women) performed a fatiguing isometric contraction with the elbow flexor muscles at 20% of maximal voluntary contraction (MVC) force on three occasions. Endurance time for session 3 [1,718 +/- 1,189 (SD) s] was longer than for session 1 (1,225 +/- 683 s) and session 2 (1,410 +/- 977 s). Five men and four women increased endurance time between session 1 and 3 by 60 +/- 28% (responders), whereas two men and three women did not (-3 +/- 11%; nonresponders). The MVC force was similar for the responders and nonresponders, both before and after the fatiguing contraction. Fatiguing contractions were characterized by an increase in the electromyogram (EMG) amplitude and number of bursts during the fatiguing contractions. The responders achieved a similar level of EMG at exhaustion but a reduced rate of increase in the EMG across sessions. The rate of increase in EMG across sessions declined for the nonresponders, but it remained greater than that of the responders. The increase in burst rate during the contractions declined across sessions with a negative relation between burst rate and endurance time (r = -0.42). Normalized force fluctuations increased during the fatiguing contractions, and there was a positive relation (r = 0.60) between the force fluctuations and burst rate. Changes in mean arterial pressure and heart rate during the fatiguing contraction were similar for the responders and nonresponders across the three sessions. The results indicate that those subjects who increased the endurance time of a submaximal contraction across three sessions did so by altering the level and pattern of muscle activation.

[1]  J. Dowling,et al.  The relationship between voluntary electromyogram, endurance time and intensity of effort in isometric handgrip exercise , 1995, European Journal of Applied Physiology and Occupational Physiology.

[2]  J. Taylor,et al.  Autonomic mediation of the pressor responses to isometric exercise in humans. , 1988, Journal of applied physiology.

[3]  A. J. Lloyd,et al.  Surface electromyography during sustained isometric contractions. , 1971, Journal of applied physiology.

[4]  M. White,et al.  Training‐induced adaptations in the central command and peripheral reflex components of the pressor response to isometric exercise of the human triceps surae , 1999, The Journal of physiology.

[5]  L. Rowell,et al.  Reflex control of the circulation during exercise: chemoreflexes and mechanoreflexes. , 1990, Journal of applied physiology.

[6]  L. Rowell.  Human Cardiovascular Control , 1993 .

[7]  E Cafarelli,et al.  Behavior of coactive muscles during fatigue. , 1993, Journal of applied physiology.

[8]  D. Kernell,et al.  Organized variability in the neuromuscular system: a survey of task-related adaptations. , 1992, Archives italiennes de biologie.

[9]  R M Enoka,et al.  Gender differences in the fatigability of human skeletal muscle. , 1999, Journal of neurophysiology.

[10]  G. A. Robinson,et al.  Behavior of motor units in human biceps brachii during a submaximal fatiguing contraction. , 1994, Journal of applied physiology.

[11]  K. Jørgensen,et al.  Modification of myo-electric power spectrum in fatigue from 15% maximal voluntary contraction of human elbow flexor muscles, to limit of endurance: reflection of conduction velocity variation and/or centrally mediated mechanisms? , 2004, European Journal of Applied Physiology and Occupational Physiology.

[12]  M Hagberg,et al.  Muscular endurance and surface electromyogram in isometric and dynamic exercise. , 1981, Journal of applied physiology: respiratory, environmental and exercise physiology.

[13]  J. Duchateau,et al.  Motor unit behaviour and contractile changes during fatigue in the human first dorsal interosseus , 2001, The Journal of physiology.

[14]  V Fialka,et al.  EMG fatigue patterns accompanying isometric fatiguing knee-extensions are different in mono- and bi-articular muscles. , 1998, Electroencephalography and clinical neurophysiology.

[15]  A. del Valle,et al.  The role of motor unit rate modulation versus recruitment in repeated submaximal voluntary contractions performed by control and spinal cord injured subjects. , 2001, Journal of electromyography and kinesiology : official journal of the International Society of Electrophysiological Kinesiology.

[16]  J. Sallis,et al.  Physical activity assessment methodology in the Five-City Project. , 1985, American journal of epidemiology.

[17]  Electromyographic responses of the human triceps surae and force tremor during sustained submaximal isometric plantar flexion. , 1994, Acta physiologica Scandinavica.

[18]  A. R. Lind,et al.  Amplitude of the surface electromyogram during fatiguing isometric contractions , 1979, Muscle & nerve.

[19]  R. Enoka,et al.  Limb immobilization alters muscle activation patterns during a fatiguing isometric contraction , 2000, Muscle & nerve.

[20]  A J Fuglevand,et al.  Impairment of neuromuscular propagation during human fatiguing contractions at submaximal forces. , 1993, The Journal of physiology.

[21]  R H Westgaard,et al.  Motor unit substitution in long-duration contractions of the human trapezius muscle. , 1999, Journal of neurophysiology.

[22]  A. Kossev,et al.  Human motor unit recruitment and derecruitment during long lasting intermittent contractions. , 2001, Journal of electromyography and kinesiology : official journal of the International Society of Electrophysiological Kinesiology.

[23]  Erik B. Simonsen,et al.  Motor unit recruitment during prolonged isometric contractions , 2004, European Journal of Applied Physiology and Occupational Physiology.

[24]  J. Mitchell,et al.  The exercise pressor reflex: its cardiovascular effects, afferent mechanisms, and central pathways. , 1983, Annual review of physiology.

[25]  R M Enoka,et al.  Sex differences in the fatigability of arm muscles depends on absolute force during isometric contractions. , 2001, Journal of applied physiology.

[26]  S J Day,et al.  Experimental simulation of cat electromyogram: evidence for algebraic summation of motor-unit action-potential trains. , 2001, Journal of neurophysiology.

[27]  W ROHMERT,et al.  [Determination of the recovery pause for static work of man]. , 1960, Internationale Zeitschrift fur angewandte Physiologie, einschliesslich Arbeitsphysiologie.

[28]  J. Mitchell,et al.  Effects of static muscular contraction on impulse activity of groups III and IV afferents in cats. , 1983, Journal of applied physiology: respiratory, environmental and exercise physiology.

[29]  D. Denny-Brown,et al.  Interpretation of the electromyogram. , 1949, Archives of neurology and psychiatry.

[30]  B. Bigland-ritchie,et al.  Fatigue of intermittent submaximal voluntary contractions: central and peripheral factors. , 1986, Journal of applied physiology.

[31]  R M Enoka,et al.  Long-term activity in upper- and lower-limb muscles of humans. , 2001, Journal of applied physiology.

[32]  W. Rohmert.  Ermittlung von Erholungspausen für statische Arbeit des Menschen , 1960, Internationale Zeitschrift für angewandte Physiologie einschließlich Arbeitsphysiologie.

[33]  F. Smirk,et al.  Observations in man upon a blood pressure raising reflex arising from the voluntary muscles , 1937, The Journal of physiology.

[34]  R. Enoka,et al.  Motor-unit synchronization increases EMG amplitude and decreases force steadiness of simulated contractions. , 2000, Journal of neurophysiology.

[35]  T S Miles,et al.  Control of motor units in human flexor digitorum profundus under different proprioceptive conditions , 1997, The Journal of physiology.

[36]  R. Enoka,et al.  Motor unit physiology: Some unresolved issues , 2001, Muscle & nerve.

[37]  M. Kaufman,et al.  Pressor reflex response to static muscular contraction: its afferent arm and possible neurotransmitters. , 1988, The American journal of cardiology.

[38]  G. Borg.  Psychophysical bases of perceived exertion. , 1982, Medicine and science in sports and exercise.

[39]  O. Lippold,et al.  The relation between force and integrated electrical activity in fatigued muscle , 1956, The Journal of physiology.

[40]  D. Kernell,et al.  Motor unit categorization on basis of contractile properties: An experimental analysis of the composition of the cat's m. peroneus longus , 2004, Experimental Brain Research.