Abstract
The extent to which an individual can voluntarily produce maximal muscular force can be estimated using the interpolated twitch technique. Incompleteness of activation is typically attributed to either incomplete recruitment, suboptimal firing rates or both of these mechanisms. The purpose of this study was to assess the relationship between muscle activation and maximal motor unit firing rates. Measures of muscle activation and motor unit firing rates during maximal effort contractions were obtained from 15 subjects (8 young, 7 older) throughout a 6-week strength training program for the knee extensors. High resolution interpolated twitch responses were obtained using a circuit that removes the force level of the maximal voluntary contraction prior to amplification of the additional evoked force. Maximal firing rates were obtained using a four-wire needle electrode. Electrical stimulation of the knee extensors during maximal effort contractions evoked a transient increase in the force and the amplitude of this additional force was correlated with maximal firing rates at r = −0.62 (p < 0.05). Central activation ratio and activation level, two indexes of activation, were correlated with maximal firing rates at r = 0.58 (p < 0.05) and r = 0.68 (p < 0.05), respectively. The training program elicited parallel increases in muscular activation and maximal firing rates. These results provide direct support for maximal firing rate as a significant factor limiting maximal force production.
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References
Adams GR, Harris RT, Woodard D, Dudley GA (1993) Mapping of electrical muscle stimulation using MRI. J Appl Physiol 74:532–537
Allen GM, McKenzie DK, Gandevia SC (1998) Twitch interpolation of the elbow flexor muscles at high forces. Muscle Nerve 21:318–328
Bampouras TM, Reeves ND, Baltzopoulos V, Maganaris CN (2006) Muscle activation assessment: effects of method, stimulus number, and joint angle. Muscle Nerve 34:740–746
Behm DG, St Pierre DM (1997) The muscle activation–force relationship is unaffected by ischaemic recovery. Can J Appl Physiol 22:468–478
Cohen J, Cohen P, West S, Aiken L (2003) Applied multiple regression/correlation analysis for the behavioral sciences, 3rd edn. Lawrence Erlbaum Associates, Mahwah
DeSerres SJ, Enoka RM (1998) Older adults can maximally activate the biceps brachii muscle by voluntary command. J Appl Physiol 84:284–291
Felmus MT, Patten BM, Hart A, Martinez C (1977) Catecholamine-induced muscle weakness. Arch Neurol 34:280–284
Gandevia SC, McKenzie DK (1988) Activation of human muscles at short muscle lengths during maximal static efforts. J Physiol 407:599–613
Hales JP, Gandevia SC (1988) Assessment of maximal voluntary contraction with twitch interpolation: an instrument to measure twitch responses. J Neurosci Methods 25:97–102
Heckman CJ, Binder MD (1991) Computer simulation of the steady-state input–output function of the cat medial gastrocnemius motoneuron pool. J Neurophysiol 65:952–967
Herbert RD, Gandevia SC (1999) Twitch interpolation in human muscles: mechanisms and implications for measurement of voluntary activation. J Neurophysiol 82:2271–2283
Hultman E, Sjoholm H, Jaderholm-Ek I, Krynicki J (1983) Evaluation of methods for electrical stimulation of human skeletal muscle in situ. Pflugers Arch 398:139–141
Jones RE (1962) Reliability of muscle strength testing under varying motivational conditions. J Am Phys Ther Assoc 42:240–243
Kamen G, Knight CA (2004) Training-related adaptations in motor unit discharge rate in young and older adults. J Gerontol A Biol Sci Med Sci 59:1334–1338
Kamen G, Sison SV, Du CC, Patten C (1995) Motor unit discharge behavior in older adults during maximal-effort contractions. J Appl Physiol 79:1908–1913
Kendall TL, Black CD, Elder CP, Gorgey A, Dudley GA (2006) Determining the extent of neural activation during maximal effort. Med Sci Sports Exerc 38:1470–1475
Kent-Braun J, LeBlanc R (1996) Quantitation of central activation failure during maximal voluntary contractions in humans. Muscle Nerve 19:861–869
Knight CA, Kamen G (2001) Adaptations in muscular activation of the knee extensor muscles with strength training in young and older adults. J Electromyogr Kinesiol 11:405–412
Leong B, Kamen G, Patten C, Burke JR (1999) Maximal motor unit discharge rates in the quadriceps muscles of older weight lifters. Med Sci Sports Exerc 31:1638–1644
Lloyd AJ, Voor JH (1973) The effect of training on performance efficiency during a competitive isometric exercise. J Mot Behav 5:17–24
Merton PA (1954) Voluntary strength and fatigue. J Physiol 123:553–564
Patten C, Kamen G (2000) Adaptations in motor unit discharge activity with force control training in young and older human adults. Eur J Appl Physiol 83:128–143
Ryan ED (1961) Effect of differential motive-incentive conditions on physical performance. Res Q 32:83–87
Voor JH, Lloyd AJ, Cole RJ (1969) The influence of competition on the efficacy of an isometric muscle contraction. J Mot Behav 1:210–219
Yue GH, Ranganathan VK, Siemionow V, Liu JZ, Sahgal V (2000) Evidence of inability to fully activate human limb muscle. Muscle Nerve 23:376–384
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Knight, C.A., Kamen, G. Relationships between voluntary activation and motor unit firing rate during maximal voluntary contractions in young and older adults. Eur J Appl Physiol 103, 625–630 (2008). https://doi.org/10.1007/s00421-008-0757-z
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DOI: https://doi.org/10.1007/s00421-008-0757-z