Counterpoint: The interpolated twitch does not provide a valid measure of the voluntary activation of muscle
2009; American Physiological Society; Volume: 107; Issue: 1 Linguagem: Inglês
10.1152/japplphysiol.91220.2008a
ISSN8750-7587
AutoresA. de Haan, K. H. L. Gerrits, Cornelis J. de Ruiter,
Tópico(s)Advanced Sensor and Energy Harvesting Materials
ResumoPOINT-COUNTERPOINTCounterpoint: The interpolated twitch does not provide a valid measure of the voluntary activation of muscleA. de Haan, K. H. L. Gerrits, and C. J. de RuiterA. de Haan, K. H. L. Gerrits, and C. J. de RuiterPublished Online:01 Jul 2009https://doi.org/10.1152/japplphysiol.91220.2008aMoreSectionsPDF (105 KB)Download PDF ToolsExport citationAdd to favoritesGet permissionsTrack citations The interpolated twitch technique (ITT) was first used by Merton (17) who superimposed an evoked stimulation on a voluntary contraction to detect muscle fibers not activated by the voluntary effort. The method has since been applied, mainly during isometric contractions, to determine how much of the muscles' potential can be used voluntarily. When no extra force is produced on a superimposed stimulus, this is usually taken as sign that "full activation" was achieved. Different methodological issues related to the measurement of voluntary activation have been addressed in the past, such as timing of, potentiation of, and type (single, doublet, multiple pulses) of the superimposed stimulus (6, 12, 18, 19), intermuscular differences (5), the method of extrapolation used to determine maximal muscle capacity (linear or curvilinear (4, 12), the influence of series elastic structures, synergistic and antagonist activation (2, 16), etc. Overall, these methodological reports indicate that the method can provide a valid estimation of voluntary activation, but that it should be used with care. In this Point:Counterpoint, we want to point out some major limitations of the ITT technique. The main issue addressed in this Counterpoint is what "full activation" signifies.The first point is the low sensitivity of the ITT method at near maximal contraction intensities (13). It should be noted that in accordance with the s-shaped stimulation frequency-force relation, the superimposed stimulus leads to relatively large increases in isometric force during submaximal contractions (with nonsaturating intracellular calcium concentrations, [Ca2+]i), but that only very small increases can be expected when maximal effort is approached. Clearly there is a low sensitivity of the ITT method at near maximal contraction intensities (13). "Full activation" in relation to the ITT method thus seems defined as the level of excitation ([Ca2+]i) needed to produce maximal force under the given contractile conditions. Surface EMG data show that for many subjects the EMG-force relationship is linear up to the near maximal voluntary isometric force (23). However, subjects with a high ability to "voluntarily activate" their muscles (e.g., with a high neural drive) show steep increases in EMG during contractions >80% of their maximal force, illustrating that much more extra excitation ([Ca2+]i ) is needed to completely use the force generating potential of the muscle (1, 14). Therefore, it can be expected that under those conditions (>80% maximal force capacity) in many experimental set ups a single superimposed twitch does not result in a measurable force increase. Consequently the force at which "full activation" is obtained is substantially underestimated in many studies.The second point is that it is not allowed to generalize findings based on a specific measurement of voluntary activation. Usually, measurements of voluntary activation have been carried out under isometric conditions and at a single specific joint angle. Such data cannot be extrapolated to other conditions. For instance, voluntary activation at one joint angle can be different from other angles, especially at angles where muscle length is short. The stimulation frequency-force relation shifts to the right at shorter muscle lengths (8), consequently higher levels of excitation are needed to obtain maximal force. It will therefore probably be more difficult to reach high levels of voluntary activation at shorter muscle lengths. Indeed, voluntary activation was measured to be lower at shorter muscle length in large muscle groups, such as the quadriceps femoris, which are relatively difficult to activate (3, 15). The poor ability to generalize voluntary activation established under isometric conditions is also illustrated by data during rapid fast "explosive" force attempts. Muscle excitation, inferred from surface EMG, is very short and much higher during jumping (10) and during fast isometric force rise (9, 22), compared with excitation at the plateau of an isometric contraction. The capacity to voluntarily activate muscles during explosive isometric contractions can be measured by comparing the initial fast-increasing force response during voluntary and electrically evoked contractions. In subjects who all had "high" voluntary activation, as established with ITT at the isometric force plateau during MVCs of the quadriceps muscles (96.5 ± 2.9%), large variations were seen in the capacity of initial activation during maximal fast contractions (9). The observed variation in this capacity was highly correlated with the initial EMG, indicating that differences in neural drive underlie this variation (9). Moreover, these findings show that the level of excitation, high enough for a near 100% activation at the plateau of an isometric contraction (full activation), is far from "full" under other conditions.The third point is that the implicit consequence of the measurement of voluntary activation is that maximal capacity is obtained at 100% voluntary activation. For the estimation of the maximal capacity, linear and curvilinear extrapolations of the superimposed force-voluntary force relation are used (7, 11). The differences in optimal curve fitting may be a consequence of variation in the ability of subject groups to "voluntarily activate" their muscles, leading to linear or curvilinear EMG-force relationships (1, 14). In almost all reports, curve fitting is performed for data of whole groups of subjects, which may mask relationships for individual subjects. Data from Kooistra et al. (14) indicate that for single subjects there might be a linear phase in the relationship between superimposed and voluntary torque, which in subjects with a very high neural drive is followed by a second (linear) phase (14 resp. Fig. 1), during which increases in EMG are much steeper than increases in voluntary force. For subjects with a very high neural drive, high voluntary activation levels (>95%) can be calculated using linear extrapolations from submaximal contraction levels, resulting in maximal capacity estimations, which are far below their real maximal capacity. From the inset in Fig. 2 it can be seen that the (very small) superimposed forces decline linearly and similarly with increasing voluntary forces on two different days. The consequence of the data for the subject displayed in Fig. 2 is that for the measurement with a torque of ∼150 Nm and extrapolating the linear relation between voluntary and superimposed torque below 150 Nm, a voluntary activation level of ∼96% is calculated, whereas in reality 150 Nm is only ∼75% of the measured maximal torque, which would indicate a voluntary activation level of ∼75%. Clearly, there is a mismatch between calculated voluntary activation levels and maximal torque capacity. Thus, even at the isometric force plateau, an almost full activation may be far from complete, at least in some subjects and in large muscle groups. In the scarce reports that show examples of data for individual subjects, linear relationships can be seen for the submaximal contractions, but there are additional data points having a low superimposed force at near maximal voluntary forces. The voluntary forces of these latter data points seem to be higher than the maximal force capacity calculated from the submaximal data (2, 19–21). Although only for subjects who are consistently very well able to activate their muscles, overestimation of voluntary activation can be demonstrated, it is conceivable that this in fact occurs for all subjects. Many subjects will not be able to further increase muscle excitation and despite measured levels of voluntary activation of ∼95% most of them do not approach their real maximal force capacity. The ITT method clearly overestimates the true voluntary activation and therewith underestimates maximal force capacity in many subjects. Fig. 2.Schematic illustration of the relationship between the amplitude of the superimposed twitch evoked by twitch interpolation and contraction strength. An inverse linear relationship is ideal. Nonlinearity can be introduced by various methodological and physiological factors.Download figureDownload PowerPointREFERENCES1 Alkner BA, Tesch PA, Berg HE. Quadriceps EMG/force relationship in knee extension and leg press. Med Sci Sports Exerc 32: 459–463, 2000.Crossref | PubMed | ISI | Google Scholar2 Allen GM, McKenzie DK, Gandevia SC. Twitch interpolation of the elbow flexor muscles at high forces. Muscle Nerve 21: 318–328, 1998.Crossref | PubMed | ISI | Google Scholar3 Becker R, Awiszus F. Physiological alterations of maximal voluntary quadriceps activation by changes of knee joint angle. 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Horstman1 July 2009 | Journal of Applied Physiology, Vol. 107, No. 1Last Word on Point:Counterpoint: The interpolated twitch does/does not provide a valid measure of the voluntary activation of muscleA. de Haan, K. H. L. Gerrits, and C. J. de Ruiter1 July 2009 | Journal of Applied Physiology, Vol. 107, No. 1 More from this issue > Volume 107Issue 1July 2009Pages 355-357 Copyright & PermissionsCopyright © 2009 the American Physiological Societyhttps://doi.org/10.1152/japplphysiol.91220.2008aPubMed19567806History Published online 1 July 2009 Published in print 1 July 2009 Metrics
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