Variability in the interpolated twitch torque for maximal and submaximal voluntary contractions

Abstract

The superimposed twitch technique is frequently used to study the degree of motor unit activation during voluntary effort. This technique is one of the preferred methods to determine the activation deficit (AD) in normal, athletic, and patient populations. One of the limitations of the superimposed twitch technique is its variability under given contractile conditions. The objective of this research was to determine the source(s) of variability in the superimposed twitch force (STF) for repeat measurements. We hypothesized that the variability in the AD measurements may be caused by the timing of the twitch force relative to the onset of muscle activation, by force transients during the twitch application, by small variations in the actual force from the nominal target force, and by variations in the resting twitch force. Twenty-eight healthy subjects participated in this study. Sixteen of these subjects participated in a protocol involving contractions at 50% of their maximal voluntary contraction (MVC) effort, whereas the remaining 12 participated in a protocol involving contractions at 100% of their MVC. Doublet-twitch stimuli were superimposed onto the 50 and 100% effort knee extensor muscle contractions, and the resting twitch forces, voluntary knee extensor forces, and STFs were then measured. The mean resting twitch forces obtained before and after 8 s of 50% of MVC were the same. Similarly, the mean STFs determined at 1, 3, 5, and 7 s into the 50% MVC were the same. The variations in twitch force were significantly smaller after accounting for the actual force at twitch application than those calculated from the prescribed forces during the 50% MVC protocol (P < 0.05). Furthermore, the AD and the actual force showed statistically significant negative correlations for the 50% MVC tests. The interpolated twitch torque determined for the maximal effort contractions ranged from 1 to 70%. In contrast to the protocol at 50% of MVC, negative correlations were only observed in 5 of the 12 subjects during the 100% effort contractions. These results suggest that small variations in the actual force from the target force can account for the majority of the variations in the STFs for submaximal but not maximal effort contractions. For the maximal effort contractions, large variations in the STF exist due to undetermined causes.

Neuromuscular performance and bone structural characteristics in young healthy men and women

“…Our results indicated that activation level played no role in explaining the bone strength. There seemed to be some variation in activation level among individuals, but evidently activation level is not a major determinative factor in force production in young adults (Merton 1954;Oskouei et al 2003;Stackhouse et al 2000). As has been shown previously, our results suggest that, in addition to variation in muscle volume, there is significant inter-individual variation in specific tension (Ikai and Fukunaga 1968;Maughan and Nimmo 1984), which also plays a role in determining the bone strength.…”

Is the notion of central fatigue based on a solid foundation?

“…Therefore, the variability observed in the estimates of voluntary drive was solely a consequence of the physiological synaptic noise included in the model that characterizes motor unit firing behavior and that influences the summation of their force twitches. In actual empirical conditions other factors, such as the level of stimulation intensity, the number and type of stimulated motor units, or the level of voluntary input excitation, may introduce even greater variability in the estimates, as discussed by Arampatzis et al (2007), Babault (2009), Herzog (2009), Folland and Williams (2007, and Oskouei et al (2003), among others. As an example, consider that only some studies (Bilodeau 2006;McKenzie et al 1992;Schillings et al 2003;Todd et al 2003) but not others (Bigland-Ritchie et al 1978;Schillings et al 2007) have observed exercise-induced central fatigue during voluntary fatiguing contractions.…”

Effect of stimulation intensity on assessment of voluntary activation

“…Doublet stimulation was chosen to increase the signal-to-noise ratio 2 and to decrease effects of potentiation. 26 Experimental Protocol. After a warm-up of 10 submaximal isometric extensions of increasing intensity, subjects performed one MVC for the knee extensors to determine target levels for the subsequent submaximal contractions with superimposed electrical stimulation.…”

“…Electromechanical delay was taken into account when voluntary torque and superimposed torque were calculated. 26 7 In addition, we calculated VA in an alternative way (VA 60-100% ) as suggested by others, 7,27 by dividing MVT over MTC 60-100% , which was obtained by extrapolation of the linear regression line fitted on the superimposed torques obtained for voluntary torques greater than 60% MVC. Figure 2A illustrates the calculation of MTC 100% and MTC 60-100% .…”