Motor unit properties of rapid force development during explosive contractions

A motor unit is comprised of an alpha motor neuron and all of the muscle fibres it innervates. Motor unit activation properties (i.e. recruitment, firing rate and/or synchronization) are considered primary mechanisms in the modulation of force production, and have received much attention in the literature. Traditionally, motor units are studied using fine‐wire electrodes or more recently, surface electromyographic (sEMG) sensors. Motor units may be studied via sEMG electrodes by using a variety of signal decomposition algorithms (i.e. blind source separation, precision decomposition III) during ramped, trapezoidal, isometric contractions at low to moderate levels of intensity (i.e. 20–70% maximal voluntary contraction, MVC). Rapid, or explosive, strength is the ability to produce force/torque quickly and is commonly examined from an isometric force‐ or torque‐time curve (Maffiuletti et al. 2016). Examining rapid strength has grown in popularity, with a recent review (Maffiuletti et al. 2016) discussing key physiological and methodological considerations for this work. Furthermore, rapid force production has been suggested to be more related to functional tasks (Mota et al. 2018) and more sensitive in detecting chronic neuromuscular changes (e.g. ageing) (Gerstner et al. 2017) than maximal muscle force. Although previous studies have suggested that muscle activation is one of the key determinants of rapid force production, investigations into specific motor unit properties and their relationship to rapid strength are sparse.

A recent study published in The Journal of Physiology by Del Vecchio and colleagues (2019) expands the motor unit physiology literature by using a high‐density sEMG array in conjunction with a blind source separation signal decomposition algorithm to identify a more robust number of motor units (compared to indwelling EMG techniques) to estimate the neural drive to the muscle during the rapid phases of explosive, isometric contractions. The authors (Del Vecchio et al. 2019) reported that: (1) the initial 35 ms of motor unit activity (i.e. firing rate, recruitment speed) may heavily influence the early phase of rapid force, (2) different motor control schemes may be present during rapid contractions compared to traditional MVCs, and (3) for the first time, motor unit function was characterized during rapid, isometric contractions in the tibialis anterior muscle. The findings of this study suggest that the maximal rate of force development (RFD) was significantly correlated with the speed of recruitment of the identified motor units, which supports previous research showing a relationship between muscle fibre conduction velocity and RFD. Interestingly, the authors only found a relationship between the initial 35 ms of motor unit activity and the explosive force estimates, with no further relationships found following the initial 35 ms of motor unit activity. Thus, Del Vecchio et al. (2019) indicate that the initial neural drive to the muscle before force onset probably determined the explosive force production.

The RFD may be investigated as peak RFD (i.e. the steepest slope during the onset of force production), as well as in two separate phases of the muscle contraction: early (0–100 ms) and late (100–200 ms) (Maffiuletti et al. 2016). These separate time intervals have been shown to demonstrate different neuromuscular contributions (e.g. neural vs. muscular) to rapid strength (Maffiuletti et al. 2016; Gerstner et al. 2017). For instance, previous studies (Maffiuletti et al. 2016) have suggested that early rapid force intervals are primarily influenced by muscle activation, which is strongly supported by Del Vecchio et al. (2019), as evidenced by a relatively high coefficient of determination for each RFD variable and the motor unit firing rate (R ² = 0.62–0.68) for the initial 35 ms of motor unit activity. In their current work, Del Vecchio et al. (2019) measured the peak RFD, denoted as RFD0‐X_MAX, two overlapping phases, denoted as RFD0‐60 and RFD0‐100, and the integral of the force‐time curve from force onset to 250 ms, denoted as ‘Impulse’. A unique finding was that all of the above rapid force variables were not related to neural variables following the first 35 ms of motor neuron activity. However, a later non‐overlapping phase of rapid force, often noted in the literature, was not measured (e.g. 100–200 ms) (Gerstner et al. 2017). The later phase is thought to be more related to maximal strength, and thus muscle‐specific factors (i.e. intrinsic contractile properties) may contribute to changes in late phase RFD (Maffiuletti et al. 2016). It would be interesting to see if a non‐overlapping later phase of rapid force (>100 ms) was also related to the initial 35 ms of motor unit activity. Del Vecchio et al. (2019) present findings which suggest the majority of the variation in rapid force production is explained by early motor unit properties (i.e. firing rate, recruitment speed), which is consistent with the previous literature (Maffiuletti et al. 2016). While the coefficient of determination was relatively high for RFD (Del Vecchio et al. 2019), this still may not discount other variables that factor into rapid force production. Future studies using high‐density sEMG signal decomposition could incorporate a non‐overlapping later phase RFD measurement (>100 ms) in addition to examining factors known to influence RFD, such as muscle architecture, echo intensity, or tendon stiffness (Maffiuletti et al. 2016; Gerstner et al. 2017), which may provide a more comprehensive perspective on rapid force.

A number of decomposition techniques have been used to extract the constituent motor unit action potentials from the EMG signal. Compared to indwelling EMG techniques, sEMG has risen in popularity due to the increased number of motor units that can be studied during a given contraction. While the topic of sEMG signal decomposition has been debated over the last two decades and continues to evolve, a commonly utilized method of signal decomposition is the blind source separation algorithm. The blind source separation decomposition technique has been previously validated during ramped, trapezoidal, isometric contractions using up to 70% MVC. In their study, Del Vecchio et al. (2019) utilized the blind source separation approach in conjunction with a high‐density sEMG array during rapid contractions reaching 75% MVC. As discussed above, the authors reveal that the motor unit firing rate and recruitment speed may explain the majority of variance associated with rapid force development, which confirms the findings of previous works suggesting that the central nervous system plays a key role in the ability of individuals to produce force rapidly (Maffiuletti et al. 2016). However, as this may in fact be one of the first papers examining this specific issue, additional studies are warranted to further unveil aspects of motor unit function on motor control during the rapid phases of isometric contractions. For instance, the interpulse interval (IPI) is an alternative to the mean firing rate calculation and serves as a metric of motor unit discharge variability. Furthermore, it is known that the first IPI within a given motor unit action potential train influences force development, but its effect on the early and/or late phases of rapid force during rapid contractions warrants further study.

Perhaps the most interesting aspect of the recent study by Del Vecchio et al. (2019) is that the onion‐skin phenomenon was not found to be present during the rapid phases of force production. The authors (Del Vecchio et al. 2019) suggest that the presence of the onion‐skin phenomenon may only be a factor in ramped, isometric contractions (i.e. slowly generated force) due to the compressed motor unit recruitment range which may occur during rapid contractions. Thus, it is possible that a different motor control mechanism could be present when rapid contractions are used, instead of ramped, isometric contractions. Indeed, it was noted in the original work (De Luca & Erim, 1994) that rapid or explosive contractions were not considered at the inception of the onion‐skin control theory. Futures studies are needed to investigate the various motor control theories (e.g. onion‐skin, common drive) that may govern rapid contractions.

In conclusion, this study by Del Vecchio et al. (2019) presents novel insights into the motor control properties of motor units active during rapid contractions. Additional insights into motor unit function (e.g. IPI), examination of the later phases of rapid force development, and the investigation of other muscle characteristics (i.e. architecture, echo intensity) may increase our understanding of the neuromuscular mechanisms associated with explosive force production and should be considered in future works. In this Journal Club article, we have explored a few considerations when interpreting the novel findings of Del Vecchio et al. (2019), how they compare to the current body of rapid force and motor control literature, and additional variables that may prove to be valuable in future studies. In the coming years, we look forward to exciting new insights into motor unit function and how it may contribute to rapid strength.

Additional information

Competing interests

The authors declare that they have no competing interests.

Author contributions

All authors have approved the final version of the manuscript and agree to be accountable for all aspects of the work. All persons designated as authors qualify for authorship, and all those who qualify for authorship are listed.

Funding

None.