Reassessment of Widely Accepted Motor Control Notions
Our dEMG technology has provided us with the means to query the validity of commonly accepted notions of motor unit control. A brief summary of our work follows, with reference to relevant published work listed in Publication list below.
Muscles are not controlled to maximize muscle force production — For the past six decades it has been widely promulgated that motor units are controlled to maximize muscle force production. This notion was based on the long-standing belief that earlier recruited motor units fire at lower firing rates than later recruited ones. This construct would enable all motor units to fire at the rate needed to produce their maximal force output. However, if this were so, the later recruited higher-firing rate motor units would fatigue quickly and the contraction could not be sustained for a prolonged time.
By applying our dEMG technology we have now shown conclusively that the opposite firing rate construct is true. That is, earlier recruited motor units have greater firing rates than later recruited ones at any force level and at any time during voluntary contractions. Consequently, motor units are not controlled to maximize muscle force. Instead, they appear to be controlled to balance a combination of force magnitude and contraction duration, a construct that is more conducive to evolutionary survival.
Synchronization is not caused by Common Inputs to Motor Units — For over nine decades and continuing to the present day, there has been a communal acceptance of the notion that common inputs from the central and peripheral nervous system to the motor units that comprise a muscle are responsible for causing some of the motor unit firings to synchronize. That is, they display the tendency to fire in the near proximity of each other. This was originally postulated as a conjecture and remains a conjecture to this day. No empirical evidence has ever been put forth to substantiate this point. With the assistance of a newly developed, statistically robust methodology for measuring synchronization we have recently provided empirical evidence indicating that the common inputs do not explain the observable behavior of synchronization.
Does muscle fatigue originate in the muscle or in the brain? – Exercise-induced muscle fatigue is manifested as the reduced capacity of muscles to produce force. There is evidence that fatigue is, in large part, due to peripheral factors that impair the muscle fiber contractile mechanisms. However, for over a century factors arising within the Central Nervous System have also been hypothesized to induce muscle fatigue, but no direct empirical evidence has yet been reported. We developed a simulation model to investigate whether peripheral factors of muscle fatigue are sufficient to explain the muscle force behavior observed during empirical studies of fatiguing voluntary contractions. We found that the force behavior commonly attributed to Central Fatigue could be explained solely by peripheral factors. It also revealed important flaw in the methods currently used for assessing Central Fatigue.
Transposition of Motor Unit Firings — It has been generally held that as the contraction force increases new motor units are recruited and the firing rate of active motor units simultaneously increases; with the inverse occurring as the contraction force decreases. This behavior is true for slowly varying contractions. With our dEMG technology we are able to study the behavior of motor unit firings during rapid cyclic contractions wherein we see a dramatically different firing behavior. Depending on the frequency of the cyclic contraction, it is possible to simultaneously recruit higher threshold motor units and decrease the firing rate of active lower threshold motor units. This revolutionary discovery holds promise for designing novel exercise programs that separately target relatively low-threshold motor units and relatively higher-threshold motor units.
- Kline JC, De Luca CJ. Synchronization of Motor Unit Firings: An Epiphenomenon of Firing Rate Characteristics Not Common Inputs. J Neurophysiol. Oct 2015. PMID: 26490288. (pdf)
- De Luca CJ, Contessa P. Biomechanical Benefits of the Onion-skin Motor Unit Control Scheme. Journal of Biomechanics, 48(2), Jan 2015. PMID: 25527890. (pdf)
- De Luca CJ and Kline JC. Statistically rigorous calculations do not support Common Input and Long-Term synchronization of motor unit firings. Journal of Neurophysiology. 112(11). Dec 2014. PMID: 25210152. (pdf)
- De Luca CJ and Contessa P. Hierarchical Control of Motor Units in Voluntary Contractions. Journal of Neurophysiology, 107(1): 178-195, 2012. PMID: 21975447. (pdf)
- De Luca CJ and Kline JC. Influence of proprioceptive feedback on the firing rate and recruitment of motorneurons. Journal of Neural Engineering, 9(1):016007, 2012. PMID: 22183300. (pdf)
- De Luca CJ, Hostage EC. Relationship between firing rate and recruitment threshold of motoneurons in voluntary isometric contractions. Journal of Neurophysiology, 104: 1034-1046, 2010. PMID: 20554838. (pdf)