We have developed a technology capable of automatically decomposing surface EMG signals into the constituent individual action potentials. The technology can identify firings of up to 50 concurrently active motor units with an accuracy that reaches 97% in contraction force up to 100% maximal voluntary force (MVC).

With this technology we studied the behavior of motor unit recruitment and firing rate as a function of force. The results describe a hierarchical inverse relationship between the recruitment thresholds and the firing rates, including the first and second derivatives (i.e., the velocity and the acceleration of the firing rate). This relationship describes an “operating point” for the motoneuron pool that remains constant at all force levels and is modulated by the excitation. This relationship differs only slightly between subjects and more distinctly across muscles. These results support the “onion skin” property that suggests a basic control scheme encoded in the physical properties of motoneurons, which respond consistently to the “common drive” of the motoneuron pool.

We also studied the behavior of firing rates of motor units throughout contractions, which were sustained to exhaustion while maintained at a constant force level. We found that the firing rate of motor units first decreased and then subsequently increased throughout contractions. This observation was accompanied with a potentiatiation and an ensuing decrease of the whole muscle force twitch. The two phenomena are complimentary. The variance of the firings and the synchronization of firings between the pairs of motor units did not change with increasing fatigue. The cross-correlation of the pairs of motor units’ firing rates, and the recruitment of new motor units increased with sustained contraction time, which ostensibly accounted for the increased fluctuation of the force during the progression of the contraction.

Armed with the findings, we have developed a model that included these observations of the motor unit firing rates and force twitches along with a feedback loop to maintain a constant output force. The required changes in the firing rates forced by the model were consistent with the physiological observations of the firing rates during constant-force isometric contractions.

Further explorations of the model will help us understand; a) Why different muscles have different firing rate and recruitment characteristics. b) Why the firing rates first decrease and later increase during fatiguing contractions. c) Why motor unit firing rates appear to decrease in the elderly in comparison to young adult at the same level of MVC. d) Why force variability increases with age and during fatiguing contractions.

In Sports Science, the model will be able to test hypotheses concerning the behavior of motor units in contractions performed during training, exercise, and skilled activities. It will be particularly useful in monitoring the behavior of the high-threshold motor units which cannot be conveniently monitored by other means.

Dr. Carlo J. De Luca
Delsys Inc., Boston USA
NeuroMuscular Research Center, Boston University, Boston USA

Dr. Carlo J. De Luca is the Director, NeuroMuscular Research Center, Professor of Biomedical Engineering, Research Professor of Neurology, Boston University, Boston, MA.

Dr. De Luca focuses on the application of engineering principles to the understanding of motor control and the development of objective patient treatment procedures. Specifically, he is interested in:

• Using innovative technology to identify motor unit action potentials from the surface EMG signal to explore the control scheme that regulates their activation and firing in the healthy neuromuscular system. This is a continuation of work that Dr. De Luca has done over the past four decades.
• Leading a team developing algorithms for identifying dysfunctional movements in neurologically impaired patients for the purpose of providing clinicians with high-resolution reports on mobility and medication-effect for improving patient management.
• Developing a physiologically-based mathematical model describing the generation of muscle force that allows for hypothesis testing of the influence of firing characteristics and muscle fiber mechanics.
• Exploring the development of technology for detecting and analyzing surface EMG signals for the purpose of facilitating and expanding the use of the EMG signal in clinical, movement science, sport science, and ergonomics environments.

Dr. Paola Contessa
NeuroMuscular Research Center, Boston University, Boston USA

Dr. Paola Contessa received a M.Sc. and a Ph.D. in Biomedical Engineering from University of Padova, Padova, Italy. During her Ph.D., she was appointed as visiting Research Assistant at the NeuroMuscular Research Center, Boston University, Boston, USA.

She is currently a Research Associate at the Neuromuscular Research Center at Boston University, where her research focuses on motor control and muscle force generation strategies.

For a complete listing of publications click here.

Findings

De Luca CJ, LeFever RS, McCue MP, and Xenakis AP. Behavior of human motor units in different muscles during linearly-varying contractions. Journal of  Physiology, 329: 113-128, 1982.

De Luca CJ, LeFever RS, McCue MP, and Xenakis AP. Control scheme governing concurrently active human motor units during voluntary contractions. Journal of Physiology, 329: 129-142, 1982.

De Luca CJ, Foley PJ, and Erim Z.  Motor unit control properties in voluntary isometric isotonic contractions.  Journal of Neurophysiology, 76:  1503-16, 1996.

Adam A, De Luca CJ, Erim Z. Hand dominance and motor unit firing behavior.  Journal of Neurophysiology, 80: 1373-1382, 1998.

Erim Z, Beg MF, Burke DT, and De Luca CJ.  Effects of aging on motor unit control properties. Journal of Neurophysiology, 82: 2081-2091, 1999.

De Luca CJ and Erim Z.  Common drive in motor units of a synergistic muscle pair. Journal of Neurophysiology, 87: 2200-2204, 2002.

Adam A and De Luca CJ. Recruitment order of motor units in human Vastus Lateralis muscle is maintained during fatiguing contractions. Journal of Neurophysiology, 90: 2919-2927, 2003.

Adam A and De Luca CJ. Firing rates of motor units in human vastus lateralis muscle during fatiguing isometric contractions. Journal of Applied Physiology,  99: 268-280, 2005.

De Luca CJ, Adam A, Wotiz R, Gilmore LD, and Nawab SH. Decomposition of surface EMG signals. Journal of Neurophysiology, 96: 1646-1657, 2006.

Nawab SH, Wotiz RP, and De Luca CJ. Decomposition of indwelling EMG signals. Journal of Applied Physiology, 105: 700-710, 2008.

De Luca CJ, Gonzalez-Cueto JA, and Adam A. Motor unit recruitment and proprioceptive feedback decrease the common drive. Journal of Neurophysiology, 101:1620-1628, 2009.

Contessa P, Adam A, and De Luca CJ. Motor unit control and force fluctuation during fatigue. Journal of Applied Physiology, 107: 235-243, 2009.

Technology

De Luca CJ, Adam A, Wotiz R, Gilmore LD, and Nawab SH. Decomposition of surface EMG signals. Journal of Neurophysiology, 96: 1646-1657, 2006.

Nawab SH, Wotiz RP, and De Luca CJ. Decomposition of indwelling EMG signals. Journal of Applied Physiology, 105: 700-710, 2008.

Nawab SH, Chang SS, and De Luca CJ.  High-yield decomposition of surface EMG signals.  Clinical Neurophysiology – in press, DOI # 10.1016/j.clinph.2009.11.092.