Review on Resistance Training and Average Motor Unit Firing Rates

Review on Resistance Training and Average Motor Unit Firing Rates

            In 2011, Beck et al. conducted a study looking at the average motor unit firing rates of untrained men after an 8-week strength program. Their findings, done by electromyography (EMG) measurements, found that the program had no effect on average motor unit firing rates and their relationship to the recruitment threshold. In order to better understand this study, some key terms will be defined. Along with the definitions, some suggestions will be made as to why untrained subjects may not have been the optimal participants, as well as some possible changes. Finally, the actual mechanisms of strength gain during this 8-week program will be opined and looked into.

Discussion

            Some key terms must be defined in order to help the reader better understand the article itself and this article review. The first is the vastus lateralis, which is the lateral portion of the quadriceps muscle. The next is electromyography, referred to henceforth as EMG. EMG is the measure of the muscle’s amplitude of contraction measured in electrical signals. This is referred to by Beck et al. (2011) as a combination of “the number of active motor units and their firing rates” (p. 1). Personally, I have worked with this concept as a neurological intra-operative monitoring technician. During lumbar spinal fusions, needle electrodes were place in the vastus lateralis muscle any time a surgeon was working in the vicinity of the L2 to L5 motor nerve root area. By reading EMG signals, the technician is able to give the surgeon feedback is the motor nerve root is touched or irritated by responses in the muscle. This was also accentuated by heavy EMG activity matching up to being able to see the patient physically move on the table. The most important aspect of this was the surgeon placing pedicle screws into the vertebral column. By placing an electrode on the screw and stimulating it, the surgeon was able to see a threshold voltage required to elicit a muscular EMG response. The lower the threshold, the closer the screw was to the motor nerve root. Powers and Howley (2012) define the motor unit as a “motor neuron and all the muscle fibers that it innervates” (p. 153). The term motor unit firing rate refers to the rate at which the motor unit is able to conduct action potentials to the muscle. High and low threshold motor units can be defined simultaneously, once again referring to the 2012 text by Powers and Howley. The threshold refers to the point at which a stimulus to the motor neuron reaches a sufficient strength and enough sodium has diffused into the cell. Once it reaches a critical “threshold” level, the all or none action potential begins. For lower threshold motor units, it refers to a lower level of stimulus and sodium diffusing into the cell; whereas high threshold motor units refer to a higher level of stimulus needed to cause the action potential. This goes hand-in-hand with a term called the size principle, which will be explored at a later section of this paper. Isometric contraction refers to a contraction that is held in place, the muscle is neither shortening, as in concentric contraction, or lengthening, as in eccentric contraction (Powers & Howley, 2012). The term “80% max voluntary contraction rate” refers to the participant contracting their muscle voluntarily at about 80% of their maximum contraction.
            The study by Beck et al. (2011) used eleven untrained men, which may not have been optimal due to delayed onset muscle soreness (DOMS). The participants in the study had not done any weight training in the six months prior to the study. Unsurprisingly, the first few weeks of the study saw a decrease in the maximum force, measured in Newtons, that the subjects were able to produce (Figure 4). The decrease in force could be due to DOMS, which is typical for novice weightlifters (Powers & Howley, 2012). DOMS usually manifests within 24 to 48 hours after training, and can be caused by a number of steps in the process. The steps are outlined by Powers and Howley (2012). It usually involves structural damage to the fibers, membrane damage, calcium leaking out of the sarcoplasmic reticulum, breakdown of cellular proteins, inflammatory response, and finally edema and pain (p. 492, Figure 21.4). Trained subjects would have been less likely to exhibit a DOMS response, and may have resulted in a better look at motor unit firing rates and recruitment threshold without the initial decreases in force production seen in the untrained subjects. For future studies with regard to subjects alone, it would be interesting to conduct the same study with an untrained group and an untrained group. With this, one could see the changes in both, as well as any differences in force production throughout the study, average motor unit firing rates, and the relationship to recruitment threshold.
            It is opined that there are two primary principles for strength increases. The first is the size principle, mentioned about. The size principle is the body’s recruitment of more and more motor neurons as force production needs increase (Powers & Howley, 2012). This is also linked to the subject better learning the movement pattern, allowing the motor units to synchronize better, and may also remove some of the motor unit inhibitions (Powers & Howley, 2012). The other adaptation that results in strength increases is muscular hypertrophy. Physiologically, the more myosin cross-bridges that are available to attach to actin, the more force is able to be produced (Powers & Howley, 2012). Larger muscles would obviously contain more actin and myosin, and thus be able to produce more force.

Conclusion

            The article by Beck et al. (2011) shows an interesting look at strength training adaptations in untrained subjects. It is a study that makes sense in the broad overview of strength and conditioning physiology, and opens up numerous future studies of interest.

References

Beck, T. W., DeFreitas, J. M., & Stock, M. S. (2011). The effects of a resistance training
            program on average motor unit firing rates. Clinical Kinesiology, 65(1), 1-8.
            Retrieved from http://www.delsys.com/decomp/2011_Beck_et_al.pdf

Powers, S. K., & Howley, E. T. (2012). Exercise physiology: Theory and application to fitness
            and performance. New York, NY: McGraw-Hill.