Concentric and Eccentric Contractions
Motor unit recruitment in concentric and eccentric contractions
All forms of muscular contractions rely on the activation of motor units. This recruitment enables muscles to complete the activity while preventing severe muscular injury. Concentric and eccentric contractions differ in motor unit recruitment. Motor unit recruitment in eccentric contraction is lower when compared to concentric contractions. When a force is applied to a muscle, it exceeds the instantaneous force produced by that muscle. The muscle-tendon system is forced to extend while contracting, resulting in an eccentric (lengthening) muscle contraction. During an eccentric contraction, more force is generated. The muscle absorbs the energy generated by an external load during this process.
In contrast to concentric (shortening) contraction or "positive work," eccentric movement is also known as "negative work." Eccentric muscular contractions are a necessary feature of most daily or athletic movements. Skeletal muscles contract eccentrically to sustain the body's weight against gravity, absorb shock, and store elastic recoil energy in preparation for concentric (or faster) contractions. Downhill running or going down the stairs, which emphasizes the eccentric effort of the knee extensor muscles, is a classic example of contractions. Eccentric contractions require less motor unit activation and burn less oxygen and energy than concentric contractions for a given muscle force. Eccentric contractions are controlled by different neural methods than concentric or isometric contractions. Eccentric contraction is gaining popularity due to its unique physiological and mechanical features. The benefits of eccentric exercise in certain groups of older adults or patients with chronic conditions such as neuromuscular disorders are accumulating. Eccentric exercises positively affect a neural drive and other health-related characteristics. Eccentric training has been widely employed in athletics to develop maximal physical strength, power, and coordination. Its widespread use in athletic rehabilitation, particularly in the treatment of tendinopathies. Furthermore, eccentric training has been shown to be helpful in preventing sports injuries such as hamstring strain in athletes. It appears to be especially well-suited to train those with medical problems that cause muscle wasting and decrease muscle strength, mobility, and aerobic capacity. Patients with cardiorespiratory issues, sarcopenia, cachexia, type 2 diabetes, neurological and musculoskeletal illnesses are increasingly being recommended eccentric training. Aside from improving muscle function, aerobic eccentric exercise has specific effects on muscle energy metabolism, insulin resistance, and blood lipid profile, which may reduce these risks. As a result, it has been identified as a promising lifestyle element in the fight against obesity and dyslipidemia (Hody et al., 2019).
How does task affect motor unit recruitment patterns?
Skeletal muscles are made of various motor unit types, each contributing to the muscle's mechanical and physiological features. A muscle must activate an appropriate number and mix of motor units to generate force over a sufficient time period in order to successfully contribute to smooth, coordinated movement. The motor units are turned on in sequential order, from the slowest to the fastest. The motor unit, defined as a motoneuron and all the muscle fibers it innervates, is the most fundamental functional unit of the neuromuscular system. It has been discovered that the size of motor units varies both within and between muscles. Slow-twitch muscle fibers make up smaller motor units, while fast-twitch fibers make up bigger motor units. Because small motor units have smaller diameter nerve axons, action potential conduction velocities are slower. As a result, the size principle predicts this. The fast motor units will be recruited after the slow motor units have been engaged. The shape and conduction velocity of action potentials from different types of motor units differ. However, deactivation of motor units should be done in reverse order, starting with the quickest and ending with the slowest. Slower motor units are more fatigue resistant and can deliver enough force for a variety of activities, including posture maintenance and walking. Fast motor units are more prone to fatigue and would be unable to maintain force generation for an extended amount of time. Motor unit recruitment patterns may have been modified by activation–deactivation kinetics due to a recruitment strategy to maximize metabolic efficiency. According to studies, single fibers based on myosin isoforms and slow muscle fibers have a lower metabolic cost during isometric and shortening contractions
References:
Hodson-Tole, & Wakeling, J. M. (2008). Motor unit recruitment patterns 1: responses to changes in locomotor velocity and incline. Journal of Experimental Biology, 211(Pt 12), 1882–1892. https://doi.org/10.1242/jeb.014407
Hody, S., Croisier, J. L., Bury, T., Rogister, B., & Leprince, P. (2019). Eccentric Muscle Contractions: Risks and Benefits. Frontiers in physiology, 10, 536. https://doi.org/10.3389/fphys.2019.00536
