Muscle Contraction, Part 3

posted by Malachy P. McHugh
filed under fitness postings
Based on their activation threshold and the shape of the twitch response to a single action potential, motor units can be categorized into three basic types: slow, fatigue resistant, and fast fatigable. Slow motor units are composed of slow-twitch fibers, do not generate a lot of force, and are extremely resistant to fatigue. Fatigue-resistant motor units are composed of intermediate fibers, can produce moderate force, and are somewhat resistant to fatigue. Fast fatigable motor units, which are composed of fast-twitch fibers, produce high forces, but fatigue rapidly. The plantar flexors consist of the soleus muscle, which works repetitively during walking and running, and the gastrocnemius, or calf muscle, which is more active during jumping and sprinting. The soleus has a predominance of slow motor units, and thus, has a high proportion of slow-twitch fibers. This muscle has high endurance, but is not very powerful. The gastrocnemius has a mixture of slow, fatigue-resistant and fast fatigable motor units, and therefore, has a greater mix of fiber types. While very powerful, this muscle has limited endurance. During maximal voluntary contractions it is the fast fatigable motor units that are difficult to recruit.

From the Message Boards

  • I'm 48 and had been thru several years of perimenopause when I developed endometrial cancer and underwent a hysterectomy in March 2009. The sudden drastic changes in my body have been profound. I gained 30 pounds!!!

Follow Us

Explore FitClick
Browse this section for quick links to our calorie counter and other popular diet and fitness features. From diet plans to weight loss programs, FitClick has the content you need to lead a healthy life. Find workout routines, a calorie calculator and more at your source for diet and fitness information.
We have updated our Privacy Policy, effective May 25, 2018. We have done this in preparation for the EU's new data privacy law, the General Data Protection Regulation (GDPR). Please take the time to review our updated documentation by clicking on the Privacy Policy link at the bottom of this page. By continuing to use this service on or after May 25, 2018, you agree to our updated Privacy Policy.