The Onset of Low-Frequency Fatigue in Slow Twitch Soleus Muscle Fibers

Amy R. Stephens
Marquette University
Mentor:  Dr. Robert Fitts

In order to prevent a decline in performance following prolonged or intense exercise, muscle fatigue must be studied extensively. Muscle fatigue, which is defined as a decline in power output, has many contributing factors. One such factor is low-frequency fatigue (LFF). LFF has been defined as a reduced tetanic tension at low frequencies of stimulation (10-30Hz) while tetanic tension at high stimulus frequencies (>50Hz) is close to control conditions. LFF has been previously produced only in Type fast II muscles (ie. plantaris). Therefore, the purpose of this experiment was to produce LFF in a slow Type I muscle (ie. soleus). The cause of LFF is believed to be due to a reduced intracellular [Ca2+] during tetani which is thought to be caused by a reduction in Ca2+ release from the sarcoplasmic reticulum (SR). The decline in Ca2+ release from the SR is currently thought to be caused from either an abnormally functioning T-tubule voltage sensor (DHPr) or a SR Ca2+ release channel (Ryr). 

For each experiment a male Sprague-Dawley rat was anesthetized, and the muscles of the left leg exposed and the soleus isolated. The rat was then placed on a stage and the soleus muscle was attached to a force ergometer. The muscle was stretched to optimal length (Lo) and peak twitch force (Pt) determined. The muscle was warmed-up by administering one tetanus (120 Hz, 500 ms train) per minute for ten minutes. A pre-fatigue force-frequency curve was then determined by recording force in response to 10, 20, 40, 60, 80, 100, 120 Hz stimulations. The pre-fatigue low to high frequency ratio (20/120) Hz was calculated. Fatigue was induced by administering 120 Hz, 350 ms train with a pulse every 4 s. The duration between pulses was reduced every 2 minutes. In the first fatigue test, stimulation continued until force had declined to 40% of initial. Recovery was monitored by recording the 20/120 Hz ratio and if LFF was not induced the fatigue test was repeated. For the second and any additional test the muscle was fatigued to 30% of initial force. LFF was followed by determining the 20/120 ratio at 30 min, 1hr, 2hr, and 3hr. After 3 hours, the force-frequency curve was re-determined. LFF was determined by a decreased 20/120 Hz ratio which ranged from 0.16 to 0.20 in comparison to the pre-fatigue ratio of 0.55 to 0.60. The reduced ratio caused the post-fatigue force-frequency relationship to shift to the right. Previous studies regarding LFF have been reported to last from hours to days which suggests that it is caused by a structural alteration rather than altered metabolic (low ATP) or ionic (H+, inorganic phosphate) conditions. Therefore, future studies will asses the cellular causes of LFF.
 
 

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