Abstract
The force–frequency relationship presents the amount of force a muscle can produce as a function of the frequency of activation. During repetitive muscular contractions, fatigue and potentiation may both impact the resultant contractile response. However, both the apparent fatigue observed, and the potential for activity-dependent potentiation can be affected by the frequency of activation. Thus, we wanted to explore the effects that repetitive stimulation had on the force–frequency relationship. The force–frequency relationship of the rat medial gastrocnemius muscle was investigated during consecutive bouts of increasing fatigue with 20 to 100 Hz stimulation. Force was measured prior to the fatiguing protocol, during each of three levels of fatigue, and after 30 min of recovery. Force at each frequency was quantified relative to the pre-fatigued 100 Hz contractions, as well as the percentage reduction of force from the pre-fatigued level at a given frequency. We observed less reduction in force at low frequencies compared to high frequencies, suggesting an interplay of fatigue and potentiation, in which potentiation can “protect” against fatigue in a frequency-dependent manner. The exact mechanism of fatigue is unknown, however the substantial reduction of force at high frequency suggests a role for reduced force per cross-bridge.
Highlights
The force–frequency relationship presents the amount of force a muscle can produce as a function of the frequency of activation
The history of activation can affect subsequent force generation in two general ways; after previous contractile activity, decreased force output will be the result of muscular fatigue, whereas increased force output will be the result of activity dependent potentiation
On the other hand, is essentially the opposite: “an enhanced contractile response which results from prior contractile activity”[3]
Summary
The force–frequency relationship presents the amount of force a muscle can produce as a function of the frequency of activation. Fatigue and potentiation may both impact the resultant contractile response Both the apparent fatigue observed, and the potential for activity-dependent potentiation can be affected by the frequency of activation. The magnitude of force a muscle can produce at any given time is dependent on a variety of factors, including: sarcomere length, velocity of shortening, frequency of stimulation, as well as the history of activation. Many of these factors may be acting simultaneously, leading to a complex interplay in which the muscular force output is dependent upon multiple mechanisms, some of which are cooperative, and some of which counteract one another. The specific “downstream” effects of these factors can be broadly categorized into two groups: (1) reduced number of engaged
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