Low frequency fatigue in endurance trained, sedentary, and spinal cord injured subjects
Mahoney, Edward Thomas
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This study examined low frequency fatigue (LFF) in endurance trained (ET) and sedentary (SED) able-bodied subjects, and in individuals with spinal cord injury (SCI). ET and SED performed two separate neuromuscular electrical stimulation (NMES) protocols to evoke fatigue of the quadriceps of one thigh (experimental leg) with the un-fatigued leg as a control. Protocol 1 (‘15 Min’) lasted 15 min and the duty cycle was 33%. For protocol 2, fatigue in SED was matched to ET during the 15 min protocol (‘Low Matched’) while fatigue in ET was matched to the SED 15 min protocol (‘High Matched’). Force was assessed at 20 Hz (P20) and 100 Hz (P100) and the ratio of P20/P100 was used to evaluate LFF in both thighs before and up to 24 hours following fatigue. The SCI group performed only one protocol in which fatigue was matched to SED during the 15 min protocol, and evaluation of LFF was the same. Results indicated that SED had a greater magnitude of LFF compared to ET with the 15 Min (p<0.001) and High Matched (p<0.020) comparisons. The ET group did not recover faster than the SED group for any of the comparisons. Muscle pain 24 hours after the fatigue tests may have affected LFF values. For SCI, the magnitude of LFF was not significantly different compared to SED. Recovery of LFF was faster in SED compared to SCI in both the experimental (p<0.001) and control leg (p<0.001). SCI did not recover from LFF over 24 hours in either leg. When LFF values in the experimental leg were corrected for LFF values in the control leg, no difference in recovery existed between SED and SCI (p=0.064). In summary, ET had less LFF than SED even when total fatigue was matched, suggesting that ET muscle is more ‘protected’ from LFF. Although ET did not recover faster from LFF, other factors such as muscle injury may make interpretations of recovery difficult. When paralyzed muscle is stimulated sufficiently, LFF will be substantial for at least 24 hours. In addition, assessing LFF with NMES in SCI causes a progressive increase in LFF, which is likely due to muscle injury.