Hey Cath thanks for your post!

With regards to your inquiry regarding the discrepancy that exists between our listed frequency of 75Hz versus the idea that the frequency should be low in order to minimize early onset of fatigue, we did some follow-up on that and found some interesting evidence.  But before getting into the early onset of fatigue, does low versus high frequency NMES illicit different effects?  As we know, results regarding the effectiveness of NMES vary due to the wide range of parameters that can be adjusted and customized.  These range from the frequency, intensity, impulse width, on/off times, number of electrodes and electrode placement.  In a study conducted by Rebai and colleagues (2002), they looked specifically at the effects of two NMES frequencies in the thigh muscle after knee surgery in order to evaluate any differences in recovery.  Their NMES parameters were setup as follows;

	Group 1: 20Hz	Group 2: 80Hz
Pulse Duration:	300 microseconds	300 microseconds
On/off time:	15s/10s	15s/75s
Treatment time:	60 minutes	54 minutes
Voluntary exercises:	Both groups given same standard exercise program.  2 hours of voluntary exercise and 1 hour of NMES per rehabilitation session.
*Number of impulses were standardized for both groups*

 

Results of their study revealed that quadriceps peak force, specifically Quadriceps Femoris (Figure 1) strength, for the 20Hz group had better results in the operated versus non-operated limb, although some studies refute this finding stating that no significant differences exists between the low and high frequency settings (Bax et al., 2005). The 20Hz group recovered ~93% of thigh muscle volume versus ~89% for the 80Hz group thus indicating less atrophy (in both type I and II fibers).  Furthermore, subcutaneous fat measurements were found to be significantly higher in the 80Hz group.  Why is this important?  For two reason; 1. Fat acts as an insulator in the body, it blocks/acts as a barrier against the electrical impulses reaching deeper tissues emitted by the NMES device thereby reducing its effectiveness.  2. Greater accumulation of subcutaneous fat reveals that the area is not being worked very hard, which supports this studies first finding that the 20Hz group gained more strength as a result.

Figure 1.  Quadriceps muscle group - Quadriceps/Rectus Femoris highlighted in red.  

 

 

 

Now getting back to the original question regarding muscle fatigue with low versus high frequency, increasing frequency does not increase muscle recruitment; it simply increases the rate of firing. Therefore, in protocols that have little rest time and longer durations of stimulation (as outlined by Rebai and colleagues) lower frequencies are selected to minimize electrical fatigue of muscles and improve effectiveness of treatment as the results demonstrate above.