AN ALTERNATIVE TECHNIQUE TO EXAMINE THE FORCE-FREQUENCY RELATIONSHIP IN HUMAN SKELETAL MUSCLE

Abstract

We devised a method to assess the force-frequency relationship in human skeletal muscle that can be administered in 3s, is well tolerated by subjects (n = 10) and is sensitive to low frequency fatigue. The method consists of a single 3s train of shocks delivered directly to the femoral nerve. The frequency of the train increases rapidly and exponentially over 3s from 5 to 100 Hz. The force response of the knee extensors to this increasing frequency train (IFT) was compared to that of a standard series of constant frequency trains (CFT), where is trains of shocks are delivered at single frequencies, ranging from 5 to 100Hz. Both force-frequency curves were examined in two conditions known to alter contractile properties. First, we changed the knee angle to alter muscle length and, in separate experiments, induced muscular fatigue by having subjects perform repeated isometric contractions until the target force was unattainable. Because the frequency required to produce 50% of peak force (50%) was not different between stimulation protocols, we conclude that there was no leftward shift in the curve when the knee extensors were shortened. In contrast, we observed a rightward shift in the curve after fatigue with both protocols; the f50% increased by 48% (p < 0.01) with CFT and 58% (p < 0.001) with an IFT. Low frequency fatigue, as assessed by the ratio of forces produced at 20 and 50Hz stimulation (20:50 ratio), was marked during recovery. The CFT produced an irregular pattern of low frequency fatigue recovery, but the IFT 20:50 ratio, which was most depressed immediately following fatigue, increased smoothly and by 300s was not different from control. These experiments show that the IFT stimulation protocol reveals the same alterations in muscle performance as the traditional CFT. Moreover, the IFT requires only 3s to administer and was judged more tolerable by 70% of our subjects. This suggests that the technique may be an effective alternative for determining the force-frequency relationship in human muscle for clinical and experimental purposes. Supported by NSERC