The relationship between isometric force requirement and forelimb tremor in the rat

To explore the effects of isometric force of rodent forelimb contraction on forelimb tremor, rats were trained to press downward on an isometric force transducer to raise a water-filled dipper cup and maintain force to keep the dipper in the raised position while licking. Force requirements were then manipulated parametrically to measure the effects of escalating force output on forelimb tremor and other variables. In the Peak-Force greater than Hold-Force (PF > HF) manipulation, the forces required to raise the dipper were 20, 40, and 60 g (each condition for about 2 weeks), while the force required to maintain the dipper in the raised position remained 6.7 g for all three conditions. In the Peak-Force equal to the Hold-Force (PF = HF) manipulation, rats were required to maintain the "dipper-raising" force throughout the response. The forces required were 20 g, 40 g, and 60 g (each for 2 weeks). For all force requirement manipulations, data were analyzed within and across conditions. As expected, force output increased with increased force requirements. Spectral analysis of force-time records revealed that during all manipulations, high-frequency (>10 Hz) forelimb tremor increased with increased force output, an effect that is consistent with human studies, and that may reflect increases in the number of motor units firing at higher rates. Additionally, with the exception of the 60-g PF = HF condition, there were within-condition decreases in tremor and increases in task engagement, evidence suggesting increased muscle strength as a function of experience (i.e., "physical training"). Taken together, the results suggest that the rodent-based method may provide a valuable, noninvasive functional assay for animal models of disorders that affect skeletal muscle control in humans.