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 t
he dipper in the raised position while licking. Force requirements were the
n manipulated parametrically to measure the effects of escalating force out
put 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 r
equired to maintain the dipper in the raised position remained 6.7 g for al
l three conditions. In the Peak-Force equal to the Hold-Force (PF = HF) man
ipulation, rats were required to maintain the "dipper-raising" force throug
hout 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 withi
n 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 studi
es, and that may reflect increases in the number of motor units firing at h
igher rates. Additionally, with the exception of the 60-g PF = HF condition
, there were within-condition decreases in tremor and increases in task eng
agement, evidence suggesting increased muscle strength as a function of exp
erience (i.e., "physical training"). Taken together, the results suggest th
at the rodent-based method may provide a valuable, noninvasive functional a
ssay for animal models of disorders that affect skeletal muscle control in
humans. (C) 2000 Elsevier Science Inc. All rights reserved.