Potential errors in fiber length measurements resulting from lever arm rotation during mechanical testing of muscle cells

In single muscle cell preparations fibers are often suspended between connectors, extending perpendicularly from a force transducer, and the lever arm of a torque motor. The fiber does not move along a horizontal plane when shortened or lengthened by lever arm rotation. An error from the true length (TL) is introduced if the expected length (EL) is calibrated along this horizontal optical plane. Lever arm length (LAL), initial fiber length (FL(i)), connector length (CL), and the magnitude of EL all contribute to this error. A mathematical model was used to determine the TL during shortening (0.96-0.80 FL(i)) and lengthening (1.10-1.50 FL(i)) at a constant LAL of 13.6mm. CL had the greatest impact on error. For FL(i) = 2mm at the longest CL modeled (15 mm), an expected shortening of 0.20 FL(i) produced a true shortening of ～ 0.17 FL(i), and an expected stretch to 1.50 FL(i) resulted in a true stretch to almost 1.60 FL(i). Under these conditions, the true sarcomere length would be 4% and 6% longer than expected during shortening and lengthening, respectively. Because of their non-linear nature, length errors at long CL's may result in an over-estimation of unloaded shortening velocity during slack tests and a left-ward shift in the passive tension-fiber length relationship at long fiber lengths. Measurement errors decreased dramatically with shorter CL's, becoming negligible (<1%) at CL = 3mm. We recommend that investigators keep CL as short as possible. Alternatively, we provide a method for adjusting the magnitude of the EL to yield a desired TL.