IN-VIVO MEASUREMENT OF HUMAN WRIST EXTENSOR MUSCLE SARCOMERE-LENGTH CHANGES

1. Human extensor carpi radialis brevis (ECRB) sarcomere length was me asured intraoperatively in five subjects using laser diffraction. 2. I n a separate cadaveric study, ECRB tendons were loaded to the muscle's predicted maximum tetanic tension, and tendon strain was measured to estimate active sarcomere shortening at the expense of tendon lengthen ing. 3. As the wrist joint was passively flexed from full extension to full flexion, ECRB sarcomere length increased from 2.6 to 3.4 mu m at a rate of 7.6 nm/deg joint angle rotation. Correcting for tendon elon gation during muscle activation yielded an active sarcomere length ran ge of 2.44 to 3.33 mu m. Maximal predicted sarcomere shortening accomp anying muscle activation was dependent on initial sarcomere length and was always <0.15 mu m, suggesting a minimal effect of tendon complian ce. 4. Thin filament lengths measured from electron micrographs of mus cle biopsies obtained from the same region of the ECRB muscles were 1. 30 +/- 027 (SE) mu m whereas thick filaments were 1.66 +/- 027 mu m lo ng, suggesting an optimal sarcomere length of 2.80 mu m and a maximum sarcomere length for active force generation of 4.26 mu m. 5. These ex periments demonstrate that human skeletal muscles can function on the descending limb of their sarcomere length-tension relationship under p hysiological conditions. Thus, muscle force changes during joint rotat ion are an important component of the motor control system.