Construction of Constant-Load (Isotonic) and Constant-Velocity (Isokinetic) Torque-Velocity-Power Profiles In vivo for the Rat Plantar Flexors

We aim to develop a simple, non-invasive in vivo system in rats to test mechanical function, specifically power, which is the product of force and velocity. This will provide insight on how diseases impact the muscular system. Our lab has been instrumental in the understanding of sarcomerogenesis and aging.

Specifically how maximal eccentric contractions impairs sarcomerogenesis, but some maximal contractions in h rodents improve serial sarcoma number. Right now I'm using the system for eccentric overload training in OVX and ovary intact rats, to investigate the effects of estrogen on muscle contractile function and the response to training. After anesthetizing the rat, position the rat with its nose and mouth placed securely in a nose cone on a heated platform.

Continuously monitor the rat's breathing and pulse rate to maintain a safe and consistent depth of anesthesia. Using a sterile applicator, spread ophthalmic ointment over both eyes of the rat to prevent dryness during the procedure. Using an electric razor, shave all hair from the rat's left leg.

Next, apply a commercial hair removal cream evenly over the entire leg using a cotton swab. After waiting for three minutes, scrape off the cream using multiple cotton swabs as needed to ensure the leg is as hair-free as possible. Then wet cotton balls with distilled water and use them to wipe the entire surface of the leg, removing any remaining cream or debris.

Pat the leg dry using a paper towel to prepare it for electrode placement. Now wrap surgical tape tightly around the rat's foot and the foot pedal to secure the foot in place. Ensure the heel is fixed securely into the slot at the base of the foot pedal.

Adjust the foot pedal position using adjustment knob three to bring it closer or farther away as needed, so that the knee is extended as much as possible. Then set up the tibial clamp by pushing it firmly into the mid proximal region of the tibia to stabilize the lower leg. Using adjustment knobs one and two, align the foot pedal with the clamp to ensure proper positioning.

Using tweezers spread conductive gel over the surfaces of both electrodes and onto the posterior side of the rat's leg. To identify the electrode placement sites, palpate the muscle bulge on the posterior aspect of the lower leg. Place the distal electrode just below the gastrocnemius muscles and the proximal electrode at the top end of the gastrocnemii where they meet the knee joint.

Then turn the gear to raise the electrodes gently into contact with the leg, ensuring consistent stimulation contact. Do not apply excessive upward force and verify that there is a slight bend in the axle holding the electrodes to keep them stable during stimulation. Once the 100 hertz at 90 degree protocol is loaded on the stimulator, set the current to 20 milliamperes to provide low level stimulation and verify electrode or gel placement.

Click on start test to initiate the protocol, observing visible muscle contractions. Once the stimulation completes, click on analysis and ensure the baseline correction box is selected to evaluate the torque produced. After allowing a two minute rest period, increase the stimulation current to 30 milliamperes.

Run the protocol again and evaluate the torque output in the analysis section, confirming that baseline correction remains selected. Increase the stimulation current to 40 milliamperes and run the protocol again. After a two minute rest, if the torque increases at 30 milliampere, raise it to 40 milliampere and repeat.

If the torque decreases at 40 milliampere, set 30 milliampere as the optimal stimulus. Create a stimulation protocol that induces a 500 millisecond isometric contraction at 100 hertz with the ankle positioned at a 70 degree angle. Run the protocol and record the maximum active torque, produced during contraction.

In the protocol screen, create a new protocol that moves the ankle into a flexed position, applies a 500 millisecond stimulation, and then returns the ankle to a neutral position. Calculate isotonic load clamp values corresponding to 10, 20, 30, 40 50, 60, 70, and 80%of the previously recorded maximum active torque. For each load clamp value, add the baseline passive torque to determine the total clamp force.

Run the stimulation protocols using these load clamps in a randomized order. In the DMC version 5.5 software, go to the offset force box. Enter the calculated torque value for the first load clamp from the list generated previously.

Load the isotonic created earlier and the new offset force will be applied automatically during the run. Click on Start test to run the isotonic protocol, allowing an isotonic contraction to occur. All three methods exhibited the characteristic hyperbolic torque velocity relationship with angular velocity decreasing as torque increased, and with peak power occurring at intermediate torque and velocity values.

The isotonic curve predicted a significantly higher maximal velocity than both ISOK kinetic curves. Peak power was higher in the isotonic curve, compared to the isokinetic curve using average torque, but not significantly different when using peak torque. Torque at peak power was overestimated by the isokinetic curve using peak torque, whereas the isokinetic curve using average torque closely matched the isotonic curve.

Velocity at peak power was underestimated by the isokinetic curve using average torque, and was slightly underestimated even when using peak torque. The curvature of the torque velocity curve was significantly reduced when peak torque was used in the isokinetic curve compared to using average torque. All models had strong fits to the hill equation, although the isotonic curve had a slightly lower R squared value than both isokinetic curves.