Induction and Assessment of Exertional Skeletal Muscle Damage in Humans

The overall goal of this procedure is to safely induce and quantify exertional skeletal muscle damage to the quadriceps femoris muscles in human subjects using voluntary eccentric contractions. This muscle damage protocol can help researchers answer key questions regarding the mechanisms of muscle adaptation and regeneration following injury. The main advantage of this technique it that we can study the processes of muscle adaptation in human subjects, eliminating the need for interspecies translation of the results.

While this method can be used to study muscle damage in the context of exercise, it can also be used to study damage caused by overt injury. Demonstrating the procedure will be Jake Sorensen, a PhD student from my laboratory. To create the isokinetic strength test protocol, begin by opening the dynamometer control software on the computer.

Select new protocol from the dropdown list, then on the unilateral tab, program one set of three repetitions at a speed of 60 degrees per second for both the away and toward contractions by entering one into the number sets field, typing three in the end by reps row, and typing 60 in both the speed away and speed toward rows. Next, create an isometric strength test with one set of three isometric contractions lasting five seconds each at a joint angle of 70 degrees by typing a one into the positions field on the unilateral tab and typing a three into the end by reps row under the position one column. Then select 70 in the row labeled angle.

Next, program the muscle damaging eccentric exercise protocol by clicking the unilateral tab and typing 10 into the number of sets field. In the first column, type 10 in the row labeled end by reps. Then type 180 in the next row, labeled speed away and 120 in the row labeled speed toward.

Lastly, in the row labeled torque, type 200 to 600 depending on the anticipated strength of the subject, and enter 60 into the field labeled rest time in seconds. Begin by making a visual analog scale by drawing a 100 millimeter line horizontally across a page. On the left hand side of the line, indicate no soreness, and on the right hand side of the line, indicate extreme soreness.

Next, escort the subject into the testing room, and instruct them to do two body weight squats to collect baseline soreness measurements. Perform both squats with arms help straight out in front of the shoulders, and feet shoulder width apart, in which proper depth is reached once the upper legs are parallel to the floor. Then, ask the subject to indicate the intensity of the soreness he or she felt in the quadriceps femoris muscles during the squats by drawing a vertical line at the appropriate spot on the visual analog scale.

Finally quantify the subjects level of soreness by measuring the distance in millimeters from the no soreness end of the visual analog scale to the subject's mark. After a brief warmup on a cycle ergometer, ask the subject to sit in the chair of the dynamometer. Adjust the dynamometer position settings such that the rotational axis of the knee aligns with that of the lever arm, and secure the leg to the shaft arm.

Record these settings for repeatable set up in the future. Stabilize the subject with shoulder, hip, ankle, and thigh straps. Next, inform the subject that they will be contracting against the lever arm that will remain stationary.

You should instruct the subject according to the prompts on the computer screen to complete three contractions lasting five seconds each. Start the isometric protocol on the computer. Set the ROM limits, calibrate the dynamometer, and allow the subject to practice the test two to three times with submaximal effort in order to familiarize the subject to the test.

Once the subject is ready, ask them to cross their arms across their chest. Start the test, and verbally encourage the subject to provide maximal effort for each contraction. Record baseline measurements of torque, power, and work.

Allow the subject two to three minutes to rest, and then load the isokinetic strength test protocol on the computer. Inform the subject that they will complete three extension and flexion contractions against the lever arm that will move at a fixed rate. Let them know that the lever arm will stop when they reach the end of the range of motion, and the goal is to contract as forcefully as possible throughout the entire range of motion for each contraction.

After a few submaximal practice tests to familiarize the subject, ask them to cross their arms across their chest, and start the test. Verbally encourage the subject to provide maximal effort, and to breathe normally. Record baseline measurements of torque, power, and work.

One to three days after collecting the isometric and isokinetic baseline measurements, have the subject return to the lab to complete the muscle damaging eccentric exercise protocol. After a warmup, orient and secure the subject in the dynamometer using the previously recorded adjustment settings and load the exercise protocol. Instruct the subject to contract against the lever arm to initiate the movement of the shaft arm for each contraction.

Then, explain that as the shaft arm moves toward them at a fixed rate, their goal is to contract against it as forcefully as possible. When the subject is ready, start the first set of contractions by pushing the start button on the screen, and as they perform the eccentric contractions, verbally encourage them to maintain maximal effort for each contraction. Finally, immediately after the muscle damaging exercise, repeat both isokinetic and isometric strength tests as well as at 24, 48, 72, and 96 hours after.

Additionally, repeat the soreness measurement at 24, 48, 72, and 96 hours after the exercise test. Results from seven untrained young men show that compared to pre-exercise, isometric and isokinetic force was decreased out to 24 and 48 hours post exercise, respectively, and returned similar to pre-exercise values thereafter. Conversely, soreness was significantly increased at 24, 48, and 72 hours after exercise.

Once mastered, this technique can be done safely and reliably if it's performed properly. Remember that it's important to consider the physical limitations of each subject and adapt the parameters accordingly. The joint angle parameters in the protocol should be reduced for individuals with limited range of motion and the target force should be reduced for weaker individuals.

Following this procedure, other methods like obtaining a blood or muscle sample can be performed in order to answer additional questions related to the cellular and molecular responses to damage. After watching this video, you should have a good understanding of how to safely and ethically induce and quantify exertional muscle damage in human subjects. Lastly, don't forget that this protocol has the small potential to cause rhabdomyolysis.

Subjects should be screened for signs and symptoms of rhabdomyolysis, which include dark brown urine, nausea, and extreme muscle weakness during the followup visits.