elitefts™ Sunday Edition

Muscle fatigue is defined as a decline in maximal force production and affects the ability to do work. Muscle fatigue can occur in response to short-term, high-intensity exercise or prolonged, submaximal exercise. Muscle fatigue may be dependent on the type of exercise and the mode of contraction performed.

Muscular endurance is quite different than muscular strength. Muscular endurance measures how long a person can maintain a submaximal contraction while muscular strength measures the total force produced. One major physiological difference between endurance and strength is fiber type. Type I fibers are more fatigue resistant than type II fibers, and during the course of this lab, we will examine fatigue during a muscle endurance test using hand dynamometer and EMG measurements. The expected outcome of this experiment is that the subjects who had the highest muscular strength in lab six will perform the worst in the muscular endurance lab because they will be type II fiber dominant. Also, I expect that women will most likely perform better than men in this lab because generally they are type I dominant compared to men.

Electromyography (EMG) is measured over the muscle and a summary action potential is recorded, which is referred to as an M-wave. The amplitude of this M-wave is taken to indicate the neural excitation delivered to the muscle. Muscle force is recorded at this time, and the relationship between M-wave amplitude and force can be examined. The greater the M-wave amplitude, the greater the force, which represents the greater number of alpha motor axons excited by the electrical stimulation. Increasing excitation will increase the force generation to a point. The frequency of stimulation will also regulate the amount of force generated. Increased stimulation frequency will cause increased force production.

Over the course of this lab, the researchers studied muscle fatigue and endurance, respectively. Prior to conducting this lab, the lab technicians determined an individual’s muscular strength and then looked at how his muscles respond to fatigue. We then took the three strongest male and female participants from the lab and used them as our participants for the study to see who among them would have the greatest muscle endurance.

Methods

In this lab, we had six subjects perform a submaximal voluntary contraction at 30 percent of their maximum voluntary contraction (MVC). The time to fatigue was measured for the subjects as an indication of their muscle endurance. Equipment required included a Biopac computer system, a hand dynamometer, and EMG leads (sticky pads and wires).

Procedure

The class divided themselves based on gender—males on the right side of the room and females on the left. The participants for this lab were the top three of each gender who produced the most force in lab six. The computer operator opened up the muscle endurance lab on Biopac under the BSL PRO tab and made sure that the hand dynamometer was plugged into channel one and the EMG was plugged into channel two. EMG sticky pads were attached to the subject’s dominant arm. One pad was placed over the distal region of the radius, one was placed over the proximal region of the ulna, and one was placed on the back of the elbow. The white cord was attached to the pad over the ulna, the red cord was attached to the pad over the radius, and the black cord was attached to the pad on the elbow. The operator hit start. The subject picked up the hand dynamometer and produced an MVC for four seconds. The operator then hit stop. This was done for both the clench force and the EMG. The subject performed at least two more MVC to get an accurate measure of maximal force output, with two minutes rest between each MVC.

Once all groups performed the MVC, each group calculated 5 percent and 30 percent of the subject’s MVC. The operator then scaled the force measurements to this value. Finally, to test muscle endurance, each subject squeezed the hand dynamometer at 30 percent of his or her MVC for as long as possible. Failure was defined as force dropping to 25 percent of the MVC at any time. To examine the initial and final EMG, we found the max EMG measurement in the first five seconds and last five seconds of the endurance test.

Results

Table 1. Subject Characteristics 
Test Subject Number Height Weight (lbs) Age Gender Training Status
Subject 1  6’1”  234 21 Male Elite powerlifter
Subject 2 5’10” 170 28 Male Walking 30 minutes/day
Subject 3 5’10” 157 21 Male Lifting 3 times/week
Subject 4 5’1” 130 21 Female Varsity softball
Subject 5 5’5” 145 21 Female Varsity field hockey
Subject 6 5’5” 150 21 Female Moderate/high physical activity

 

Table 2. Endurance Results
  Max MVC (Newtons) 30% MVC (Newtons) Time to Failure (Minutes) Initial EMG Final EMG
S1  794 222 3:33 1.56 5.9
S2  520 156 4:02 1.32 8.2
S3 382 114 4:04 0.69 2.4
S4 413 124 3:09 1.23 5.7
S5 338 101 9:29 1.07 3.3
S6 267 80.1 5:10 1.71 6.6

 

Table 3. Recovery from Fatigue for Subject One

Peak Baseline

Post MVC

30s

60s

120s

240s
Force

794

564

568

574

592

638
Fatigue Index

71.0%

71.5%

72.3%

74.6%

80.4%

Sample calculations

Fatigue index

Discussion

The data shown illustrates almost exactly what was hypothesized. Subject one produced the most force and was the first male to reach failure. This makes perfect sense, seeing as his training status says that he is an elite powerlifter, and powerlifters are only concerned with their maximal force production (i.e., lifting the most amount of weight possible). He is type II fiber dominant and produced 220 N more force than the second strongest male participant. The time to failure was 3:33, which was about half a minute faster than the next strongest participant. Subject four was the first female out and this wasn't a surprise either. She produced 75 N more force than the second strongest female participant with 413 N. Her time to failure was 3:09, which was a full two minutes faster than the next quickest female to fatigue. Her training background of being a varsity softball player coincides with her type II fiber dominance, considering that softball relies heavily on the ATP-PC energy pathway.

Subject six had the longest time to fatigue with 9:29, and she had the second lowest MVC force of 338 N. This time was almost double the second longest time to fatigue of 5:10. Her training background of varsity field hockey would make her type I fiber dominant because field hockey is a very aerobic sport, relying heavily on the electron transport chain energy pathway.

The subjects' post-MVC measurement was a decent predictor of their time to fatigue. Subject one was 29 percent fatigued after the two-minute MVC test and only recovered about 10 percent after four minutes. The subjects who were more fatigued after the MVC test had a shorter time to failure than those who showed less fatigue after the MVC test.

The data can be applied to many different sects of exercise physiology. Sports teams can use protocols like this to assess how quickly the players fatigue and develop training protocols to increase their strength or endurance based on the results. Although this isn't the most practical and cost-effective test to administer on a large scale, it is one of the most accurate and quantifiable, so it will most likely be reserved for professional teams that can afford the cost of administering the test.