Laboratory #4: Contractility and Fatigue of Skeletal Muscle

Objectives

• Observe and measure muscle contraction dynamics while understanding the acute effects of fatigue on muscle strength and performance over time.

Recommended Reading

• Chapter 12: Skeletal Muscle of your textbook provides a foundational understanding necessary for this experiment, including the physiological basis of muscle contraction, types of muscle fibers, and their functions.

Background

• Motor Unit: A motor unit consists of a motor neuron and all muscle fibers it innervates. The size and number of active motor units significantly impact the strength of muscle contractions.

• Electromyogram (EMG): This tool represents electrical activity in muscles, appearing as spike-like signals during contraction. The duration and frequency of these signals correspond to contraction duration and intensity.

• Measurement Challenges: Accurately measuring electrical activity is complex and requires the transformation of raw EMG data into usable information. A common method involves integrating the absolute values of EMG spikes, where the area under the integrated graph is linearly correlated with contraction strength.

• In this experiment, grip strength is measured using a hand dynamometer, concurrently recording EMG activity from the forearm muscles. The resulting data will facilitate a comparative analysis of grip strength versus EMG activity, aiding in understanding muscle fatigue.

• The experiment will also compare subjects’ dominant and non-dominant forearms concerning strength, electrical activity, and fatigue rates, providing insights into muscle performance variability based on limb dominance.

Exercise 1: EMG Intensity and Force in Dominant Arm

Aim

• To determine the relationship between EMG intensity and muscle contraction force specifically in the dominant arm, exploring how electrical activity correlates to physical force generation.

Procedure

1. With the subject at rest, perform four increasing strength muscle contractions on the dynamometer, holding each for 2 seconds followed by 2-second relaxations.

2. Save recordings under the title "Increasing Grip Force-Dominant" and subsequently analyze EMG waves and muscle force patterns based on the collected data.

Data Analysis

• Graphs depicting EMG intensity and muscle force for the four contractions will be analyzed. Record absolute integrals of the signals, ensuring both EMG amplitude and contraction strength are documented in your Journal.

Questions

1. Correlate absolute integrals of EMG with muscle contractions to explore the strength of their relationship.

2. Determine if linear relationships exist between EMG signals and the respective muscle contractions.

3. Investigate the potential for muscle fibers exhibiting a refractory period during successive contractions.

4. Analyze factors contributing to increased EMG signal amplitude versus muscle force, focusing on fiber firing frequency versus recruitment of additional muscle fibers.

Exercise 2: EMG Intensity and Fatigue in Dominant Arm

Aim

• To observe the relationship between muscle contraction duration and strength concerning EMG activity during the onset of fatigue.

Procedure

1. The subject will grip the dynamometer as tightly as possible, maintaining the contraction for as long as feasible until muscle strength drops below half of the initial maximum strength. Throughout this period, relevant data will be continuously recorded.

Data Analysis

• Important parameters such as peak muscle force and the time taken to reach half-max fatigue will be measured meticulously and recorded in the Journal for analysis.

Exercise 3: EMG Intensity and Force in Non-Dominant Arm

Procedure

1. Follow the same procedures as outlined in Exercise 1 to evaluate performance metrics in the non-dominant arm, allowing for a comprehensive comparison with the dominant arm's results.

Data Analysis

• Compare EMG intensity and muscle force metrics for both arms, highlighting any notable differences in performance output.

Exercise 4: EMG Intensity and Fatigue in Non-Dominant Arm

Procedure

1. Repeat the previous fatigue protocols specifically for the non-dominant forearm to identify differences in fatigue resistance compared to the dominant forearm.

Questions

1. Compare maximum forces generated by both arms, calculating percentage differences to quantify strength variations.

2. Examine circumference differences between the arms and correlate them with the maximum forces recorded.

3. Explore timing differences in muscle fatigue between dominant and non-dominant forearms to understand the impact of muscle training and use.