Muscle Performance Analysis: Clench Force, Motor Unit Recruitment, and Fatigue in Dominant and Non-Dominant Forearms

The fundamental role of muscles is to convert chemical energy into mechanical energy, leading to muscle contraction and movement. Skeletal muscles are stimulated by somatic motor neurons, which transmit nerve impulses from the central nervous system to the muscles. Motor units, consisting of a motor neuron and its associated muscle fibers, play a crucial role in muscle contraction. The brain determines the number of active motor units needed for a specific task, and motor unit recruitment involves the sequential activation of these units to increase contraction strength.

Even in a relaxed state, muscles maintain a level of contraction known as muscle tone. This sustained partial contraction, along with tonus, leaves the muscle in a state of readiness for the next contraction. Summation occurs when the muscle, after performing a small task, removes slack, resulting in a stronger second contraction. As muscle fibers contract, they consume energy (ATP and oxygen) and generate force. Fatigue sets in when energy is depleted faster than it can be replenished, leading to a decrease in contraction force.

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Muscle fatigue and the ability to generate ATP depend on the type of muscle fibers. Larger muscle fibers fatigue slower and have greater endurance using aerobic energy, while smaller fibers provide quicker energy (glycolic energy) for fast contractions but fatigue faster. Electromyography (EMG) measures electrical activity in muscles, and dynamometry assesses power.

In the presented experiments, the focus was on determining clench force, motor unit recruitment, and fatigue in different forearm muscles. The dominant forearm generally exhibited greater clench force and power compared to the non-dominant forearm.

The EMG values increased with higher clench intensity in both dominant and non-dominant forearms, indicating increased motor unit recruitment. The experiments aimed to explore the variations in muscle performance and activity between different muscle groups and dominant and non-dominant sides.

The primary function of muscles involves the conversion of chemical energy into mechanical energy, resulting in muscle contraction and movement. This process is stimulated by somatic motor neurons, which transmit nerve impulses from the central nervous system to the skeletal muscles. Motor units, comprising a motor neuron and associated muscle fibers, play a vital role in this contraction process. The brain dictates the number of active motor units required for a specific task, leading to motor unit recruitment, where these units are sequentially activated to increase contraction strength.

Even in a relaxed state, muscles maintain a level of contraction known as muscle tone. This sustained partial contraction, along with tonus, keeps the muscle in a state of readiness for subsequent contractions. Summation occurs when the muscle, after performing a small task, removes slack, resulting in a stronger second contraction. Muscle fibers consume energy (ATP and oxygen) and generate force during contractions. Fatigue sets in when energy is depleted faster than it can be replenished, causing a decrease in contraction force.

The ability to generate ATP and resist fatigue depends on the type of muscle fibers. Larger muscle fibers fatigue slower and have greater endurance using aerobic energy, while smaller fibers provide quicker energy (glycolic energy) for fast contractions but fatigue faster. Electromyography (EMG) measures electrical activity in muscles, and dynamometry assesses power.

In the presented experiments, the focus was on determining clench force, motor unit recruitment, and fatigue in different forearm muscles. The dominant forearm generally exhibited greater clench force and power compared to the non-dominant forearm. The EMG values increased with higher clench intensity in both dominant and non-dominant forearms, indicating increased motor unit recruitment.

Laboratory Design:

Experiment 1: Clench Force in Dominant and Non-Dominant Forearms

Objective: To measure clench force and assess motor unit recruitment in dominant and non-dominant forearms.

Procedure:

1. Have the subject, Maggie, clench the dynamometer at four assigned increment levels.
2. Record EMG values and calculate integrated EMG for both dominant and non-dominant forearms.

Formulas:

1. Calculate the difference in EMG values for each clench: EMG Difference=EMGfinal−EMGinitialEMG Difference=EMGfinal​−EMGinitial​
2. Calculate the difference in integrated EMG values for each clench: Integrated EMG Difference=Integrated EMGfinal−Integrated EMGinitialIntegrated EMG Difference=Integrated EMGfinal​−Integrated EMGinitial​

Discussion:

The results from Experiment 1 indicate that the dominant forearm generally exhibited greater clench force and integrated EMG values compared to the non-dominant forearm. This suggests that the dominant muscle group may have higher motor unit recruitment and, consequently, greater force production.

Experiment 2: Comparing Clench Force and Power between Dominant Biceps Brachii and Dominant Flexor Digitorum Muscles

Objective: To compare clench force and power between the dominant biceps brachii and the dominant flexor digitorum muscles.

Procedure:

1. Have the subject, Maggie, clench the dynamometer using both muscle groups.
2. Record EMG values and calculate integrated EMG for both muscle groups.

Formulas:

1. Calculate the difference in EMG values for each clench in both muscle groups: EMG Difference=EMGfinal−EMGinitialEMG Difference=EMGfinal​−EMGinitial​
2. Calculate the difference in integrated EMG values for each clench in both muscle groups: Integrated EMG Difference=Integrated EMGfinal−Integrated EMGinitialIntegrated EMG Difference=Integrated EMGfinal​−Integrated EMGinitial​

The results from Experiment 2 will provide insights into the clench force and power differences between the dominant biceps brachii and the dominant flexor digitorum muscles. It will help determine if certain muscle groups exhibit superior performance in generating force and power.

In conclusion, these experiments aim to elucidate the electrical activity, force, and fatigue patterns in different muscles. The laboratory design incorporates measurements such as EMG and integrated EMG values to quantify muscle activity. The results and calculations provide valuable insights into the dynamics of motor unit recruitment, clench force, and power generation in various muscle groups, contributing to our understanding of muscle physiology and performance.

Table 1: EMG (mV) and Integrated EMG (mV-s) Measurements of Clench Force of Dominant and Non-dominant forearms using electrodes to measure four increasing clutches of increasing intensity.

Tonus, in this context, was indicated by the region between clenching periods. Both forearms exhibited varying EMG mV measurements. The dominant forearm initiated at .41 mV, reached its peak at .43 mV, and concluded at .36 mV. Integrated EMG values remained constant at .04 mV-s. On the other hand, the non-dominant forearm's EMG mV started at .30 mV, peaked at .39 mV, and then dropped sharply to .20 mV. Integrated EMG values fluctuated between .02 mV-s and .03 mV-s before concluding at .02 mV-s (refer to table 2).
Table 2: EMG (mV) and Integrated EMG (mV-s) Tonus state measurements represented by the area between the four clenches (clusters).

In the second phase of the experiment, the participant clenched a dynamometer to record the motor unit recruitment of force (kg) in incremental steps. However, each incremented clench in the dominant forearm fell short of reaching the desired assigned force. For instance, at the assigned force of 5kg, her actual clench force was 4.46kg, and at the assigned force of 25kg, her clench force remained at 4.46kg. The raw EMG values exhibited fluctuations, ranging from 4.68 mV to 8.14 mV, ultimately ending at 5.93 mV. Integrated EMG values in the dominant forearm started at 0.20 mV-s, increased to 0.34 mV-s, and concluded at 0.19 mV-s.

In contrast, the non-dominant forearm exhibited an increase in clench force but failed to reach the assigned forces. At the assigned force of 5kg, the dynamometer recorded a clench force of 8.34kg. The clench force value reached 9.46kg for the assigned force value of 10kg. However, it fell short of the 25kg target, ending at 11.49kg. Raw EMG values for the non-dominant forearm commenced at 3.27 mV, decreased to 2.18 mV during the second clench, and concluded at 4.02 mV. Integrated EMG values began at 0.21 mV-s and ended at 0.37 mV-s, with a difference of 0.16 mV-s (refer to table 3).

Muscle fatigue was assessed by measuring the maximum clench force (kg) in both the dominant and non-dominant forearms. The dominant forearm exhibited a maximum clench force of 16.64 kg and experienced fatigue at 35.93 seconds with a force of 8.32 kg. On the other hand, the non-dominant forearm demonstrated a maximum clench force of 18.29 kg and fatigued at 39.67 seconds with a force of 9.15 kg (refer to table 4).

Our group introduced an additional experiment wherein Maggie served as the test subject to compare the clenching strength of her biceps brachii, specifically while flexing, against the clench force of the dominant forearm observed in the previous experiment. Maggie's dominant forearm in this experiment was her right arm, specifically the biceps brachii. To assess the clenching strength, she flexed her arm against resistance provided by another student's hand, repeating this action four times. The recorded EMG values demonstrated a continuous increase, starting at 3.68 mV and concluding at 9.48 mV, showing a difference of 5.80 mV. Similarly, the integrated EMG values began at 0.37 mV-s and ended at 0.80 mV-s, resulting in a difference of 0.43 mV (refer to table 5).

In the discussion, it is noted that muscles in the body contract in response to stimulation from somatic motor neurons transmitting impulses from the brain. As a muscle generates force and contracts, action potentials fire motor units along axons, and these impulses are detected by EMG electrodes placed on the skin.

The electrical activity observed in the dominant forearm was consistently higher than in the non-dominant forearm. This discrepancy is attributed to Maggie's elevated physical activity levels, particularly in sports like tennis and weightlifting, resulting in a stronger clench force in her dominant forearm. The failure to reach the assigned force in either forearm during the dynamometer task is discussed. The non-dominant forearm, although weaker, exhibited a more consistent and precise increase in force. It is suggested that the brain actively recruited more motor units in the dominant arm, contributing to the fluctuating data observed in the experiment (refer back to table 3).

During the tonus stage, both dominant and non-dominant forearms displayed a steady rate of rest, indicating a state of readiness for the next clench. While there was minimal difference in tonus measurement between the forearms, any variance is attributed to the weaker, non-dominant forearm having more slack during rest (refer back to table 2).

The discussion also delves into the contraction phase, where muscle fibers consume energy to generate force and eliminate slack in the relaxed muscle. As fatigue sets in, more motor units are recruited, but mental effort does not necessarily contribute to increased clench force. Continued clenching until total fatigue would result in a decline in the muscle's ability to generate force due to the exhaustion of energy sources like ATP and oxygen.

The additional experiment comparing the dominant biceps brachii clench force to the dominant forearm clench force demonstrates that a stronger and larger muscle group can achieve a higher clench force (refer back to table 5). Maggie's physically developed biceps brachii, being stronger than her forearm, exhibited more electrical activity due to the muscle's size and the mechanical energy required for the task of flexing against resistance.