Effects of the Addition of Weight on the Force Produced by Muscles Undergoing Isometric and Isotonic Contraction

Investigating the Effects of the Addition of Weight on the Force Produced by Muscles Undergoing Isometric and Isotonic Contraction.
 

Abstract:

The forces produced by isometric contractions within the human biceps and triceps muscles as well as isotonic contractions within the cane toad Sartorius muscle may be altered through the addition of weight. This investigation provides an insight into the production of the force of contraction by differing muscles via the contractile process, influenced by calcium, acetylcholine and ATP in response to the stress of the weight placed upon the muscle. The results demonstrate that force produced by the biceps and triceps through muscle contraction increased as the weight was shifted further from the biceps insertion point. Electrical activity in the biceps increased from 0.15mV when the weight was 54 cm from the insertion point, to 0.81mV when the weight was placed 104 cm away, whereas the triceps increased from 0.07 to 0.24 respectively. The maximum lifting velocity of the cane toad Sartorius muscle drastically decreased as increasing weight was applied to the muscle, beginning at 0.193 mm/ms at 5g, and falling to 0.047 mm/ms when 50g was applied. Therefore, the results gathered indicate that the force generated by muscles may be altered in order to accommodate different stresses, which may further influence such things as its maximum velocity of lift.

Introduction:

Isotonic and Isometric muscle contractions occur due to the interaction of thick myosin and thin actin filaments within myofibrils, known as a cross-bridge interaction (Speranza, 2019). The sliding filament mechanism initiates the cross-bridge interaction between these thick and thin filaments, causing the filaments to overlap, shortening the sarcomeres as the Z lines are drawn towards each other (Speranza, 2019). At the neuromuscular junction, acetylcholine is released, which binds to sarcoplasmic reticulum and thus opens ion channels that allow for calcium ions to flow out (Speranza, 2019) (Johnstone, 2019). Ca²⁺ works as an initiator for this interaction as it binds to troponin molecules within the thin actin filaments, releasing tropomyosin, and thus allowing for the heads of myosin molecules to attach to the actin binding site. ATP binds to the myosin head during this interaction and as myosin utilises this ATP for energy, it pulls the thin filament towards the M line, allowing the myosin head to detach from the initial actin and onto the next actin, and thus the process repeats. This creates contraction within the muscle (Rice et al., 2008) (Edman and Grieve, 1964) (Speranza, 2019). The contraction of skeletal muscles is dependent on the length-tension relationship and is thus able to be altered through changes in the force produced through contraction as well as the number of motor units recruited (Speranza, 2019).

Figure 1: Overlap of thin actin and thick myosin filaments as a result of the change of length of the sarcomere. Adapted from Reconditi, M., Brunello, E., Fusi, L., Linari, M., Martinez, M., Lombardi, V., Irving, M. and Piazzesi, G. (2014). Sarcomere-length dependence of myosin filament structure in skeletal muscle fibres of the frog. The Journal of Physiology, 592(5), pp.1119-1137.

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Experiment A was conducted in order to determine the effects of the addition of various weights to the force generated by isometric contractions in the human biceps muscle. Additionally, experiment B was done in order to analyse the effects of the addition of weights to the maximum velocity of lift and the force produced by isotonic contractions in cane toad Sartorius muscles. As a result of this, the molecular processes that alter the contraction of the muscle and the force it is able to produce may be analysed. The hypothesis tested stated that as the distance of the weight was increased from the bicep insertion point, the electrical activity of the muscle would increase within the human bicep and triceps muscles as more motor units are recruited (Experiment A). The null hypothesis produced for this experiment stated that there would be no increase in the electrical activity of these two muscles upon increasing the distance of the weight, as no more motor units would be recruited. The other hypothesis that was tested stated that the maximum velocity of lift of the cane toad Sartorius muscle (Experiment B) would decrease upon the addition of weight to the muscle. The null hypothesis formulated for this experiment thus stated that there would be no change in the maximum velocity of lift as weight was added to the muscle.

Methods:

The experimental methods used were gathered from the MEDS2001/PHSI2x07 University of Sydney kuracloud website (syd1.kuracloud.com, 2019)

During experiment A, the subject sat at the table and rested their arm along the edge of the table at a 90-degree angle. The distance of the bicep insertion point to the end of the hand was then recorded. The subject was handed a stick with cable ties placed at 10cm intervals along the stick and was instructed to hold it at the area just before the initial cable tie. A 1kg weight was then moved 10cm along the stick every few seconds whilst the subject attempted to hold the stick stably. The RMS values of the biceps and triceps was then gathered by the EMG and was tabulated.

Experiment B utilised an isolated cane toad Sartorius muscle, whose distal end was connected to a lever so that weights may be applied, and pelvic end connected to a Perspex chamber that was held in place by a palmer stand. The lever was then connected to a fulcrum transducer. 2mm mechanical stops were put in place to avoid overstretching. The weights were then added to the distal end of the muscle in increasing intervals whilst the muscle was stimulated with electrodes. The initial length of the muscle was kept the same for all tests. The fulcrum transducer then measured the lift done by the muscle against the time taken for the lift to occur. The maximum velocity was then calculated and tabulated.

Results:

Analysis of the results gathered from experiments A and B allowed for the investigation of the effects of the addition of weights to the electrical activity, and thus force produced by isometric contraction within the bicep and triceps muscles as well as the maximum velocity of lift produced by the cane toad Sartorius muscle through isometric contraction. As can be viewed in Table 1, both the electrical activity of the bicep and triceps muscles increased as the 1kg weight was moved further from the bicep insertion point. The bicep muscle electrical activity rose drastically more so than that of the triceps muscles, beginning at 0.15mV at 54 cm from the bicep insertion point, and rising to 0.81mV at 124cm from the bicep insertion point, whereas the triceps respectively rose from 0.07mV to 0.24mV. This relationship can be observed through Figure 2. Experiment B however, demonstrated that as the weight placed on the muscle was increased, the maximum velocity of lift produced by the muscle decreased and the muscle was unable to lift the weight as it grew heavier. This can be observed through Table 2, whereby the maximum velocity of lift fell from 0.193 mm/ms at 5 g, to 0.047 mm/ms at 50g.

Table 1: The force generated by the Biceps and Triceps when weight is applied at certain distances from the bicep insertion point.

Figure 2: A comparison of the force generated by the biceps and triceps when weight is applied at increasing length intervals from the elbow joint. A) The electrical activity of the bicep muscle increased dramatically as the weight was moved further from the bicep insertion point. B) The electrical activity of the triceps muscle increased less dramatically than the biceps muscle.

Table 2: The decrease in maximum lift velocity produced by isotonic contraction of cane toad Sartorius muscles when varying levels of weight are applied.

Figure 3: The maximum velocity of lift produced by the force of the isotonic contraction within the cane toad Sartorius muscle when subjected to various different weights. A) The muscle was provided with an electrical stimulus in order to ensure the contraction of every muscle fibre and thus allowing for cross-bridge cycling to occur. As the weight was increased, the maximum velocity of lift decreased from 0.193mm/ms at 5g of weight to 0.047mm/ms at 50g of weight.

Discussion:

Through analysis of the data gathered from experiments A, it can be observed that the electrical activity of both the biceps and triceps muscles increased as a result of the weight being moved further from the bicep insertion point. This occurred as a result of the need to recruit more motor units to oppose the torque created by the placement of the weight away from the biceps insertion point. The isometric contraction, whereby no muscle lengthening is occurring, that is transpiring within the muscle is caused by the process of excitation-contraction coupling (Speranza, 2019). This process utilises motor neurons to innervate muscle fibres within the body. This causes the release of acetylcholine at the neuromuscular junction, where it binds with the sarcoplasmic reticulum, opening ion channels that allow for the movement of calcium and sodium ions within the muscle. (Speranza, 2019) (Edman and Grieve, 1964). The amount of motor neurons innervating smaller muscle fibres within the body may be increased as a result of the need to increase force produced by the muscle. Sarcomeres within the muscle shorten as a result of this process, whilst the muscle itself doesn’t contract, increasing tension within the muscle and allowing it to produce enough force to counter the load being placed on it (Reconditi et al, 2014) (Speranza, 2019). Thus, electrical activity in the muscle is increased. This result is consistent with the alternative hypothesis that was generated for Experiment A.

The isotonic contraction that occurs within the cane toad Sartorius muscle however involved the lengthening and shortening of the muscle (eccentric and concentric contractions) to create the force needed to lift the weight that was applied. The maximum velocity of lift of the muscle decreased substantially as the weight applied to the muscle was increased (syd1.kuracloud.com, 2019). The isotonic contraction occurred through electrical signalling, similar to Experiment A. The release of acetylcholine was triggered through constant stimulation to ensure all muscle fibres were producing force. The binding of acetylcholine to the sarcoplasmic reticulum allowed for calcium and sodium to move through the ion channels that were opened (Edman and Grieve, 1964). The calcium allowed for the creation of cross-bridge cycling through its interaction with troponin in the thin actin filaments, removing tropomyosin as a result (Edman and Grieve, 1964). The heads of myosin molecules were thus able to bind to the actin at the actin binding site, utilising ATP hydrolysis as a fuel source at the myosin ATPase site (Reconditi et al, 2011). This creates a power stroke, causing the sarcomeres within the muscle fibres to shorten, thus producing contraction (Speranza, 2019). As the initial length of the muscle was kept the same before each weight was added, the force produced by the muscle through stimulation was the same throughout. However, the increasing weight that was added produced an increase in downward force, up to a point that exceeded the force generated by the muscle (syd1.kuracloud.com, 2019). The release of optimal amounts calcium within the muscle also decreased as the weights were added, increasing the latency period that occurs before contraction, and thus resulting in a decrease in the maximum velocity of lift (Edman and Grieve, 1964). The results gathered from this experiment supported the alternative hypothesis that was generated.

As a result of the experiments being performed one time per individual group, the results gathered may not be valid. This prevented the use of statistical tests to determine the significance of the results that were gathered. In future experiments, larger sample sizes should be used, in conjunction with a minimum of 100 repetitions to provide sufficient data for statistical analysis. This will allow for increased validity and reliability as the significance of the results gathered may be determined and contrasted against the results produced by other studies. As a result of all of this, further research may be undertaken towards this subject to increasing our understanding and to develop new methods and applications of these processes.

The physiological mechanisms involved in the process of muscle contraction may influence the force produced by the muscle as well as the velocity at which the muscle may lift a certain object through the introduction of weights that counteract the force created. The results gathered from this study indicate that as weight is added to a muscle, the force produced by the muscle will increase to counteract the weight, eventually decreasing the velocity of its lift as the weight begins to exceed the force produced by the muscle.

References:

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