Isotonic Contraction and the Effect of Load on Skeletal Muscles

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Lab #3: Isotonic contraction and the effect of load on skeletal muscles

Abstract
The job of the motor nervous system is to control certain elements in muscles simultaneously to ultimately produce movement. Movement of the body is the result of specialized cells directly associated with skeletal muscle. Skeletal muscles are voluntary muscle and must contract before movement can occur. We know the muscle team moving the arm is formed at the biceps and triceps. Biceps can bend the elbow, but by itself can not extend the arm. Biceps contract and triceps relax to flex the elbow. When the elbow is straightened, the reverse takes place; the biceps relax and triceps contract.

However, what happens during skeletal muscle contraction? For instance, what happens to the muscles in the leg when one lifts weights? We used two variables, muscle length and resistance; to explore how skeletal muscle speed and contraction is affected during an isotonic contraction when these variables are manipulated. We found if resistance is too light skeletal muscle contracts with ease and at a faster rate. However, if the resistance is heavy muscle contraction has a much slower rate. These findings give good insight into safety precautions, maintenance, and medical diagnoses’ of our body. For instance when lifting weights, or determining underlining causes of a heart condition. Introduction

The muscular system has more than 600 muscles (cardiac, skeletal and smooth muscle) throughout the human body. Contraction of these muscles is generated by specialized muscle cells. Skeletal muscles are voluntary and move the body by pulling on the bones, for instance, when throwing a ball or walking. This involves a series of steps in which bones are moved at the joint by a complex electrochemical and mechanical process of contraction and relaxation of skeletal muscles (Kendal et al., 2000). First, skeletal muscle fibers communicate with the nervous system at the neuromuscular junction (NMJ) by stimulating the sarcolemma. Through a process called excitation-contraction coupling where acetylcholine (ACh) is released into the synaptic cleft, opening sodium ion channels (Na+) and generating an action potential (AP). The AP causes the sarcoplasmic reticulum (SR) to release calcium ions (Ca2+) where cross bridges are formed and the muscle contraction cycle is initiated. During the contraction phase, the skeletal muscle shortens producing tension on the ends of the muscle. Next, the relaxation phase, ACh is broken down by acetylcholinesterase (AChE) and the AP is ended. The SR reabsorbs the Ca2+ and with no more cross-bridge interaction, the contraction ends returning the muscle to its resting length (Martini et al., 2012).

Muscles experience two basic types of contractions called isometric and isotonic. Isometric contractions occur when there is a rise in muscle tension, but the length of the muscle stays the same. Isotonic contractions occur when tension in muscle rises and the length of the muscle changes. This is usually associated with muscle moving something that is of a fixed weight. Our purpose is to “describe the effects of resistance and starting length on the initial velocity of shortening and discover why muscle force remains constant during isotonic shortening” (Marieb et al. 2009). Our findings will provide improved understanding of how resting length will result in maximum force production in human muscles (Marieb et al. 2009). Materials

Materials used in the experiment include:
Data collection unit, electrical stimulator, electrodes, force transducer, hooks, muscle support stand, myograph, oscilloscope display, platform height simulator, simulated muscle, voltage control simulator, and weights (grams): 0.5-g, 1.0-g, 1.5-g, and 2.0-g.

Methods
Experiment 1: We began the experiment by placing a hook through the upper tendon of the muscle connecting it to the force transducer. Next, we suspended the muscle...
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