2.5 Motor Activity.pdf

[00:00:00] >> Muscle activity. These are the objectives. An isometric contraction is also called a static or holding contraction. Isometric contraction occur when the muscle produces force with no apparent change in the joint angle. Isometric contractions provide a static contraction with a variable...

[00:00:00] >> Muscle activity. These are the objectives. An isometric contraction is also called a static or holding contraction. Isometric contraction occur when the muscle produces force with no apparent change in the joint angle. Isometric contractions provide a static contraction with a variable and accommodating resistance without producing any notable muscle length change. [00:00:29] A muscle strength is defined as the force generated about the joint during a maximum isometric contraction. An isotonic contraction is when the muscle tension remains constant as the muscle shortens or lengthens. Truly, isotonic contractions seldom ever occur when muscles are acting through the lever system of the body. [00:00:50] But often the term is used incorrectly to refer to a contraction that causes a joint to move through some range of motion, such as in flexing the elbow while holding a weight in the hand. Even though the weight remains the same throughout the movement, the tension requirements of the muscle change. [00:01:04] With the changing leverage and the torque exerted by the weight changes with changing joint angles. A concentric contraction shortens the muscle when the muscle tension is sufficient to overcome an external resistance. And move the body segment of one attachment toward the segment of its other attachment. This is when the shortening of the muscle occurs to move a body segment. [00:01:29] An eccentric contraction occurs when the muscle slowly lengthens. And the muscles' points of insertion move away from each other as it gives into an external force greater than the contractile force that is exerting. In reality, the muscle does not lengthen, it merely returns from its shortened position towards normal resting length. [00:01:46] Eccentric muscle contractions are capable of generating greater forces than either isometric or concentric contractions, and are involved in activities that require deceleration. For example, the quadriceps activates eccentrically when the body moves from a standing to sitting. And eccentric activity decelerates body segments and provides shock absorption as when landing from a jump or walking. [00:02:10] An isokinetic contraction occurs when the rate of movement is constant. In the 1916 a device as shown in the picture was developed that limits the rate of movement of a crank arm or a pulley to some preset angular velocity regardless of the force exerted by the contracting muscles. [00:02:29] The axis of rotation of the crank arm of the isokinetic device is aligned with the axis of rotation of the moving joint. And the device lever is matched to the skeletal level. The subject contracts the muscle being exercised, and the device controls the speed of the body segment without permitting acceleration to occur. [00:02:46] The resistance accommodates to the external force so that the muscle maintains maximum output throughout the full range of motion. Recording muscle output activity using surface needle or indwelling wire electrodes is called electromyography or EMG. Each pair of electrodes is connected to a channel of the recording apparatus. And the use of multi-channel instruments allows the contraction and relaxation patterns of several muscles to be recorded simultaneously during a particular movement. [00:03:16] Using EMG, the sequence of activation and relaxation as well as the relative amount of activity of specific muscles can be studied as they perform various isolated or coordinated functions. So you can see in this picture that you have the surface electrodes and they are placed over the muscles that we're trying to detect. [00:03:38] And then they go through equipment that amplify those signals. And so that we'll see signals like this so we can see where this person is resisting. In the biceps, we see activation. And in this second picture where they're resisting in the triceps, we see activation of the triceps. [00:03:56] So you will see these EMG recordings often in research papers. Muscles can also be described as agonist and antagonist or synergist. An agonist is a muscle that's the principal muscle producing a motion or maintaining a posture. An agonist actively contracts to produce a concentric, eccentric, or isometric contraction. [00:04:17] Agonist are sometimes referred to as a prime movers. The antagonists is a muscle or muscle group that provides the opposite anatomic action of the agonist. During functional activities, the antagonist is usually inactive during the activity so it neither contributes to nor resists activity. But it's passive elongation or shortening allows the desired activity to occur. [00:04:37] For example, in this picture, the biceps is the agonist as one is lifting the ball and the triceps is the antagonist. You can also have synergist. A synergist is a muscle that contracts as the same time as agonist. The muscle may provide synergistic action in different ways. One way is, it may provide identical or nearly identical activity to that of the agonist. [00:05:02] An example would be the brachioradialis or the brachialis working along with the biceps for flexion. Muscles more often work as synergists than as either agonists or antagonists. In fact, without synergists or stabilizers, agonistic movement is inefficient and ineffective. One needs to know the anatomic attachments and actions and muscles. [00:05:30] The anatomic attachments and actions can predict muscle function when the following are present. When the proximal attachments stabilized, when the distal attachment moves towards the proximal attachment so you have a concentric contraction. When the distal segment moves against gravity or resistance and when the muscle acts alone. However, these circumstances really occur in normal function. [00:05:56] So it's necessary to identify a muscles proximal and distal insertions and its actions, but it only provides part of the picture. You also have to consider times where you might have a proximal attachment that moves towards a fixed distal attachment, like an enclosed kinematic chain exercise. Consider that contractions can be concentric, eccentric or isometric. [00:06:22] Movement of a distal segment is often assisted by the force of gravity, and muscles seldom, if ever, act alone. They often act as synergists with other muscles. So all of these things combined do change the activity of muscles in different circumstances. Thus, it's good to realize that the relationships of muscles as agonists, antagonists, and synergists are not constant. [00:06:49] They vary with the activity one is doing, the position of the body and the direction of resistance which the muscle must overcome. You also must take into account gravity in situations when a muscle is activated. So all of these things together help us describe and think about movement. [00:07:09] Let's look at these EMG images to see how muscle activation might change in different positions. In this one, you can see that the person is moving bending the elbow with a weight against gravity. Thus, the biceps is contracting as the person lifts the weight and then eccentrically controlling the motion against gravity as the person lowers the weight. [00:07:35] When the person moves to a supine position and is going to bend the elbow, the triceps is controlling that in an eccentric fashion. And then when the person is going to straighten the elbow, the triceps is then working in a concentric fashion. However here, where the person is lying on their back and starting from a straight position, the biceps is initially going to activate to bend the elbow. [00:08:05] And then gravity is going to kick in a little bit and there is a little bit of a triceps moving in an eccentric fashion there. Once the person is now going to straighten the elbow, you have the triceps working in to cause that elbow extension and then the biceps is controlling that motion at those later stages. [00:08:34] Thus you can see, that as you vary the activity the position in the body and the direction under the resistance the muscle must overcome then the EMG recordings change. These are the references. Thank you.