Explanation
In this module, we cover the topics of muscle strength and joint range of motion as fundamental elements in understanding indirect training for dysphagia patients. The goal is to learn anatomy and physiology so that you can create a treatment plan for muscle weakness and contractures. Although indirect training includes motor learning-focused exercises to improve motor skills as well as exercises intended to strengthen muscles, we don't cover motor learning in this module.
Explanation
Muscles are a collection of muscle fascicles, and fascicles are a collection of muscle fibers (muscle cells). Muscle fibers are composed of myofibrils, and the myofibrils contract when actin filaments slide past myosin filaments.
Muscle fibers are classified as type I fibers or type II fibers by how they stain. Muscles with a lot of type I fibers look red, so they are called red muscles. Type I fibers have a lot of mitochondria, and use aerobic metabolism extensively. They have a slow contraction rate but are also slow to fatigue, which makes them well suited to endurance exercise. Muscles with a lot of type II fibers are called white muscles. They have a lot of glycolytic enzymes and contract quickly, which makes them better for generating explosive power but they tire easily.
When a weak force is generated, type I fibers are recruited first, and then type II fibers.
Explanation
The first step in muscle contraction is firing of motor neurons in the motor areas of the cerebral cortex. The axons of these motor neurons extend to the brainstem through the pyramidal tract (corticospinal tract), and then switch to the opposite side at the pyramidal decussation. The axons then continue down the lateral funiculus of the spinal cord up to the anterior horn (central [primary] motor neurons). After that, the synapses of these axons connect with anterior horn cells, and the axons of those anterior horn cells extend to the muscles (peripheral [secondary] motor neurons).
The axons of the anterior horn cells branch off and connect with anywhere from a few muscle cells to a few hundred muscle cells. A motor neuron and the group of muscle fibers it innervates are collectively called a motor unit. The number of muscle fibers in a muscle divided by the number of motor units in that muscle is called the innervation ratio. Small muscles such as the finger muscles have a small innervation ratio, and large muscles that support the body (eg, the quadriceps) have a large innervation ratio. This is because the higher the number of muscle fibers that contract or relax when a single motor neuron is switched on or off, the less precisely that muscle is controlled.
Explanation
Muscle contractions are classified as isometric, isotonic, or isokinetic depending on how resistance is applied. They are also classified by the direction of movement at the joint: movement in the direction of muscle contraction is called concentric contraction, lengthening of the muscle despite engagement of that muscle is called eccentric contraction, and contraction without movement is called isometric contraction. Eccentric contraction works best for building muscle strength, but also poses a higher risk of muscle injury if improper technique is used.
Explanation
Manual muscle testing (MMT) is used to evaluate muscle strength. Its most important feature is that it tests muscles against gravity. The MMT score is 3 if the person can move the muscle while in a position that causes the distal end of the body part to face upward when the tested muscle is contracted. The score is 2 if the person can move the joint in the horizontal plane.
Be aware that a single point difference, such as between scores of 0 and 1 or scores of 3 and 4, does not correspond to an equivalent difference in absolute strength. The difference in absolute strength between scores of 0 and 1 is much smaller than the difference between scores of 3 and 4.
Explanation
Muscle strength decreases when a muscle is not used. Loss of strength varies depending on the extent of inactivity, but one study showed a 20% loss in muscle strength after 2 weeks of inactivity. This is why muscles must be used preventively.
Explanation
To strengthen a muscle, we need to apply a load to that muscle. This triggers various reactions such as mechanical stimulation and secretion of growth hormone due to muscle activity. The muscle strengthening effect changes depending on the load, contraction time, frequency, and period of time.
Just because a muscle is engaged to contract, it does not mean that all of the motor units are recruited. The reason why muscle strength quickly increases after starting training is believed to be that the number of motor units recruited increases. Another way of saying this is that the cerebral excitation level increases. When training is continued for about 4 weeks, muscle hypertrophy occurs and muscle strength increases even more. Muscle hypertrophy is due to increases in both the size and number of myofibrils. No clear conclusion has been reached as to whether muscles strengthen by division of muscle fibers.
Explanation
It is important to use isometric and isotonic strengthening exercises in their appropriate applications. Isometric exercises are better for building explosive strength, whereas isotonic exercises are better for endurance-focused training. When working with people with a chronic illness, we need to consider the effects of that illness. If they have exercise restrictions due to a heart-related illness, they should be instructed to avoid isometric exercises due to the stress these exercises put on the heart and instead do isotonic exercises such as pedaling a bicycle. Conversely, if they have osteoarthritis or another such joint problem, we should avoid them doing overhead exercises that wear down articular cartilage and instead emphasize isometric exercises.
Explanation
Range of motion (ROM) refers to the movement potential at a joint in a particular direction. There are 2 types of ROM measurement: passive and active. Passive ROM is measured by movement of a person's muscle by the evaluator, and active ROM is measured by having the person move their own muscle.
The shape of the joint determines what movements it allows. Uniaxial joints allow only for flexion and extension, but multiaxial joints allow for complex movements.
Explanation
It's good to follow the terms agreed upon by the Japanese Orthopaedic Association and the Japanese Association of Rehabilitation Medicine when discussing joint range of motion.
Explanation
A condition called a contracture can develop when someone doesn't move a joint and the periarticular connective tissue then attaches to the joint, causing its passive ROM to decrease below the normal range. Many people with contracture also have muscle atrophy because muscles shorten when they aren't stretched. This tends to occur particularly often when spasticity of the hand or foot muscles is left untreated.
It's important not to mix up the terms ankylosis, spasticity, rigidity, and contracture.
Evaluation of ROM at the rib cage, shoulders, and neck is particularly important in patients with dysphagia.
Explanation
Contractures are treated by stretching. However, lack of movement for more than 1 month can cause irreversible changes in the tissue that make recovery of original function highly unlikely.
The 2 stretching techniques are ROM training (repeated flexion/extension or other such movements) and prolonged stretching. One example of a prolonged stretch would be stretching the triceps surae (the muscle at the posterior aspect of the calf) by having the person stand on a triangular board with the toe side higher than the heel side.
ROM training is painful and strenuous, so it's important to do preventive ROM exercises to avoid contractures-rather than allowing contractures to happen and having to treat them. To prevent contractures, 5 repetitions of ROM exercises are recommended for each joint twice daily.
Recommended readings
- Chino N Ed.: Modern Textbook of Rehabilitation Medicine, 3rd.ed.Tokyo Kanehara, 2009
- Yamada S & Fukunaga T Ed.: Training effects on skeletal muscles from the viewpoint of biochemistry and physiology, 2nd.ed. Nap, 2003
- Nara I、Hamamura Ed.: Prevention and treatment of contractures, 2nd.ed.Tokyo Igakushoin, 2009
