Isometric Muscle Contraction
Imagine doing a strength exercise where you hold a position without any movement. In isometric muscle contraction, the joint angle and muscle length remain the same during the exercise. For instance, when performing weight shifting exercises or leg lifts, you engage in isometric muscle contractions. To encourage isometric contractions on a specific hind limb, a hydrotherapist may lift the opposite limb, putting more load on the targeted limb without causing any motion through its joints. This helps stimulate muscle contraction without joint movement.
Eccentric Muscle Contraction
When a muscle actively extends while under a load, it’s called an eccentric muscle contraction. This means the muscle is elongating in response to a greater opposing force. An everyday example of this is when you slowly lower yourself while lying down. This motion requires your muscles to gradually contract to control and lower the weight of your body. Without this controlled eccentric muscle contraction, there’s a risk of sudden collapse.
Concentric Muscle Contraction
Imagine a muscle shortening to generate force, like when you’re lifting a weight. This is a concentric muscle contraction. In a hydrotherapy setting, you can experience concentric contraction by flexing or protracting your hip while using an aquatic treadmill with the water level at the level of your greater trochanter. The increased resistance from the water requires you to use more force compared to exercising on land to propel your body through the water. This helps build muscle strength and endurance.
The Sliding Filament
In hydrotherapy, we primarily focus on skeletal muscles, which are responsible for voluntary movements in the body. Skeletal muscles contract in response to signals from the central nervous system (CNS). The neuromuscular junction is the site where communication occurs between a nerve and a muscle fiber.
Muscle fibres are composed of myofibrils, which contain contractile units called sarcomeres. These sarcomeres run adjacent to each other along the length of the myofibril and give skeletal muscles their striped appearance, known as striated muscles. The sarcomere consists of alternating thick and thin protein filaments. The thick filaments, composed of myosin, are anchored at the center of the sarcomere, known as the M line. The thin filaments, composed of the protein actin, are anchored to the Z lines, located on the outer edges of the sarcomere.
Muscle contraction occurs when these filaments slide past each other. The actin filaments are anchored to the Z lines, causing the sarcomere to shorten from both sides when the actin filaments are pulled along the myosin filaments. This sliding filament mechanism is responsible for muscle contraction.
The contraction process begins when an ATP molecule, bound to myosin, is hydrolyzed into ADP and inorganic phosphate. This causes the myosin head to extend toward the actin, forming a cross-bridge. The power stroke is then triggered, allowing myosin to pull the actin filament toward the M line, resulting in sarcomere shortening. During the power stroke, ADP and inorganic phosphate are released, but myosin remains attached. A new ATP molecule binds to myosin, freeing it to go through another cycle of binding or remain unattached. If myosin remains unattached, the muscle relaxes.
The regulation of muscle contractions involves two regulatory proteins on the thin filaments (actin): troponin and tropomyosin. Tropomyosin normally blocks the cross-bridge binding sites on actin when the muscle is relaxed. Muscle contractions are controlled by the actions of calcium ions. When calcium ion levels are high enough, and ATP is present, calcium ions bind to troponin, displacing tropomyosin. This exposes the myosin binding sites on actin, allowing myosin to attach and form cross-bridges, initiating the muscle contraction cycle.
Calcium ions necessary for muscle contraction are stored in the sarcoplasmic reticulum. When the CNS sends an electrical impulse to contract a muscle, it travels down the T-tubules, releasing calcium ions from the stores. The calcium ions flow to the myofibrils, triggering muscle contraction. As actin and myosin “slide” along each other, the sarcomere shortens, and the Z lines draw closer to the M line.
When sarcomeres within myofibrils contract, the muscle fiber as a whole shortens. When multiple muscle fibres contract in unison, a muscle can generate enough force to move the body. This complex process of muscle contraction is fundamental to our ability to perform various voluntary movements, making it a vital aspect of canine hydrotherapy and rehabilitation.