Skeletal Muscle Fiber Types and Function

Study Tip: When distinguishing between these types on an exam, look for the relationship between myoglobin/mitochondria and fatigue resistance. High myoglobin = High mitochondria = High fatigue resistance (Type I).

ATP Production Pathways

Muscle contraction is an energy-intensive process that requires Adenosine Triphosphate (ATP). Because the body's stores of ATP are extremely limited, muscles must utilize several different pathways to replenish it depending on the duration and intensity of the activity:

1. Immediate Energy (2–3 seconds): The enzyme adenylate kinase provides a near-instantaneous burst of energy by converting two ADP molecules into one ATP and one AMP molecule.
2. Short Bursts (5–6 seconds): The enzyme creatine kinase facilitates the rapid transfer of a high-energy phosphate group from creatine phosphate to ADP, creating ATP.
3. Anaerobic Respiration (30–40 seconds): Through glycolysis, the muscle breaks down glucose without the need for oxygen. This process yields a net of 2 ATP molecules per glucose and results in the production of lactate.
4. Aerobic Respiration (Hours): For prolonged activity, the mitochondria utilize oxygen to break down glucose through glycolysis, the citric acid cycle, and the electron transport chain. This is highly efficient, yielding up to 36 ATP molecules per glucose.

Oxygen Deficit and EPOC

During the transition from rest to intense exercise, there is a lag between the onset of activity and the point where aerobic respiration can meet the ATP demand; this is known as oxygen deficit. Once exercise ceases, the body does not immediately return to its resting metabolic rate. Instead, it enters a state of excess postexercise oxygen consumption (EPOC), also known as "oxygen debt." This elevated oxygen intake is necessary to restore metabolic homeostasis, replenish creatine phosphate and glycogen stores, and convert lactate back into glucose.

Muscle Anatomy and Functional Groups

To understand how muscles move the body, we must look at how they are attached and how they interact with one another.

Attachments and Terminology

Muscles are attached to the skeleton via connective tissues. A tendon is a fusion of collagen fibers that connects muscle to bone, while aponeuroses are broad, sheetlike tendons. In certain areas, such as the wrist or ankle, a retinaculum (a band of connective tissue) holds tendons in place.

When describing muscle movement, we use specific anatomical terms:

• Origin: The most stationary or fixed attachment end of a muscle.
• Insertion: The end of the muscle attached to the bone that undergoes the greatest movement.

Functional Groups in Movement

Muscles rarely act in isolation; they work in coordinated groups to produce smooth, controlled movements:

• Agonist: The primary muscle responsible for a specific movement.
• Antagonist: The muscle that opposes the action of the agonist.
• Synergists: Muscles that work together to produce a coordinated movement. Within this group, the prime mover is the most significant synergist.
• Fixators: Muscles that stabilize the origin of a prime mover, holding a bone in place so the movement can occur efficiently.

Muscle Fatigue and Contracture

Fatigue is a temporary reduction in the work capacity of a muscle caused by prolonged activity. It can be driven by several factors, including acidosis (lowered pH), local ATP depletion, oxidative stress from reactive oxygen species, and inflammatory responses.

In extreme cases, the depletion of ATP leads to serious physiological conditions:

• Physiological Contracture: When ATP is completely depleted, the muscle can no longer relax. This is because ATP is required for the myosin heads to detach from the actin filaments during the cross-bridge cycle.
• Rigor Mortis: This occurs after death. As sarcoplasmic membranes break down, calcium leaks into the sarcoplasm, triggering cross-bridge formation. However, because no more ATP is being produced, the myosin heads remain permanently locked to the actin, resulting in the rigid stiffening of the body.

Types of Contraction

The way a muscle changes length during contraction defines the type of work being done: