Muscles contract in various ways, from shortening to lengthening under tension. These contractions depend on the overlap of thick and thin filaments in sarcomeres. Understanding muscle contraction types helps explain how we move and generate force in everyday activities.

Muscle twitches have distinct phases, from the initial stimulus to relaxation. Tension control mechanisms like wave summation and tetanus allow for sustained contractions. Neuromuscular control, involving motor units and acetylcholine, orchestrates muscle movements in response to nerve signals.

Muscle Contraction Types and Characteristics

Types of muscle contractions

• Concentric contraction increases muscle tension causing the muscle to shorten (biceps contracting to lift a weight)
• Isotonic contraction maintains constant muscle tension throughout the contraction and includes both concentric and eccentric contractions
• Eccentric contraction lengthens the muscle while under tension when the opposing force is greater than the force generated by the muscle (biceps lengthening while lowering a weight)

Length-tension relationship in muscles

• Maximum tension generation occurs with optimal overlap of thick and thin filaments at a sarcomere length of approximately 2.0-2.2 μm
• Decreased tension occurs when the sarcomere is either too short due to less overlap of thick and thin filaments or too long due to decreased overlap and fewer cross-bridges formed
• Passive tension generated by the elastic components of the muscle increases as the muscle is stretched beyond its resting length

Muscle Twitch and Tension Control

Phases of muscle twitches

• Latent period is the time between the arrival of the action potential and the start of muscle contraction when calcium ions are released from the sarcoplasmic reticulum
• Contraction phase increases tension as the muscle shortens and cross-bridges form between actin and myosin filaments
• Relaxation phase decreases tension as the muscle returns to its resting length and calcium ions are actively pumped back into the sarcoplasmic reticulum

Mechanisms of muscle tension control

• Wave summation occurs when a second stimulus is applied before the muscle has fully relaxed from the first stimulus resulting in a higher peak tension than a single twitch
• Tetanus is a sustained muscle contraction resulting from rapid, repeated stimulation and can be either:
  1. Incomplete tetanus with slight relaxation between stimuli
  2. Complete tetanus with no relaxation between stimuli resulting in a smooth, continuous contraction
• Treppe (staircase phenomenon) progressively increases muscle tension with repeated stimulation at a constant frequency due to increased calcium ion availability and enhanced contractile mechanism efficiency

Neuromuscular Control

• Motor units consist of a motor neuron and all the muscle fibers it innervates
• The neuromuscular junction is the site where a motor neuron communicates with a muscle fiber
• Acetylcholine is the neurotransmitter released at the neuromuscular junction to initiate muscle contraction
• The all-or-none principle states that a muscle fiber either contracts fully or not at all in response to stimulation