NPB 101: Lecture 16 - Muscle Physiology 2: Mediators of Force

Learning Objectives: Interpret active, passive, and total length-tension relationships, relating active force to sarcomere filament overlap. Describe a muscle twitch and explain how increasing stimulation frequency produces summation and tetanus. Distinguish isometric, concentric, and eccentric contractions by changes in muscle length during force production. Use the force-velocity relationship to predict how load affects muscle shortening velocity. Explain how sarcomeres arranged in parallel versus series contribute to muscle force, shortening velocity, and excursion. Predict how differences in physiological cross-sectional area and fiver length affect the functional capabilities of muscles.

Muscle cramps

Involuntary tetanic contraction of skeletal muscle.

• Causes are…

  • Many times unknown (still currently an active area of research)

  • But are often related to ion imbalances

• Hypocalcemic Tetany

  • Low extracellular Ca²⁺

    • There is a lower membrane potential (easier to depolarize)

    • More sensitive to stimuli and will more easily fire APs

Length tension curve

Describes how the force (tension) a muscle can generate varies with changes in its length.

Twitch contraction

A brief, single contraction and relaxation cycle of a skeletal muscle fiber in response to a single action potential from a motor neuron.

Isotonic contractions

A type of muscle contraction where the tension or force generated by the muscle remains constant while the length of the muscle changes to produce joint movement.

Three primary types:

• Concentric contraction

  • Muscle generates force while shortening

  • e.g. lifting weight

• Eccentric contraction

  • Muscle generates force while lengthening, acting as a “brake” to control movement

  • e.g. lowering a weight down slowly

**NOT the same as isometric contractions where length remains the same

Force-velocity curve

A curve that describes the inverse relationship between the force a muscle can generate and the speed (velocity) of its contraction.

• Inverse Relationship

  • Heavy loads move slowly (high force, low velocity), while light loads can be moved very quickly (low force, high velocity)

Concentric contraction

A type of muscle contraction where the muscle generates force while shortening.

• Occurs when the internal tension produced by the muscle exceeds the external load or resistance acting against it

• Myosin cross-bridges actively pull the actin filaments toward the center of the sarcomere, decreasing the overall length of the muscle fibers

• e.g. doing bicep curls

Isometric contraction

A type of muscle contraction where the muscle generates tension without changing its length or moving the associated joints.

• The internal tension produced by the muscle equals the external resistance of load pushing against it

• The angle of the join and the overall length of the muscle stays constant while the muscle fivers actively fire

• e.g. holding a plank

Eccentric contraction

A type of muscle contraction where the muscle lengthens while under tension because the external load applied to it is greater than the force the muscle generates

• Often called “negative work” because it acts as a brake

• Muscles can generate significantly more force during eccentric contractions (often up to 40% more than the maximal isometric or concentric forces) due to specialized proteins like titin acting as a stiff, uncoiling spring

• e.g. lowering a weight

Muscle tetanus

A sustained, maximal muscle contraction caused by a high frequency of nerve APs, leaving no time for the muscle fiber to relax between stimuli.

• Type of temporal summation!

• Two types:

  • Unfused tetanus

    • As stimulation frequency increases, the muscle goes through quick cycles of contraction with very short, partial relaxation phases

    • Generates tension about 3-4x greater than single twitch

  • Fused tetanus

    • When stimulus frequency is high enough (typically 50-100 Hz), the relaxation phase completely disappears

    • The contraction becomes a smooth, continuous plateau

• Force increases with stimulation frequency

Physiological Cross-Sectional Area (PCSA)

The total cross-sectional area of all muscle fibers perpendicular to their orientation within a muscle.

• Number of sarcomeres in parallel

• The primary architectural factor that determines a muscle’s maximum force-generating capacity (maximum tension)

Normalized fiber length

The absolute length of a muscle fiber (L_M) divided by its optimal fiber length (L_M0)— the length at which the muscle fiber can generate the maximum isometric (active) force.

• The number of sarcomeres in series

• Relates to:

  • Maximum velocity

  • Muscle excursion