How is force typically graded in skeletal muscle, and what does the all-or-none principle imply for an individual muscle fiber when activated?

How is force typically graded in skeletal muscle, and what does the all-or-none principle imply for an individual muscle fiber when activated?

• A. Force is graded by recruitment of more motor units; individual fibers contract fully when activated
• B. Force is graded by increasing firing rate in a single motor unit only; fibers contract partially
• C. Force is graded by alternating contraction of motor units; each fiber contracts partially
• D. Force is graded by recruiting fewer motor units; individual fibers contract partially when activated

Answer: (A)

Force in skeletal muscle is graded by recruiting more motor units. Each motor unit pairs a motor neuron with all the muscle fibers it controls; activating additional units brings more fibers into contraction, increasing overall tension. For an individual muscle fiber, the all-or-none principle means that once a threshold stimulus arrives, the fiber contracts with its full twitch strength. You can vary the total force by recruiting more units and by firing rate to cause summation across active units, but a single activated fiber itself contracts fully rather than partially.

Force in skeletal muscle is graded by recruiting more motor units. Each motor unit pairs a motor neuron with all the muscle fibers it controls; activating additional units brings more fibers into contraction, increasing overall tension. For an individual muscle fiber, the all-or-none principle means that once a threshold stimulus arrives, the fiber contracts with its full twitch strength. You can vary the total force by recruiting more units and by firing rate to cause summation across active units, but a single activated fiber itself contracts fully rather than partially.