Final answer:
Twitch force properties in motor units include the phases of twitch (latent, contraction, and relaxation), recruitment of different motor units for varying force production, and the effects of summation and tetanus on muscle tension.
Step-by-step explanation:
The properties of twitch force in motor units are essential for understanding how muscles produce varying amounts of force.
Each motor unit consists of a single motor neuron and the skeletal muscle fibers it innervates. When activated by an action potential, the motor unit's muscle fibers produce a contraction known as a twitch, which has three distinct phases: the latent period, the contraction phase, and the relaxation phase.
The tension generated during a twitch is determined by the number of cross-bridges between actin and myosin.
When different motor units are activated, a process known as recruitment takes place, leading to increased muscle tension. Smaller motor units with low-threshold motor neurons are activated first, followed by larger motor units for greater muscle strength.
The frequent stimulation of motor neurons can lead to summation and tetanus; incomplete tetanus involves rapid cycles of contraction with short relaxation phases, while during complete tetanus, the contractions become continuous due to the absence of relaxation phases.