Which among the following do fast-twitch fibres contain?

Correct option is C

The correct answer is (c)Fast-twitch muscle fibers, also known as Type II fibers, are one of the two main types of skeletal muscle fibers found in the human body, the other being slow-twitch fibers (Type I fibers). Fast-twitch fibers are specialized for generating rapid and powerful contractions, making them well-suited for activities requiring explosive strength and speed, such as sprinting and weightlifting.One of the key distinguishing features of fast-twitch fibers is their high myosin ATPase activity. Myosin ATPase is an enzyme that catalyzes the breakdown of adenosine triphosphate (ATP) into adenosine diphosphate (ADP) and inorganic phosphate (Pi), providing the energy necessary for muscle contraction. In fast-twitch fibers, myosin ATPase activity is significantly higher compared to slow-twitch fibers, enabling faster hydrolysis of ATP and more rapid cycling of the cross-bridges between myosin and actin filaments during muscle contraction.The elevated myosin ATPase activity in fast-twitch fibers allows for quicker cross-bridge cycling, which translates into faster muscle contraction and relaxation kinetics. This rapid cycling of cross-bridges allows fast-twitch fibers to generate high levels of force over short periods of time, making them well-suited for activities that demand explosive power and speed.In addition to high myosin ATPase activity, fast-twitch fibers also exhibit other characteristics that contribute to their fast-twitch properties. These include:Larger cross-sectional area: Fast-twitch fibers tend to have a larger diameter and cross-sectional area compared to slow-twitch fibers, allowing for greater force generation.Glycolytic metabolism: Fast-twitch fibers primarily rely on anaerobic glycolysis for energy production, generating ATP quickly but with limited endurance. This metabolic pathway is well-suited for short bursts of intense activity but can lead to rapid fatigue due to the buildup of lactic acid.Fewer mitochondria: Fast-twitch fibers contain a relatively small number of mitochondria compared to slow-twitch fibers. Mitochondria are the cellular organelles responsible for aerobic respiration and ATP production. The lower mitochondrial density in fast-twitch fibers reflects their reliance on glycolytic metabolism and their preference for rapid energy production over sustained endurance.Fast-twitch motor units: Fast-twitch fibers are predominantly innervated by motor neurons known as fast-twitch motor units. These motor units have high firing rates and are capable of rapidly activating large numbers of muscle fibers, contributing to the explosive nature of fast-twitch muscle contractions.