Question

Jim is lifting weights. Holding a 20lb weight down by his side, he bends his arm at the elbow, contracting his bicep, and holds the position until he reaches muscle fatigue. Briefly describe how the Henneman Size Principle relates to this example. List the steps of muscle fiber recruitment necessary for the initial lift and subsequent fatigue. Include a short summary of the muscle fiber types, and the properties that contribute to the speed of contraction, recruitment pattern and metabolic activity.

Answer 1

Henneman size principle - motor units are recruited from smaller to largest based on the need. Initially Fewer muscle fibers are needed, when larger motor neurons are recruited larger muscle fibers are needed to increase the muscle strength. For this many muscle twitches are needed to increase the strength from lower to higher.

When a person lift a weight he needs sufficient muscle strength to lift it, for that he has to increase smaller motor units to larger motor units. When the muscle begins to contract after a long period it needs 30 - 50 twitches to gain the strength.

when a person holds a weight muscles are contracted and motor units are recruited from smaller to larger then larger muscle fibers are utilized, energy is lost finally it reaches muscle fatigue.

low demand efforts recruit the smaller motor units, lower threshold, slower to fatigue motor units

more effort recruit the larger motor units, higher threshold, faster to fatigue motor units

Types of muscle fiber

1. Slow twitch oxidative (type I) muscle fiber

2. Fast twitch oxidative (type II A) muscle fiber

3. Fast twitch glycolytic (type IIX) muscle fiber

	Slow twitch oxidative	Fast twitch oxidative	Fast twitch glycolytic
Myosin ATPase activity	slow	fast	fast
Size	samll	medium	large
Duration of contraction	long	short	short
Fatigue	resistant	resistant	easily fatigued
Energy utilization	aerobic/oxidative	both	anaerobic/glycolytic
Mitochondria	high numbers	medium numbers	low numbers