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If that promotes the development of fast-twitch muscle fibres in growing foetuses, that could add to the Jamaican edge. The truth is, we donât have enough information to draw firm conclusions about the Jamaican dominance of sprinting. But there is good reason to dig deeper â and it has nothing to do with athletics. We now have evidence that extreme stress, heat, cold, malnutrition and pollution (including tobacco smoke) trigger changes in the way genes are activated or silenced.
When a muscle is stretched, primary sensory fibers (Group Ia afferent neurons) of the muscle spindle respond to both changes in muscle length and velocity and transmit this activity to the spinal cord in the form of changes in the rate of action potentials.he spindle is a stretch receptor with its own motor supply consisting of several intrafusal muscle fibres. The sensory endings of a primary (group Ia) afferent and a secondary (group II) afferent coil around the non-contractile central portions of the intrafusal fibres.