July 14, 2008 at 10:15 pm #9847BakeParticipant
I would like to know how the strength of a contraction of a muscle is controlled in detail.
Does a muscle contract stronger when the frequency of the nerve impulses is higher?
(Just a guess; Please enlighten me if I am wrong)
Thanks in advance!
July 15, 2008 at 10:41 am #85073NadParticipant
There are a number of factors which influence expression of *max-strength* in humans. Following enumeration is not exhaustive:
-cross sectional area of the muscle (higher CSA , higher strength)
-type I and II fiber area
-myosin heavy chain isoforms present in type II fibers
-angle of pinnation
– various metabolic factors , but is very important to understand that those factors
affect more endurance than max-strength.
-motor unit rate coding (frequency)
-motor unit synchronization
-inhibition of Golgi Tendon Organs
-inhibition of antagonist muscles ( the torque about a joint is the difference of torque
exerted by opposing muscles. Thus , lower antagonist activation will result in greater torque about the joint )
-intramuscular coordination (coordination of various synergistic muscles and primary effectors. Probably the factor which gives the biggest factor in neural components of expression of strength. Highly dependent of *movement*. Coordination is skill specific)
-facilitation of contraction through various mechanisms , which I wont break down here but you should think at stretch shortening cycle
-drive to motoneurons from the motor cortex
-various spinal cord connection plasticity, though to influence bilateral facilitation / bilateral deficit. those thing affect expression of strength in unilateral vs bilateral movements executed with homologous limbs
As for an explanation in detail of hos muscles work, is very hard to do this on a internet board, you can easily fill tenth of pages. you can find a primer in any decent biology book, but for details you will need a good dedicated book.
If you want to know ‘everything’ there is to know about single and multiple joint systems, I would recommend R. Enoka’s "Neuromechanics of human movement" 3rd edition.
July 15, 2008 at 11:06 am #85074NadParticipantquote Bake:
there are 3 primary mechanisms used by nervous system for controlling the strength of a contraction in a single muscle
– recruitment (varies contraction strength by activating and deactivating motor units )
– rate coding (frequency, changes in firing rate of motor units)
– a motor unit is either ON or OFF. There is no gradation of force produced by a motor unit by varying the level of motoneuron excitation
-gradation of force output of a single motor unit is realized through rate coding
-recruitment is the primary mechanism which determine the strength of a voluntary contraction. Recruitment of motor units follow the so called "Size principle". It means that small motor units are recruited first. Large motor units are recruited last. Large motor units are the ones having fastest and largest twitch contraction, thus producing greatest force.
July 15, 2008 at 4:17 pm #85075BakeParticipant
Thank you very much!
July 20, 2008 at 11:16 am #85144loyolite11Participant
A muscle contraction (also known as a muscle twitch or simply twitch) occurs when a muscle fiber generates tension through the action of actin and myosin cross-bridge cycling. While under tension, the muscle may lengthen, shorten or remain the same. Though the term ‘contraction’ implies a shortening or reduction, when used as a scientific term referring to the muscular system contraction refers to the generation of tension by muscle fibers with the help of motor neurons. Locomotion in most higher animals is possible only through the repeated contraction of many muscles at the correct times. Contraction is controlled by the central nervous system (CNS), which comprises the brain and spinal cord. Voluntary muscle contractions are initiated in the brain, while the spinal cord initiates involuntary reflexes.
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