The POWER of Muscle Contractions
Watt is POWER?
From a physicist’s viewpoint, power is the rate of energy transfer from one system to another. An example of energy transfer is the work done on or by an object of a system, in which an external force, f, causes the object to move some distance, s. The average work done during a certain time period, t, equals the average POWER, P, of the system. Also, because velocity is equal to distance divided by time, average POWER can be expressed as the product of an external force acting on an object and the velocity of the object.
Power = Work / Time = (Force x distance) / Time = Force x Velocity = Watts
Muscle contractions occur as the sarcomeres present in muscle fibers shorten. When muscle fibers are triggered by neuromuscular junctions, a process begins in which thick and thin filaments overlap causing the sarcomeres to shorten over a brief period of time. The thick filaments are composed of about 200 mysosin fibers that look similar to golf clubs. Along the long filament, the myosin proteins line up in opposite directions with their “heads” protruding out from the filament. The thin filaments, composed of actin, tropomyosin, and troponin, form a spiral helix with myosin binding sites on the actin. When a muscle contraction is triggered, 1) calcium ions and energy-supplying ADP cause rearrangement of the thin filaments, 2) the myosin heads bind to the actin, and 3) the myosin heads pull the thin filaments towards the center of the sarcomere. Once contraction is complete, ATP binds to myosin, the myosin-actin bond is broken, and the myosin head “re-cocks” to the starting position until the cycle repeats. This cycle occurs simultaneously in many muscle fibers present in the entire muscle tissue causing contractions in the muscles of your eye, to the movement of your arms and legs.
Skeletal muscles undergo two types of contractions: isometric and isotonic. Because isometric contractions occur when the muscle does not shorten, as when pushing against a wall, no work is done and no power is produced. In isometric contractions, myofibrils slide over each other causing the sarcomeres to shorten and external work is performed. Since POWER equals the rate at which work is done, only isotonic contractions produce POWER as applied force from the myosin heads causes the thin filaments to move some distance towards the center of the sarcomere. POWER of muscle contractions, however, are most commonly represented by the product of the strength and velocity of a muscle contraction, P = Force x Velocity.
In this relationship, POWER is proportional to the velocity of a contraction. From experimentation, the peak POWER of muscle contractions is found to occur at approximately 1/3 the maximum shortening velocity. If the speed of the contraction is increased while the force remains constant, the POWER will increase but, increased velocity and high forces cannot be developed by muscle fibers at the same time. This says that the greatest force developed in a muscle is when there is no velocity and at maximum velocity, the muscle develops little force.
Consider a muscle with a length of 0.4 meters and a cross section of 10 cm2. Assume what speed would produce the highest power output. What is that power output and how would it change if the muscle contraction speed was reduced to ˝ of the velocity which produces the maximum power?
Assume that muscles produce about 30-100 N/cm2 and the speed of a contraction is about 0.5 – 1.6 m/sec.
Using max values, F = 1000 N (= 10 cm2 x 100 N/cm2) and Vmax = 1.6 m/s find the maximum power output.
P = F x V = (0.1)(1000 N)(1.6 m/s) = 160 Watts
At ˝ the velocity (V = 0.8 m/s)
P = V x max. power = (0.8 m/s)(160 Watts) = 128 Watts
References and Additional Information
Muscle Contraction: Clinical Introduction
Muscles, Tendons, and Bones
Q & A: Muscle Mass
Muscle Pictures and Links
More on Muscles
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