opinions and comments of readers
Comments on "The Power Struggle" To the Editor: Moffroid and Kusiak have examined what is a basic problem for those wanting to measure muscular performance using a dynamometer (Phys Ther 55:1098-1104, 1975). Power has frequently been used as a measure of per formance, but is generally not understood by the practitioner. An understanding of power can only be arrived at through an understand ing of the theoretical basis of dynamometric measurement. Dynamometers such as the "Cybex II" are designed to allow a part of the body to rotate about its proximal axis and against some re sistance. As has been pointed out by Moffroid and Kusiak, the moving parts are a lever arm and a "skeletal" lever, and the instrument and anatomical axes coincide. Mechanically, this system can be described as a rigid (mechani cal) body which is constrained to rotate about a fixed noncentroidal axis. The equilibrium equation for such a system is M ( . - M r - m s gr s cos0 - l 0 a = 0
(1)
where M ( . is the moment of force due to mus cle contraction, M r is the resistance moment due to the dynamometer, m s is the mass of the system, r s is the radius from the axis of rota tion to the mass centroid of the system, g is acceleration due to gravity, 6 is the angular position of the system, l 0 is the moment of inertia of the system about the axis of rota tion, and a is the angular acceleration. The moment of force due to muscle con traction is M ( . = M r + m s gr s cos0 + l»a
Volume 56 / Number 4, April 1976
(2)
which, in words, is equal to the moments created by the resistance of the dynamome ter, the weight of the system, and the inertia of the system. In the case of the "Cybex II" dynamometer, the angular acceleration is as sumed to be zero and thus the inertial term is zero. The work produced (w) when the contrac tion moment causes the system to move from 0, to 0 2 is given by
w =
I
M^d#.
(3)
J» i Thus if the moment of force due to contrac tion and the angular displacement are meas ured for a dynamometer, the work output can be calculated as the area under the forcedisplacement curve. 1 Power is defined as the time rate of doing work 2 and at any instant is
The power at any instant is given by P = M c
Comments on "The Power Struggle" To the Editor: Moffroid and Kusiak have examined what is a basic problem for those wanting to measure muscular performance using a dynamometer (Phys Ther 55:1098-1104, 1975). Power has frequently been used as a measure of per formance, but is generally not understood by the practitioner. An understanding of power can only be arrived at through an understand ing of the theoretical basis of dynamometric measurement. Dynamometers such as the "Cybex II" are designed to allow a part of the body to rotate about its proximal axis and against some re sistance. As has been pointed out by Moffroid and Kusiak, the moving parts are a lever arm and a "skeletal" lever, and the instrument and anatomical axes coincide. Mechanically, this system can be described as a rigid (mechani cal) body which is constrained to rotate about a fixed noncentroidal axis. The equilibrium equation for such a system is M ( . - M r - m s gr s cos0 - l 0 a = 0
(1)
where M ( . is the moment of force due to mus cle contraction, M r is the resistance moment due to the dynamometer, m s is the mass of the system, r s is the radius from the axis of rota tion to the mass centroid of the system, g is acceleration due to gravity, 6 is the angular position of the system, l 0 is the moment of inertia of the system about the axis of rota tion, and a is the angular acceleration. The moment of force due to muscle con traction is M ( . = M r + m s gr s cos0 + l»a
Volume 56 / Number 4, April 1976
(2)
which, in words, is equal to the moments created by the resistance of the dynamome ter, the weight of the system, and the inertia of the system. In the case of the "Cybex II" dynamometer, the angular acceleration is as sumed to be zero and thus the inertial term is zero. The work produced (w) when the contrac tion moment causes the system to move from 0, to 0 2 is given by
w =
I
M^d#.
(3)
J» i Thus if the moment of force due to contrac tion and the angular displacement are meas ured for a dynamometer, the work output can be calculated as the area under the forcedisplacement curve. 1 Power is defined as the time rate of doing work 2 and at any instant is
The power at any instant is given by P = M c
