Biomedical Engineering Reference
In-Depth Information
Torque-Velocity Relationship
140
Isometric
120
100
80
60
40
Eccentric
Concentric
20
0
-200
-100
0
100
200
300
Velocity (deg/s)
FIGURE 6.3
Schematic representation of the torque-velocity relationship for human muscle.
muscles are lengthened while resisting external loads. For example lowering an
object, walking down stairs, or performing “negative” push-ups (i.e., the lowering
phase of a push-up) all involve eccentric contractions. In animal studies eccentric
contractions produced through electrical stimulation can produce up to 150% of
peak isometric (static) contraction forces (
Lieber, 2002
); however, in humans vol-
untary eccentric contractions typically only reach levels between 100
120% of
isometric peak torque (
Chapman et al., 2005; 2008; Griffin, 1987; Horstmann
et al., 1999; Klass et al., 2005; Kramer and Balsor, 1990
).
A side note regarding eccentric contractions is that the common experience of
developing muscle soreness 1
2 days after exercise (i.e., delayed onset muscle
soreness, DOMS) has been shown to be a result of only the eccentric and not the
isometric or concentric component of unaccustomed tasks (
Cleak and Eston,
1992; Yu et al., 2002
).
Two factors contribute to the influence of joint angle on muscle strength.
First, the length of the contracting muscle fibers has a direct influence on muscle
force produced (i.e., the length-tension relationship) and second, the varying
moment arm through a joint range of motion has an indirect influence on muscle
strength via mechanical advantage (i.e., torque
moment arm). Muscle
contraction occurs as two myofilament proteins bind and slide past one another:
the actin and myosin filaments. There is an optimal level of overlap between
these filaments which results in peak force-production from a muscle fiber.
However, if this overlap increases or decreases from the optimal length, the abil-
ity of the fiber to produce active muscle force decays, creating a curvilinear
5
force
3
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