Strength and Fatigue: Experiments and Modeling - Human Motion Simulation: Predictive Dynamics - page 128

FIGURE 6.2
Resolving a torque at a joint to muscle forces.

At the single muscle level, strength is proportional to muscle cross-sectional
area. This is commonly observed as the phenomenon of large, hypertrophied mus-
cles being stronger than small, atrophied muscles. Total muscle length is not a
critical factor in muscle strength per se, but may indirectly influence force-
production through the mechanical advantage of the muscle or the ability of the
muscle to contract rapidly (e.g., contraction velocity), which will be discussed
below. Conversely, the relative length of the muscle influences force-production
through the length-tension relationship, which will be discussed further below.
Muscle hypertrophy occurs as an adaptation to unaccustomed activity or
strength training, and strength training is often performed precisely because of
this relationship, namely to create larger muscles. The specific tension of muscle,
that is the force produced per unit area, is relatively constant across muscles, men
and women, and is often used to estimate peak force for single muscle models.
Specific tensions have been reported to range from approximately 30 N/cm 2
( Chow et al., 1999; Narici et al., 1992; Reeves et al., 2004 )to55 60 N/cm 2

( O'Brien et al., 2010 ). The variations in estimates are likely due to differences in
how physiological cross-sectional area and/or peak force are accounted for (e.g.,
fiber angle at rest or during contraction; antagonist muscle force considered, etc).
However, specific tension does not vary between men and women or children ver-
sus adults ( O'Brien et al., 2010 ). Thus, differences in strength between individuals
are largely due to differences in the muscle cross-sectional area.
Another inherent muscle property influencing force-producing capability is the
torque-velocity relationship ( Figure 6.3 ). Hill (1938) first described this nonlinear
phenomenon over 80 years ago. Essentially, muscles produce less peak force the
faster they shorten ( Hill, 1938 ). Practically, this is apparent when one tries to lift a
light versus a heavy weight in one hand as fast as the muscles can. They are able
to move quite quickly with a light hand weight (e.g., 2 lbs), but move much slower
when trying to move a heavy weight (e.g., 20 lbs). Interestingly this occurs during
shortening, or concentric, contractions, but the opposite occurs during lengthening,
or eccentric contractions (see Figure 6.3 ). Eccentric contractions occur when