Force can be measured in vivo using force transducers,
free weights, and different commercially available dynamometers.
A clinically useful measurement of strength
(defined as the maximal force generated at a specific
velocity) is the one repetition maximum (1RM), which is
the maximal amount of weight that can be lifted only once
throughout the full range of motion of a joint. Because of
the potential learning effect, this test is more reliable when
administered after significant practice or familiarization.
More than one testing session may be required. The ability
of human single fibers and single myosin molecules to
generate force has also been measured in vitro [37, 38].
These studies show that type I myosin isoform (slow)
produces less force than type II myosin isoform (fast).
However, no differences among sub-types of fast myosin
(IIa, IIx, or hybrid IIax) were reported. The level of force
generated during muscle actions is influenced by the concentration
of calcium in the sarcoplasm and the sensitivity
of the thin filament containing troponin and actin. This
relationship is known as the force–Ca2? curve.