Chapter 20:
Physiology of weight training
Weight training refers generally to any activity where muscles must produce high forces
against an external resistance (such as a dumbbell, weight machine, or rubber tubing). Due to
the high forces involved, weight training can recruit all muscle fiber types similar to interval
training. However the adaptations seen with weight training are significantly different than with
interval training and are discussed separately.
Section 1: Adaptations to weight training
In general, the adaptations to resistance training improve the body’s ability to generate
force. In the laboratory, strength is defined as the amount of force an individual can produce
during an isometric contraction (where the muscle contracts but the limbs do not move). This
measurement of strength is referred to as maximal voluntary isometric contraction (MVIC).
Many lifters are familiar with the term 1 Repetition Maximum (1RM) which is the weight which
can be lifted only once in perfect form. For all practical purposes MVIC is equal to 1RM.
As a general rule, MVIC is proportional to a muscle’s cross sectional area (CSA,
essentially its size). However, there is also a neural component of strength and some have
suggested that size and strength can be developed preferentially. Schematically, maximal
strength can be represented as (1)
MVIC/1RM = muscle CSA * neural factors
An individual with well-developed neural factors but small muscle CSA would have overall
lower maximal force capacity than an individual with the same neural factors but larger muscles.
By the same token, an individual with a large muscle CSA but poorly developed neural factors
would not achieve his or her strength potential.
Adaptations from strength training occur both centrally, in the nervous system, and
peripherally, in the muscle itself (2,3). The major nervous system adaptations to strength
training include increased Type IIb fiber recruitment, increased rate coding (the number of
signals sent to the muscle), a decrease in activity of non-involved or opposing muscles during
activity (called disinhibition), better motor unit synchronization within a single muscle, better
synchronization between several muscles involved in the same movement (i.e. pectorals, deltoids
and triceps in the bench press), and changes in muscle fiber recruitment as exercises are learned
(1, 3-10)
Peripherally, the primary adaptation is an increase in muscle CSA with preferential
growth occurring in the Type II fibers (11,12). Growth also occurs in the Type I fibers, simply to
a smaller degree. Muscle growth can potentially occur in one of two ways: hypertrophy or