Progressive exercise is thought to improve the mechanical and structural
properties of tissues; good physical fitness is also considered crucial to
avoiding sports injury. Preventive training includes training of muscle,
mobility and flexibility, and coordination. Warm-up and cool-down are
also considered to be important features of injury prevention (Kannus,
1993a), although there are few conclusive laboratory and clinical studies
to show that these do prevent injury (Best and Garrett, 1993a). Attention
needs to be paid not only to the intensity and duration of training, but also
to the repetitions within an exercise period and the rest between periods,
because of the reduced ultimate strength of tissues for repeated compared
with single loading (Nigg, 1993). Normal compressive forces, and tensile
forces caused by muscle action, create an electrical potential which induces
bone growth. This may explain why people who are physically active have
significantly greater bone densities than those who are less active (Kannus,
1993b). Long distance runners have been reported as having 20% higher
bone mineral content than controls, and local increases in the bone mineral content have been found for loaded areas of the skeleton, for example in
tennis players (Zetterberg, 1993). The long bones of the extremities, in
particular, are highly responsive to changes in mechanical loading—they
increase in both size and mineralisation and undergo substantial cortical
remodelling. How mechanical change affects remodelling, and the identity
and manner of the response of cells initially receptive to that change, remain
to be fully established. Cyclic bending strain may be a mechanism to account
for selective bone remodelling (Zernicke, 1989). It has been reported that
high intensity training leads to an increase in bone density, but that low to
moderate intensity training has no such effect. Low intensity training
promotes increases in bone length and growth in the growing athlete, but
relatively high intensity training inhibits these (Booth and Gould, 1975).
Zernicke (1989) considered that high intensity training (70–80% of
maximum oxygen uptake) inhibits bone remodelling and leads to a
significant reduction in bending stiffness and energy-to-failure.

It has often been reported (e.g. Booth and Gould, 1975) that exercise leads
to hypertrophy of ligaments and tendons, with increased stiffness, ultimate
strength and energy-to-failure, as well as some increase in mass. Junction
strength changes are related to the type of exercise regimen as well as its
duration; endurance training before trauma may lead to increased junction
strength after repair (Booth and Gould, 1975). Within its elastic limits,
cartilage increases in thickness with short-and long-term exercise, and this is
accompanied by an increased elasticity (Nigg, 1993). Connective tissue can
experience stress relaxation and creep during exercise. Cyclic loading of such
tissues with a fixed displacement, as through activities such as running and
swimming, can lead to stress relaxation and a reduction of tissue load.
Increased ligamentous laxity after exercise is an example of the creep properties
of tissue (Best and Garrett, 1993a).
Training can increase muscle strength though physiological adaptations,
related to an increase in muscle mass, an improved recruitment pattern
and a change in fibre orientation (Nigg, 1993). The physiological
mechanisms stimulated depend on the specific form of training, as this
affects the patterns of motor unit activation (Kraemer et al., 1996).
Kawakami et al. (1993), for example, found that 16 weeks of heavy
resistance training increased the physiological cross-sectional area by 33%
and the pennation angle by 29%, causing a reduction in specific tension.
The muscle force-time curve is sensitive to heavy resistance and explosive
training, which has even more effect on the force-time curve than on muscle
structure (Komi, 1989). The length-feedback component of the muscle
spindle response has been claimed to be trainable, increasing the muscle
spindle discharge for the same stretch. It has also been hypothesised that
training can decrease the force-feedback component of the Golgi tendon

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