Fundamentals of Biomechanics

78 FUNDAMENTALS OF BIOMECHANICS
Application: Flexibility and Stretching
A common health-related fitness component is flexibility. Flexibility is defined as “the intrinsic property of
body tissues, which determines the range of motion achievable without injury at a joint or group of joints”
(Holt et al., 1996:172). Flexibility can be mechanically measured as static and dynamic flexibility. Static flexibility
refers to the usual linear or angular measurements of the actual limits of motion in a joint or joint
complex. Static flexibility measurements have elements of subjectivity because of variations in testers and
patient tolerance of stretch. Dynamic flexibility is the increase in the muscle group resistance to stretch
(stiffness) and is a less subjective measure of flexibility (Knudson et al., 2000). Inactivity and immobilization
have been shown to decrease static range of motion (SROM) and increase muscle group stiffness (Akeson
et al., 1987; Heerkens et al., 1986).
Stretching is a common practice in physical conditioning and sports. Stretching exercises must be carefully
prescribed to focus tension on the MTUs and not the ligaments that maintain joint integrity (Knudson,
1998). Long-term stretching programs likely increase static range of motion by stimulating the production
of new sarcomeres in muscle fibers (De Deyne, 2001) and neuromuscular factors (Guissard & Duchateau,
2006). While much is known about the acute and chronic effects of stretching on static flexibility, less is
know about its effect on dynamic flexibility or muscle-tendon stiffness (see Knudson et al., 2000). One example
of the complications in examining the effects of stretching by measuring muscle stiffness is the
thixotropic property of muscle. Thixotropy is the variation in muscle stiffness because of previous muscle
actions. If an active muscle becomes inactive for a long period of time, like sitting in a car or a long lecture,
its stiffness will increase. Do your muscles feel tight after a long ride in the car? Enoka (2002) provides
a nice review of this phenomenon and uses ketchup to illustrate its cause. A gel like ketchup if allowed to
stand tends to “set” (like actin and myosin bound in a motionless muscle), but when shaken tends to change
state and flow more easily. Most all of the increased stiffness in inactive muscles can be eliminated with a little
physical activity or stretching.This does not, however, represent a long-term change in the stiffness of the
muscles. Some of the most recent studies suggest that long-term effects of vigorous stretching are decreases
in muscle viscosity and hysteresis, with no changes in tendon stiffness (Kubo et al., 2001a, 2002). Recent
studies of the in vivo change in length of muscle fibers and tendons using ultrasound and MRI show that that
the limits of SROM are within the toe region of the muscle and tendon load-deformation curve for the hamstrings
and gastrocnemius (Magnusson et al., 2000; Muraoka et al., 2002). Even if consistent stretching did create
long-term decreases in muscle stiffness, it is unclear if this would translate to improved performance or
lower risk of injury. More research is needed on the effects of stretching on muscle stiffness.
Interestingly, many studies have shown that the hypothesized performance-enhancing benefits of stretching
prior to activity are incorrect. Stretching in the warm-up prior to activity has been shown to decrease muscular
performance in a wide variety of tests (Knudson, 1999b; Magnusson & Renstrom, 2006; Shrier, 2004).
Muscle activation and muscular strength are significantly decreased for 15 and 60 minutes, respectively, following
stretching (Fowles et al., 2000a).The dose of stretching that significantly decreases strength may be
between 20 and 40 seconds (Knudson & Noffal, 2005).The large stresses placed on MTUs in passive stretching
create short-term weakening, but these loads have not been shown to increase protein synthesis (Fowles
et al, 2000b).
Recent biomechanical and epidemiological research has also indicated that stretching during warm-up does
not decrease the risk of injury (Knudson et al., 2000; Shirier, 1999; Magnusson & Renstrom, 2006). Several
lines of evidence now suggest that the best time to program stretching in conditioning programs is during
the cool-down phase. Recommendations on stretching and flexibility testing have been published
(Knudson,1999b) and Knudson et al. (2000). Injury risk may be reduced, however, by generalized warm-up
(Fradkin, Gabbe, & Cameron, 2006; Knudson, 2007a).
Flexibility is also strongly related to variations in body position because the passive tension increases in each
MTU, especially in multiarticular MTUs (passive insufficiency).This is why there are strict rules for body positioning
in flexibility testing.What muscles of the leg and thigh are unloaded when students try to cheat by
bending their knees in a sit-and-reach test? What calf muscle is unloaded in a seated toe-touch stretch when
the ankle is plantar flexed?