Download Biomechanics Length-Tension Relationships Real World Strength

Biomechanics
Length-Tension Relationships
The most basic relationship governing muscle performance is often said to be
the association between the length of the muscle and the corresponding
tension it produces. The easiest way to understand this is to think of an
isometric contraction. If we started with a muscle in a loose position the tension
of that muscle could be recorded using a dynamometer. As the muscle is slowly
lengthened a point would be reached where passive resistance is recorded. The
muscle would continue to be lengthened until no increase in tension was
apparent. Two independent sources of resistance could then be identified.
The active tension produced by the contractile elements.
The passive tension produced by the non-contractile elements.
The muscle tension corresponding to the maximal active tension is known as
the resting length (physiotherapists often confuse this length with the
anatomical position).
Gordon et al. (1996) stated that the length-tension relationship of the whole
muscle reflects the mechanical behavior of the individual fibers which can be
related to the number of cross bridges between the actin and myosin filaments
in the fibers (or the degree of overlap).
Real World Strength
No matter what techniques we use (either isometric, isokinetic or other) to
measure strength (not to be confused with force as this is a linear entity) we
actually look at the rotational effect generated by the force of a muscle or
muscle group. So, in other words, we actually infer the force generated by the
muscles rather than record it.
Gravitational Correction
Since the majority of isokinetic tests consist of angular motions e.g. plantar and
dorsiflexion of the ankle, the effects of gravity must be corrected for. However,
in some movements e.g. inversion and eversion of the ankle, gravity does not
have to be corrected for even though the motion is angular.
This is extremely important when considering the agonist/antagonist strength
ratio. Consider it like this. The weight of the lower leg has to be overcome by
the quadriceps to extend the knee in the sitting position, yet when the knee is
flexed by the hamstrings the weight of the leg actually assists the movement.
So gravity correction becomes important for reliable results.
Force-Velocity Relationship
There is a direct relationship between the amount of force generated and
velocity in that:For concentric contractions there is a parallel decrease in the maximal moment
developed by the muscles as speed increases. This is because of neuromuscular
recruitment patterns i.e. both type I and II fibers are activated together at
lower speeds but as speed increases less type I fibers are recruited and
eventually become inactive. At very high velocities smaller and smaller fiber
populations are recruited (Kannus 1994).
For eccentric contractions the maximal moment may rise initially but at higher
velocities it plateaus due mainly to neuromuscular facilitation (as eccentric
contractions could be theoretically very high at speed Perrin 1993).
Order of Strength
The principles mentioned above are supplemented by:At the same velocity eccentric strength is greater than concentric strength.
Elftman’s (1966) principle states that the order of strength is dependent on
contraction mode i.e. eccentric > isometric > concentric.
These are also dependent on the type of exercise performed i.e. isokinetic >
isometric> isotonic
A further expansion of these principles can be seen here.
Order of Joint Forces
When a muscle is contracted around a joint a certain amount of pressure is
created within that joint. That pressure is dependent on the type of exercise
and type of contraction i.e. isotonic> isokinetic> isometric and eccentric>
concentric> isometric.
A further expansion of these principles can be seen here.
Test Velocities
For concentric contractions most dynamometers have a maximum speed of 500
degrees per second. The use of velocities is dependent on the joint tested and
the ROM, however, higher velocities are usually only of academic interest.
Corresponding eccentric velocities are not usually possible and are generally
one-third less than concentric. Stretching an active muscle at high velocity
poses a serious threat to muscle integrity (so don’t do it). Restraint of
antagonist activation (or in other words when the antagonist muscles activate
to prevent hyper extension) is a neuromuscular phenomenon that is well
developed in athletes (Glossman et al. 1988). This is not so well developed in
untrained athletes who may find high velocities very difficult.
Increased Eccentric Contraction
Theoretically the eccentric strength could exceed the isometric strength (never
mind the concentric strength) by about 100% (Edman et al. 1979). However, in
real life this never happens. The best explanation for this phenomenon
suggests a negative feedback loop which involves peripheral and spinal
regulation in order to avoid excessive stresses on the muscle (Storber, 1989).
The only time this feedback loop can be over-ridden is when the CNS
determines that increased eccentric contraction is needed for defensive
purposes. When this happens there is usually damage to the musculo-tendinous
junction rather than the muscle.
The Eccentric/Concentric (EC) Ratio
This is expressed as the maximal eccentric moment divided by the maximal
concentric moment. Since this is dependent on velocity it should increase
proportionally with this. It has been proposed that with respect to single joint
testing (Dvir, 1994) the EC value derived from low to medium test velocities is
very likely to be within the range of 0.95 : 2.05. Trudelle-Jackson et al. (1989)
proposed that the EC ratio at slower speeds is less than 0.85. This ratio tends
to fall particularly in the presence of pathology (Conway et al. 1992). Bennett
and Storber (1986) have suggested that a particularly low EC ratio (less than
0.85) is a potential source of patella-femoral pain (based on an error in
neuromotor control). If there is a large increase in the EC ratio then excessive
connective tissue components may be the reason. As described by Dvir and
Dagan (1994) this may be due to a rare form of hereditary anemia which
should be eliminated before continuing with isokinetics.
Closed versus Open Chains
Most isokinetic machines are configured for assessing planar, single joint
motion. However, in most instances joint motion is multi planar and often
combined harmoniously with other joints. Although moment output may not be
measurable from the individual contributions of the muscles responsible for
executing multi joint motion, their use is of considerable interest. A lot of
debate rages over the use of closed chain and open chain exercises. It is true
that there are high joint loading forces during most isokinetic movements.
Obviously if a load is to be moved it must be supported by structures around
the joint. As most muscles have very short levers only a small percentage of the
muscle force is used to counteract the external load. The rest is transmitted
through the joint articular surface or taken up by anatomical structures such as
the ligaments and capsule. It should be emphasized that the forces seen during
isokinetics are not the largest the joint will be expected to support.
The following are several examples of forces:Kaufman et al (1991) indicated average tibio-femoral force of 4 times body
weight whilst testing isokinetically. This is equal to that obtained during
walking.
The anterior shear force was on average 0.3 times body weight which is higher
than that expected during walking but lower than that expected during stair
climbing (1.7 x body weight) or running (3 x body weight).
Posterior shear force was on average 1.7 times body weight which is larger
than walking but on a par with stair climbing and lower than running.
Patella-femoral forces reaching 5.1 times body weight at 60 degrees/second
were calculated. This can be compared to 0.5 times body weight during
walking, 3.3 times body weight whilst ascending stairs, 7.6 times body weight
in the squat or 20 times body weight during jumping.
Eccentric-Concentric Coupling
This phenomenon of concentric contraction potentiation following eccentric
contraction is well established (Cavanagh et al 1968, Komi and Bosco,
1978/1979, Bosco et al., 1987). This phenomenon is also termed as prestretching, stretch shortening cycle or plyometric contraction. It is based
primarily on the mechanical behavior of the series elastic element found in
contractile elements and tendons within the muscle (Svantesson et al. 1991).
Basically in eccentric contractions energy is accumulated in both mechanical
and chemical forms and is released at the beginning of the next concentric
contraction. This is relevant to isokinetic testing as the plyometric response can
be trained for. Affecting an increase in plyometric contraction using isokinetics
is more controversial.