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- PUZZLE & PARADOX 122
INTRODUCTORY NOTE
For newcomers to this forum, these P&Ps are Propositions, not facts or
dogmatic proclamations. They are intended to stimulate interaction among users
working in different fields, to re-examine traditional concepts, foster
distance education, question our beliefs and suggest new lines of research or
approaches to training. We look forward to responses from anyone who has
views or relevant information on the topics.
PUZZLE & PARADOX 122
The relevance of muscle fibre types to the production of speed or power may be
overstated and misdirected.
BACKGROUND
It is commonly believed that a predominance of a certain type of muscle fibre
in any given muscle group determines whether an individual will be genetically
endowed for either endurance or power/speed activities. While it is also
suggested that training may cause some change in the way in which any given
type of fibre may behave and that there may be a dynamic spectrum of fibre
types along a continuum between two extremes (ST and FT), it is fairly well
accepted that one must have a large percentage of fast twitch FTb (Type IIb)
fibres in order to produce rapid or powerful movements.
In other words, the ability to generate maximal speed or power is assumed to
depend primarily on the proportion of FTb fibres, which implies conversely
that someone who is slow twitch (ST) dominant will not be able to produce
great speed or power.
GENERATION OF SPEED
Before these assumptions can be analysed it is necessary to examine the
hypothesis that speed of limb movement is directly related to the intrinsic
speed of contractile ability of a group of certain muscle fibres. Are we
justified in making this assumption? If a given muscle group, say, the
quadriceps contain a predominance of FTb fibres, does it mean that leg
extension necessarily will be fast and powerful?
Similarly, since gastrocnemius tends to be largely FT in nature, does it mean
that this makes for faster, more powerful plantarflexion and that the
dominantly ST soleus muscle does not produce rapid or powerful plantarflexion?
To answer these questions, we have to analyse the factors which determine
speed of joint movement:
• the type of mechanical lever involved
• the lengths of the levers involved
• the neural processes involved (activating, inhibiting and disinhibiting)
• the types of muscle fibre involved
• the cross-sectional area and strength of the muscles involved
• the contribution of ballistic or elastic processes
• the effect of mass, structure and distribution of connective tissues
involved
• the position of the joint
• the range of joint action involved
• the pattern of movement
• the weight of the limb or implement involved
We will note that it is not simply a matter of muscle fibre speed of
contraction which determines speed of joint rotation. There are several other
important factors involved, some of which may even be more important than
muscle fibre type.
In the interests of length of commentary, this P&P focuses primarily on the
relevance of biomechanical, rather than neural factors involved. Anyone
responding to this P&P is also welcome to address the effect of any of the
other factors summarized above.
THE INFLUENCE OF LEVER TYPE
There is a very important biomechanical reason why muscle fibre type may be
misapplied with respect to the production of fast limb movement. In order to
move, the body consists of a system of different types of lever, some of which
are intended to act as speed levers and others which are intended to be force
levers.
To recap, a lever system with a fulcrum in between the load and the effort,
like two children on either ends of a seesaw is called a Class 1 lever. If
the load lies between the effort and the fulcrum, as in the case of the
wheelbarrow, this is called a Class 2 lever. The remaining lever, with the
effort exerted between the fulcrum and the load, is called a Class 3 lever.
Virtually all levers in the human body are either Class 1 or Class 3, with
Class 3 being the most prevalent. Now, Class 2 levers are always force levers
and Class 3 levers are always speed levers, which means that most joints in
the body involve speed leverage. Speed levers are characterised by large
efforts acting through short distances on short lever arms to move loads
acting on longer lever arms through large distances. Thus, small movements of
a muscle can move a limb or added load through a large distance at speed,
regardless of whether the fibres involved are fast or slow.
Interestingly, for a joint to act as a speed lever system, the muscle has to
produce a large force, so that we may conclude that the majority of joints
necessitate the production of large muscle force or tension (since most of our
joints are Class 3 or speed levers), but not necessarily the production of
very fast contractions of some special FT fibres.
Since the lever arm lengths and effective angle of muscle pull change with
joint angle, relative positions of all joints involved and range of movement,
overall leverages and involvement of synergistic muscles change. Therefore,
it would appear to be a vast oversimplification to suggest that speed of
movement is determined by only one dominant set of conditions and that these
are provided by a single muscle group having a large enough proportion of FT
fibres.
The nature of the lever systems involved suggest that FT fibres exist to
provide large forces over certain phases of joint angle so that other types of
fibre can operate under their most advantageous conditions, namely endurance
or prolonged posture. So, it might be preferable to think, as is sometimes
the case, of FT fibres being large force, rapid fatiguing muscles rather than
fibres which are intended primarily for speed production.
The existence of what some scientists consider to be a dynamic continuum of
fibre types would be consistent with the idea this would offer a smooth, non-
jerky sequential contribution of each fibre type to movement over the full
range of any joint, particularly in compound movements involving several
joints.
Obviously, a favourable genetic component of location of muscle insertions,
limb lengths and masses, and effective lever lengths may readily overpower any
disadvantage caused by a somewhat disadvantageous proportion of FT fibres.
OTHER FACTORS
All muscle fibre activation is via the nerves, so that the most fundamental
determinant of movement speed is nervous activation, be it patterned
intentional action or reflexive protective reflex. Though there is evidence
that muscle fibre type depends on the speed of nerve fibre feeding a given
nerve, the rapidity with which reflexive joints actions occur indicates that
any group of fibres, irrespective of type, can produce very fast joint action.
This observation might be countered remarks that this explosive type of brief
activation is due solely to activation of FT fibres. However, very rapid
reflexive action also takes place in ST.
Some deficits in proportion of FT fibres may be compensated for by changes in
joint positioning or carryover effects from actions in other joints which
produces momentum (or kinetic energy) which can speed up subsequent actions
involving other joints.
Increasing the relative strength of any given group of muscles should also
enable those muscles to overcome initial inertia and accelerate the limb or
load more rapidly than if the muscle were weaker.
If one does not have the recommended preponderance of FT fibres , then
individual modification of one's technique to ensure more favourable leverages
to move the body or a load more rapidly may be perfectly adequate to
compensate for such inadequacies.
CONCLUSION
Much more evaluation of the theories concerning fibre type and speed
capabilities may be derived from that list of factors itemised above, but the
preceding discussion suggests that this theory certainly warrants some
dissection. On the practical side, it has been noted that some of the world's
top marathon runners run the closing 100m of a marathon in less than 12
seconds and even in normal sprint events can complete 100m in some 11 seconds,
which is by no means slow for someone whose slow-twitching locomotor muscles
are supposed to limit him to fairly geriatric pace over short distances.
Has undue emphasis been placed on relating speed of movement to proportion of
certain fast twitch fibres or is this emphasis thoroughly substantiated by
research and practice? Should the label 'fast twitch' be discarded and
replaced by one which emphasizes the fatigue resistance or force generating
nature of these fibres instead? Do some of the questions posed earlier imply
that muscle biopsy testing is more of academic than practical value? Some
coaches even maintain that fibre typing is generally a waste of time - are
they justified in adopting this rather harsh assessment of the situation?
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