Leg Press Biomechanics?

[Guest guest]

Here are a few extracts from an article in the Aug 2002 " Personal Fitness

Professional " . I will offer only a few comments and leave the rest to list

members as a tutorial on applied biomechanics. The full article also appears

on the following website, but does not include Table 1 (which I have retyped

below to assist readers).

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<http://www.fit-pro.com/editorial2.asp?ID=147>

<Learning about the Leg Press

Rodney Corn MA PES CSCS

<Pronation is the eccentric deceleration of forces that allows the body to

store potential energy to ultimately produce force in the opposite direction.

This opposite multiplanar motion that produces force is known as supination.

Pronation can be thought of as eccentric muscle action, and supination

involves concentric muscle action.

[*** What this means is anyone's guess - it certainly isn't physics or

biomechanics. Pronation is not a special form of deceleration, eccentric or

otherwise - if we are to eliminate any political correctness for a moment,

what is written here is total nonsense. ]

Understanding the complexity of pronation and supination allows for the

simple application of these terms to describe multijoint exercises such as

the leg press. The kinetic chain must be aligned and positioned

appropriately to efficiently pronate and supinate.... >

*** He then continues to define PRONATION thus:

Table 1:

Foot/ankle

Dorsiflexion (sagittal)

Eversion (frontal)

Abduction (transverse)

Knee

Flexion (sagittal)

Adduction (frontal)

Internal Rotation (transverse)

Hip

Flexion (sagittal)

Adduction (frontal)

Internal rotation (transverse)

[***Note well that rotation occurs about an axis and does not take place in

any plane like the transverse plane.]

This is how he defines SUPINATION:

Table 1 continued:

Foot/ankle

Plantarflexion (sagittal)

Inversion (frontal)

Adduction (transverse)

Knee

Extension (sagittal)

Abduction (frontal)

External Rotation (transverse)

Hip

Extension (sagittal)

Abduction (frontal)

External rotation (transverse)

[*** Again, note that rotation occurs about an axis and does not take place

in any plane like the transverse plane. Moreover, the knee joint cannot

adduct or abduct, and its degree of medial or lateral rotation diminishes

markedly as full extension is approached. Maybe the author of this article

in good faith took this information about joint action from some textbook,

but, since he did not cite any references, we have to assume that he was

solely responsible for all that was written.]

*** Note that the photo and the description show that:

1. the leg press is performed with feet flat on the leg press platform

throughout the movement

2. the foot plate is rigid and does not tilt in any plane whatsoever

....so, the primary question arises -- how is it possible to pronate OR

supinate the foot without any lateral movement or rotation of the foot in any

direction? The knees may undergo valgus or varus tilting and hip motion may

occur, but if the soles of the feet remain fully flat on any surface, they

cannot undergo any marked degree of pronation or supination (other than a

mild tendency to attempt to do so in some individuals). There are far more

significant joint and muscle actions which occur during the different ways of

executing the leg press (narrow vs wide stance, high or low foot placement on

the plate, flatfooted vs ball of foot action.

It is important to note, too, that the definitions of eversion and inversion

are often confused with those for pronation and supination, even among many

academics and therapists, so it is vital to explain such problems when

describing and analysing human movement. For example, inversion is often

defined to be the complex combination of ankle plantarflexion, external

rotation of the knee, abduction of the hip and foot supination. The author

concerned uses another convention which classifies eversion and eversion as

subsets of pronation and supination. The biomechanical conventions regarding

foot action can be very daunting for the newcomer to this field and in any

articles it is vital to address controversial definitions before one goes any

further.

Anyway, for the general fitness professional, the attention paid here to the

largely insignificant actions of foot pronation and supination is

unwarranted. There are far more important issues to be addressed. After

all, this article stated that: " most trainers and fitness enthusiasts

believe certain foot placements have a drastic effect on the musculature

involved; however, no research has validated these claims to date. " Correct

- so why bother to focus so much attention on foot placement or minor foot

motion?

There is a great deal more to evaluate - over to the rest of you! Remember

that this free magazine apparently reaches 80,000 fitness professionals, so

that it really ought to have articles undergo some more effective review

process to ensure that readers are protected from too much misinformation, at

least in their technical articles.

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Dr Mel C Siff

Denver, USA

http://groups.yahoo.com/group/Supertraining/