A biomechanical model of the squat

[Guest guest]

I'm probably treading on dangerous ground here given the apparent

enthusiasm of many members here for the squat exercise, but I would be

grateful for any comments on or criticisms of a basic 2-dimensional

model of the squat that I have developed.

An article describing it is titled, " A biomechanical model for

estimating moments of force at hip and knee joints in the barbell

squat " , and can be accessed at:

http://www.myoquip.com.au/Biomechanical_model_squat_article.htm

<http://www.myoquip.com.au/Biomechanical_model_squat_article.htm> .

Extracts can be seen below.

Regards

Bruce Ross

Sydney, Australia

<<<Introduction

The barbell squat is a complex, mass load bearing multi-articular exercise

movement. It is the basic lower body exercise prescribed in training programs

for many sports even though it is unpopular with most athletes and is often

performed inexpertly. One of the major problems when performing a full squat

with heavy weights is that there appears to be excessive loading in the bottom

part of the movement. At the same time loading through the top range of the

movement seems inadequate.

This study examines the extent to which these effects may be attributable to

changing values of resistive torque in moving from deep flexion to full

extension of the hip and knee joints. A basic biomechanical model of the squat

has been developed to calculate moments of force or torque applied about the

transverse axes of the hip and knee joints at various angles of those joints.

The model has drawn on that used by Abelbeck (2002) to evaluate a linear motion

squat performed on a machine. I am not aware of any similar study of the

free weight squat.

The Model

A mathematically scaled model of a person of 180cm height and 100kg body weight

was created consisting of four linked segments. These were the upper body or HAT

(head, arms and trunk) assumed to be a rigid member; the thighs; the shanks; and

the feet. The lengths of the segments as a percentage of total height were 50,

24, 22, and 4 respectively. Centres of gravity for the upper body, thighs and

shanks were assumed to be at 60%, 43.3% and 43.3% respectively of segment length

measured proximally. The proportion of body weight for these three segments was

estimated as 68.6%, 20.0% and 8.6% respectively.

In order for stability to be maintained in squatting, the centre of gravity of

the system (exerciser's body plus weight bar) must remain directly over the

feet. Unless the centre of mass is constantly positioned directly above the

ground reaction force vector, a moment would exist and the system would rotate,

i.e., tip forward or backward.

To provide a determinate model and to facilitate calculation, a number of

simplifying assumptions were used, Firstly, throughout the exercise movement the

hip and knee joints move synchronously, i.e., at any point their angles are

equal. Secondly, the force vector of the weight bar (FWB) was assumed to be

located directly above that of the upper body (cgUB). Thirdly, it was assumed

that the centre of gravity of the system remains directly above the ankle joint

rather than at the midpoint of the foot as is usually assumed.

At each observation point throughout the exercise the body is evaluated in a

static or constant velocity state and therefore can be treated as rigid....

Conclusion

This study has demonstrated that throughout a squat movement with normal loading

the moments of force experienced at the hip and knee joints vary from excessive

to inconsequential. Because of this the leg extensor muscles are likely to be

effectively activated for only a minor part of the exercise movement.

It therefore seems appropriate to question the efficacy of the squat as a

general exercise for developing leg strength. In particular the wisdom of its

use in preparing athletes for participation in sports that themselves have high

incidence of back and knee injury must be doubted.>>>