veterinary Muscle & Body Biomechanics (Animal)

Muscle & Body Biomechanics

MUSCLE FIBER ARRANGEMENT

EFFECT ON STRENGTH

The amount of force that a muscle can generate is proportional to the cross-sectional area of muscle fibers (a.k.a. muscle cells) attaching to its tendon, i.e., the number of contractile proteins (actin and myosin) pulling on the tendon and contributing to

muscle force.

•  pennation design increases the number of muscle fibers (cross sectional area) attached to the tendon 

•  since force is a function of cross sectional area - a pennated muscle can generate more force than a comparable muscle with parallel fibers.

EFFECT ON SHORTENING

In this example, again consider two muscles - one with parallel fibers the other pennate

Assume each muscle fiber will contract to 50% of its resting length

Therefore:

•  with parallel-arranged muscle fibers the entire muscle can contract by 50% 

•  with the pennate arrangement each individual muscle fiber is pulling at an angle, resulting in reduced overall shortening of the entire muscle belly.

DEFINITIONS:

LINEAR FORCE:

Force can be broken down into various vectors. 

•  Vertical vectors (e.g. the downward forces due to body weight and the upward forces of the supporting surface)

•  Horizontal vectors (e.g. forces exerted to propel forward and

backward forces to brake forward motion)

With adequate force and friction (traction) the body can propel itself forward.  (practical application dictates a need for good traction to allow this forward motion – whoa to those leading a horse on ice)

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ROTATIONAL FORCE (TORQUE)  

 

Rotational force = force (F) x distance from fulcrum (d)

Limb rotation = muscle force (F) x distance from joint (d)

Torque input (muscle generated) = torque output (limb movement)

Muscles generate forces which when applied to the skeleton will generate rotation about a joint.   

MUSCLE ATTACHMENT EFFECTS 

The location of the muscle attachment (e.g. distance from joint) influences the resultant movement of that joint 

MECHANICAL ADVANTAGE VERSUS VELOCITY ADVANTAGE

Muscles that attach further from the joint have a mechanical advantage over muscles attached closer to the joint In the diagram below if muscles #1 and #2 were of equal strength (i.e., can generate the same force) then muscle #2 could produce a greater rotational force because its attachment is at a greater distance from the joint (rotational force = muscle force X distance from joint). 

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Conversely  muscles  that  attach  close  to  the point  of  rotation  are  able  to  produce  faster movement  of  the  lever  arm  than  muscle  that attach farther from the fulcrum.  In  the  diagram  to  the  right  if muscle  #1  and muscle  #2  both  contract  10% during  an identical time period  - muscle #1’s contraction would result  in a  larger movement of  the  lever arm  during  that  same  frame  of  time  than muscle  #2.    In  other  words,  muscle  #1  will result in a more rapid rotation - it has a velocity

advantage.

  

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Muscles attaching close to the joint with their velocity advantage are termed “high gear” muscles and those with a more distal attachment resulting in a mechanical advantage are termed “low gear” muscles.

It may be helpful to consider a similar gear analogy as in a car or bike.  At low gears the output force is relatively large  – allowing the vehicle to climb up a steep hill.  High gears on the other hand generates a lot of speed  – as would be advantageous in passing a vehicle. 

 

JOINT POSITIONING EFFECTS

THE BODY’S LEVER SYSTEM

Unique skeletal features result from functional adaptations over time.

In the figure below - the upper diagram is an example of an animal that uses it’s front limbs for digging; the muscles attached to the point of the elbow (olecranon) are positioned further from the elbow joint (fulcrum of movement) thereby generating large

forces for digging.  

The lower diagram with muscles attaching closer to the elbow joint is an runner adaptation that can result in a rapid rotation with muscle contraction

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For the same force and velocity input (left arrows), note the

relative magnitude of bolded arrows to the right of the diagrams –       a large downward force (F) is generated in upper diagram and

rapid rotation (V; velocity) of movement is produced in the

lower diagram.