Consequences of pennate muscle architecture Pennate muscle

1 consequences of pennate muscle architecture

1.1 physiological cross sectional area (pcsa)
1.2 relationship between pcsa , muscle force
1.3 lower velocity of shortening
1.4 architectural gear ratio

consequences of pennate muscle architecture
physiological cross sectional area (pcsa)

one advantage of pennate muscles more muscle fibers can packed in parallel, allowing muscle produce more force, although fiber angle direction of action means maximum force in direction less maximum force in fiber direction. muscle cross sectional area (blue line in figure 1, known anatomical cross section area, or acsa) not accurately represent number of muscle fibers in muscle. better estimate provided total area of crossections perpendicular muscle fibers (green lines in figure 1). measure known physiological cross sectional area (pcsa), , commonly calculated , defined following formula (an alternative definition provided in main article):

pcsa

=



muscle volume


fiber length



=



muscle mass


ρ
⋅

fiber length




,


{\displaystyle {\text{pcsa}}={{\text{muscle volume}} \over {\text{fiber length}}}={{\text{muscle mass}} \over {\rho \cdot {\text{fiber length}}}},}

where ρ density of muscle:

ρ
=



muscle mass


muscle volume



.


{\displaystyle \rho ={{\text{muscle mass}} \over {\text{muscle volume}}}.}

pcsa increases pennation angle, , muscle length. in pennate muscle, pcsa larger acsa. in non-pennate muscle, coincides acsa.

relationship between pcsa , muscle force

the total force exerted fibers along oblique direction proportional pcsa. if specific tension of muscle fibers known (force exerted fibers per unit of pcsa), can computed follows:

total force

=

pcsa

⋅

specific tension



{\displaystyle {\text{total force}}={\text{pcsa}}\cdot {\text{specific tension}}}

however, component of force can used pull tendon in desired direction. component, true muscle force (also called tendon force), exerted along direction of action of muscle:

muscle force

=

total force

⋅
cos
⁡
Φ


{\displaystyle {\text{muscle force}}={\text{total force}}\cdot \cos \phi }

the other component, orthogonal direction of action of muscle (orthogonal force = total force × sinΦ) not exerted on tendon, squeezes muscle, pulling aponeuroses toward each other.

notice that, although practically convenient compute pcsa based on volume or mass , fiber length, pcsa (and therefore total fiber force, proportional pcsa) not proportional muscle mass or fiber length alone. namely, maximum (tetanic) force of muscle fiber depends on thickness (cross-section area) , type. no means depends on mass or length alone. instance, when muscle mass increases due physical development during childhood, may due increase in length of muscle fibers, no change in fiber thickness (pcsa) or fiber type. in case, increase in mass not produce increase in force.

lower velocity of shortening

in pennate muscle, consequence of arrangement, fibers shorter if ran 1 end of muscle other. implies each fiber composed of smaller number n of sarcomeres in series. moreover, larger pennation angle, shorter fibers.

the speed @ muscle fiber can shorten partly determined length of muscle fiber (i.e., n). thus, muscle large pennation angle contract more similar muscle smaller pennation angle.

figure 2 architectural gear ratio


architectural gear ratio

architectural gear ratio, called anatomical gear ratio, (agr) feature of pennate muscle defined ratio between longitudinal strain of muscle , muscle fiber strain. defined ratio between muscle-shortening velocity , fiber-shortening velocity:

agr = εx/εf

where εx = longitudinal strain (or muscle-shortening velocity) , εf fiber strain (or fiber-shortening velocity).

it thought distance between aponeuroses did not change during contraction of pennate muscle, requiring fibers rotate shorten. however, recent work has shown false, , degree of fiber angle change varies under different loading conditions. dynamic gearing automatically shifts in order produce either maximal velocity under low loads or maximal force under high loads.