Architectural gear ratio

Azizi and Brainerd demonstrated that the gear ratio of pennate muscle can vary; dependent on external load.

Preliminary models results show that with muscle bulging, the Architectural gear ratio will increase.

[3] The rotator cuff comprises four pennate muscles, the supraspinatus, infraspinatus, subscapularis and teres minor, and their accompanying tendons.

[2] A 2011 study on human cadaveric shoulders suggests tendon tears may affect the pennation angle of the rotator cuff muscles.

Full-thickness tendon tears did not affect the pennation angle of the subscapularis or teres minor muscles.

However, a correlation between full-thickness rotator cuff tear size and the pennation angle of the supraspnatus and infraspinatus muscles was evident.

The length of the full-thickness tendon tear strongly correlated with an increase in the pennation angle of the supraspinatus muscle.

In addition, a moderately strong association between the area of the full-thickness tear and the resulting increase in pennation angle of the infraspinatus was visible.

[5] The increase in pennation angle following full-thickness tendon tears will result in a change to the PCSA of the supraspinatus and infraspinutus muscles.

[5] Azizi's observations on variable gearing in pennate muscles further suggests tendon tears will affect the AGR of the supraspinatus and infraspinutus.

The abovementioned human rotator cuff study correlates pennation angle with tear length in the supraspinatus muscle.

This suggests fiber length and pennation angle modifications occur via separate mechanical stimuli, i.e. distance of operation and muscle volume respectively.

Parameters of fascicle location and contraction type (eccentric or passive), determined the magnitude of strain experienced by differing regions of the MG.[6] Fascicle ends nearest the deep MG aponeurosis (Achilles tendon) showed an increase in strain from the proximal to distal portions of the MG muscle.

The converse was seen in the fascicle ends adjacent to the superficial aponeurosis, which decreased in fiber strain from proximal to distal portions of the MG muscle.

These trends may have been due to changes in CSA of the muscle at the proximal and distal ends of the MG, resulting in regions of high stress and strain concentration.

[6] This regional variability in strain was accompanied by a statistically significant increase in AGR and resting pennation angle from distal to proximal portions of the muscle.

The experimental AGR values modulated positively with the pennation angle as well as the distance between the deep and superficial apopneuroses and may have been affected by regional patterns in orthogonal bulging.

Figure 1 Anatomical gear ratio. The line aw represents a muscle fiber of length m with its origin at w and insertion into an aponeurosis (TT') at a. The fiber shortens to length m' and moves its insertion the distance d to point b. Note that the shortening muscle fiber does not pull the aponeurosis along the line of action of the fiber but rather rotates around its origin. This is because the 3-dimensional structure of the muscle resists inward movement of the aponeurosis so that the distance between the fiber origin and the aponeurosis remains constant. For a very small shortening of the muscle, the distance ac represents the shortening of the muscle and is equal to ab*cosΦ where Φ is the instantaneous pennation angle. For a pennate muscle, cosΦ is always less than 1, meaning that the distance ac is always shorter than the distance ab, thus the muscle fiber shortening is 'amplified' by a factor of 1/cosΦ.