Geared up to stretch: pennate muscle behavior during active lengthening

Area of Science:

• Muscle physiology
• Biomechanics
• Skeletal muscle function

Background:

• Eccentric contractions, or muscle lengthening, can cause muscle damage and reduce force output.
• Pennate muscles, with their angled fibers, may possess protective mechanisms against such damage.
• The architectural gear ratio (AGR) describes how muscle-tendon unit (MTU) velocity relates to fascicle velocity, but its role during lengthening is unclear.

Purpose of the Study:

• To investigate how the architectural features of pennate muscles protect against eccentric muscle damage.
• To determine the architectural gear ratio (AGR) of pennate muscles during both shortening and lengthening contractions.
• To understand the relationship between force, muscle deformation, and protection during active muscle lengthening.

Main Methods:

• In vitro experiments were conducted on the bullfrog plantaris muscle.
• Fascicle length and MTU length were independently measured during contractions.
• The muscle's force-velocity relationship and AGR were characterized across a range of forces (10-190% peak isometric force) during isotonic shortening and lengthening.

Main Results:

• The architectural gear ratio (AGR) varied with force during both shortening and lengthening.
• The highest AGR was observed during lengthening contractions.
• A high AGR during lengthening implies that MTU lengthening occurs with minimal fascicle stretch.

Conclusions:

• Pennate muscles exhibit a high architectural gear ratio (AGR) during lengthening, which may protect them from damage.
• Limiting fascicle strain during active lengthening is a key protective mechanism afforded by muscle architecture.
• These findings highlight the importance of muscle architecture in mitigating injury during locomotor activities involving deceleration.