Muscle Physiology and Locomotion

Muscle Physiology Quiz Recap

Muscles need a rigid skeleton to transmit force. Muscles also stabilize joints.
Muscles do exert force while shortening (concentric contraction).
Work is measured in joules (J) or millijoules (mJ). Millijoules are used because muscle length changes are small (millimeters).
Muscle cells are very long relative to their diameter, as seen histologically.
Myosin filaments have a heavy chain and two light chains (not actin).
The force that muscles can generate per unit area of activated fibers is quite uniform across different animals.
The force-length relationship suggests muscles operate optimally at intermediate sarcomere lengths to avoid overlap interference.
The amount of power a muscle can produce per unit mass is independent of sarcomere length.
During jump landing, eccentric muscle contraction occurs, with muscles lengthening for braking.
Excessive muscle stretch can contribute to injury as it pushes muscles to their force-generating limits.
Hill's force-velocity relationship (1930s) has limitations:
- Done in vitro, not in vivo.
- Idealized isometric contractions and isotonic states, unlike dynamic animal locomotion.
Counterclockwise work loops indicate net positive mechanical work, while clockwise loops indicate net negative work.
Fast glycolytic fibers generate ATP anaerobically, suitable for burst locomotion but prone to fatigue. Oxidative fibers use aerobic mechanisms, better for endurance.

Muscle Architecture

In addition to the force-length and force-velocity relationships, muscle architecture is crucial for muscle properties.

Parallel Fibered Muscles:
- Fibers run parallel to the axis of force generation (end-to-end).
- Attach directly to the skeleton.
Pinnate Fibered Muscles:
- Fibers run at an angle to the direction of force transmission.
- Attach via external tendon.
  - Unipennate: Attach to the distal tendon at one location.
  - Bipennate: Attach to the distal tendon from two locations.

For a muscle of equal size, pinnate muscles can generate greater force due to a larger fiber cross-sectional area compared to parallel fibered muscles.

Multipennate Muscles: More complex fiber architectures with fibers at a range of different angles.

Parallel fibered muscles offer a greater range of shortening ability due to their length.

Muscle work is calculated as force multiplied by change in length. Despite differences in force generation and fiber length, muscles generally have the same capacity to do work on a per-unit mass basis.

Longer parallel fibered muscles require more energy (ATP) to generate force because they need to form more cross-bridges, as per the sliding filament theory.

Breed-Specific Muscle Differences

In human literature, relationships exist between histological structure (fiber type percentage and diameter) and predisposition to specific sporting abilities (sprinting vs. endurance).

Genetic factors also play a role. Studies in humans correlate the protein alpha-actinin-3 (part of the actin protein family) with type II fast-twitch fibers. The ACTN3 gene underlies this protein and is important in skeletal muscle function and metabolism.

Equine Example (Muzeal, 2019):
- Examined allele and genotype frequencies of the ACTN3 gene in different horse breeds (native, draft, noble/sport).
- Found polymorphisms related to racing ability (native vs. racehorse breeds) and endurance (Arabian horses).
- Expression of this gene can change with training, particularly in Arabian horses. This means it is not finite across different groups.
- Further research is needed to link polymorphisms to specific performance results.
Canine Example (Kim et al, 2018):
- Compared genomes of village dog populations (unselected breeding) to sports dogs and terrier breeds for genetic differences.
- Athletic breeds showed selective pressure on the circulatory system, potentially increased cardiac output (for oxygen delivery to muscles).
- TRPM3 gene, important in the contractile response in vascular smooth muscle, was flagged as significant.
- ROBO1 gene (neuronal) may improve cognitive skills, enhancing agility and trainability.

TRPM3 Gene and Myostatin (MSTN) Mutation

TRPM3 has a strong association with racing grade in whippets and affects contraction in smooth muscle cells of blood vessels, probably related to blood flow to skeletal muscle.
MSTN mutation (myostatin) causes double-muscled animals (bully whippets), but is not beneficial for breed standards and causes health issues. This mutation is also seen in cattle.
The MSTN mutation does not appear to affect greyhounds.

Endurance may be associated with specific genes, but this study did not deeply investigate them. Sprint events may be better with pointer breeds, while endurance events may be better with malamute breeds in sled dogs.

Conclusion

Muscle work is an important consideration in different sporting disciplines. Specific locomotor activities and behaviors require high levels of muscle work, which can be linked to potential injuries.

Previously, studies have reported the prevalence of different injuries in agility dogs and showdown courses, which can further link into the muscular aspect.

Muscle Physiology Quiz Recap

Muscles need a rigid skeleton to transmit force and stabilize joints, ensuring efficient movement and preventing injury.
Concentric contraction occurs when muscles shorten while exerting force.
Work is measured in joules (J) or millijoules (mJ). Millijoules are more practical for muscle physiology due to small muscle length changes (millimeters).
Muscle cells are long relative to their diameter, allowing for efficient force transmission along their length.
Myosin filaments consist of a heavy chain and two light chains; these interact with actin during muscle contraction.
The force that muscles generate per unit area of activated fibers is uniform across different animals, indicating a consistent physiological property.
The force-length relationship shows muscles operate optimally at intermediate sarcomere lengths to maximize cross-bridge formation and force generation while avoiding overlap interference.
The amount of power a muscle can produce per unit mass is independent of sarcomere length, emphasizing efficiency in muscle performance.
During jump landing, eccentric muscle contraction occurs, with muscles lengthening to absorb impact and control movement.
Excessive muscle stretch can cause injury by pushing muscles beyond their force-generating limits, leading to strain.
Hill's force-velocity relationship (1930s) offers insights into muscle dynamics but has limitations:
- It was conducted in vitro, not in vivo, limiting its applicability to real-world conditions.
- It assumed idealized isometric contractions and isotonic states, unlike the dynamic and variable movements in animal locomotion.

Counterclockwise work loops indicate net positive mechanical work, signifying energy generation, while clockwise loops indicate net negative work, representing energy absorption.
Fast glycolytic fibers generate ATP anaerobically, making them suitable for burst locomotion but prone to fatigue due to lactic acid buildup. Oxidative fibers use aerobic mechanisms, allowing for sustained endurance.

Muscle Architecture

In addition to force-length and force-velocity relationships, muscle architecture significantly influences muscle properties.

Parallel Fibered Muscles:
- Fibers run parallel to the axis of force generation (end-to-end), optimizing the range of motion.
- Attach directly to the skeleton, enabling direct force transmission.
Pinnate Fibered Muscles:
- Fibers run at an angle to the direction of force transmission, increasing force generation capability.
- Attach via external tendon, allowing for a greater number of fibers within a given muscle volume.
  - Unipennate: Attach to the distal tendon at one location, providing strong force in one direction.
  - Bipennate: Attach to the distal tendon from two locations, balancing force distribution.

For a muscle of equal size, pinnate muscles can generate greater force due to a larger fiber cross-sectional area compared to parallel fibered muscles, enhancing their strength.

Multipennate Muscles: More complex fiber architectures with fibers at a range of different angles, allowing for versatile force application.

Parallel fibered muscles offer a greater range of shortening ability due to their length, facilitating larger movements.

Longer parallel fibered muscles require more energy (ATP) to generate force because they need to form more cross-bridges, as per the sliding filament theory, increasing metabolic demand.

Breed-Specific Muscle Differences

In human literature, relationships exist between histological structure (fiber type percentage and diameter) and predisposition to specific sporting abilities (sprinting vs. endurance). Fiber type distribution and muscle fiber size significantly impact athletic performance.

Equine Example (Muzeal, 2019):
- Examined allele and genotype frequencies of the ACTN3 gene in different horse breeds (native, draft, noble/sport).
- Found polymorphisms related to racing ability (native vs. racehorse breeds) and endurance (Arabian horses). Certain genetic variants are associated with superior athletic traits.
- Expression of this gene can change with training, particularly in Arabian horses, indicating plasticity in gene expression. This means it is not finite across different groups. Training can modify genetic expression, enhancing performance.
- Further research is needed to link polymorphisms to specific performance results, emphasizing the complexity of genetic influences.
Canine Example (Kim et al, 2018):
- Compared genomes of village dog populations (unselected breeding) to sports dogs and terrier breeds for genetic differences. Selective breeding has shaped genetic traits related to athleticism.
- Athletic breeds showed selective pressure on the circulatory system, potentially increased cardiac output (for oxygen delivery to muscles). Efficient oxygen delivery is crucial for high-performance activities.
- TRPM3 gene, important in the contractile response in vascular smooth muscle, was flagged as significant. This gene may affect blood flow regulation during exercise.
- ROBO1 gene (neuronal) may improve cognitive skills, enhancing agility and trainability, underlining the importance of neurological factors in athletic ability.

TRPM3 Gene and Myostatin (MSTN) Mutation

TRPM3 has a strong association with racing grade in whippets and affects contraction in smooth muscle cells of blood vessels, probably related to blood flow to skeletal muscle. This highlights the interplay between genetics and vascular function in athletic performance.
MSTN mutation (myostatin) causes double-muscled animals (bully whippets), but is not beneficial for breed standards and causes health issues. This mutation is also seen in cattle. Myostatin regulates muscle growth, and its mutation leads to excessive muscle mass.
The MSTN mutation does not appear to affect greyhounds, suggesting breed-specific genetic differences.

Endurance may be associated with specific genes, but this study did not deeply investigate them. Further research is needed to identify genetic factors influencing endurance capacity. Sprint events may be better with pointer breeds, while endurance events may be better with malamute breeds in sled dogs, indicating breed-specific fitness advantages.

Conclusion

Muscle work is an important consideration in different sporting disciplines. Specific locomotor activities and behaviors require high levels of muscle work, which can be linked to potential injuries. Understanding muscle function is crucial for injury prevention.

Previously, studies have reported the prevalence of different injuries in agility