ACE PERSONAL TRAINER MANUAL | RESISTANCE TRAINING: PROGRAMMING AND PROGRESSIONS CHAPTER 10 Muscle Length
Perhaps the most important factor for attaining large muscle size is muscle length relative to bone length. Muscles typically attach to bones by connective tissues called tendons. Some people have relatively short muscles with long tendon attachments, whereas other people have relatively long muscles with short tendon attachments. Those with relatively long muscles possess greater potential for muscle development than those with relatively short muscles. For example, an individual who has relatively long gastrocnemius muscles with short Achilles tendons possesses more potential to develop large calf muscles than an individual who has relatively short gastrocnemius muscles with long Achilles tendons.
Limb Length
Although limb length does not influence muscle hypertrophy, it definitely affects strength performance. Other things being equal, shorter limbs provide leverage advantages over longer limbs. The relationship between muscle force and resistance force is mediated by leverage factors, as expressed in the following formula:
Muscle force x Muscle force arm = Resistance force x Resistance force arm
The muscle force arm is the distance from the joint axis of rotation to the muscle– tendon-insertion point, and the resistance force arm is the distance from the joint axis of rotation to the resistance application point (Figure 10-1). Longer limbs provide longer resistance force arms and require more muscle force to move a given resistance. Conversely, shorter limbs provide shorter resistance force arms and require less muscle force to move a given resistance. Assuming equal biceps muscle strength and tendon insertion points, a person with a shorter forearm can curl a heavier dumbbell than a person with a longer forearm.
Figure 10-1
Example of a lever system in the human body
F X Fa Ra Tendon Insertion Point
Like limb length, the point where the tendon inserts on the bone does not influence muscle hypertrophy, but definitely affects strength performance. Based on the formula presented in the previous section, a longer muscle force arm provides a leverage advantage for moving a heavier resistance. Assuming equal biceps muscle strength and forearm lengths, an individual with a tendon insertion point farther from the elbow joint axis can curl a heavier dumbbell than an individual with a tendon insertion point closer to the elbow joint.
R
Note: X = axis of rotation; F (biceps contraction) = motive force; R (weight in hand) = resistance; Fa (biceps force x distance of biceps attachment from axis) = lever arm of the motive force; Ra (weight x dis- tance from axis) = lever arm of the resistance.
