Muscle-Joint Interactions

Maximum Strength

Examination of current physiology texts reveals a good deal of wishy-washiness regarding the definition of the joint angle that corresponds to maximum strength. Typically, it is stated that muscle force is maximum when the joint is in a neutral position, or when the muscle is at resting length. What is the basis for such statements? Unfortunately, there is very little scientific basis for such a statement. Recent studies of torque generation in animals and humans have generally agreed that the joint angle at which the muscle generates maximal force is not the same angle at which moment arm is maximum. Thus, during normal joint rotation, both moment arm and muscle force are constantly changing which results in the "shape" of the strength or torque curve. This concept has recently been addressed in detail by experimental and theoretical modeling (Hoy et al., 1990; Lieber and Boakes, 1988; Lieber and Shoemaker, 1992). What an interesting design!

Range of Motion as a Function of Architecture

We can now "combine" the muscle architectural discussion with the joint moment arm concept. We stated that muscles with longer fibers have a longer functional range than muscles with shorter fibers (Muscle Architecture). Does this imply that muscles with longer fibers are associated with joints that have larger ROMs? The answer is No. It is true that a muscle with longer fibers does have a longer working range. However, the amount of muscle length change that occurs as a joint rotates is very strongly dependent on the muscle's moment arm--the perpendicular distance from the muscle insertion to the axis of joint rotation.

Effect of MA on excursion

This idea is illustrated where we have attached a simulated "muscle" using two different moment arms. In A, the moment arm is much less than in B. This means that in A, the muscle will change length much less for a given change in joint angle compared to the same change in joint angle in B. As a result, the active ROM for the muscle-joint system shown in A will be much greater than that which is shown in B, in spite of the fact that their muscular properties are identical. In fact, in the current example, increasing moment arm decreased range of motion from 40° (A) to only 25° (B)!

This indicates that muscles that appear to be designed for speed because of their very long fibers may not actually produce large velocities if they are placed in position with a very large moment arm. The increased moment arm causes a greater joint moment, and the muscle may actually be better suited for isometric torque production. Similarly, a muscle that appears to be designed for force production due to a large PCSA, if placed in position with a very small moment arm, may actually produce high joint excursions or angular velocities. Thus, muscle design may or may not be a reflection of its actual use in the physiologic torque-generating system. It does seem, in general, that muscle fiber length and muscle moment arm are positively correlated (McClearn, 1985). Thus muscles with long fibers tend to have long moment arms, but this need not necessarily be the case. Muscle architectural features may represent muscle adaptation to kinematic criteria. However, definitive answers to these suggestions await further study.