Although the contraction of each muscle cell is all-or non, it is obvious that body movements are not. Sometimes they are forceful, other times slight. This is easily accounted for by realizing that body movements are brought about by whole muscles (groups of muscle cells), not by single cell acting alone. Increasing the force of movement may simply be a matter of recruiting more and more cells into cooperative action. However, there are also more subtle means for changing the performance of individual cells.
The strength or, more precisely, the force a muscle is capable of exerting depends on its length. For each muscle cell, there is an optimum length or range of lengths where the contractile force is strongest. This is easily explained by the sliding fillament theory. The strength of contraction depends on the number of cross bridges that can make contact with actin fillaments. When the muscle is too long, few cross bridges can make contact, and contraction is weak. When the muscle is too short, cross bridge contact can be made, but the fillaments begin to get in each other’s way and jam up. Again, contraction is weak. Maximal force develpos only at a small range of lengths where recruitment of operable cross bridges is maximal and where filaments do not interfere. For the human bicep muscle, this optimum length is attained when the forearm and upper arm are at right angles. When the arm is extended so that the angle between forearm and upper arm is 180 degrees, the bicep is stretched, and contraction is weaker. This explains a common experience of weight lifters: when performing a curl, it is most difficult to raise the weight from the bottom position with the arm extended. Progress is much easier once the weight has been lifted, and the fore and upper arm are at right angles.
When picking up a light weight, the muscles shorten and move the skeleton. We call this isotonic contraction. What happens if you attempt to pick up a weight that is too heavy? The muscle tenses but does not shorten. This is called isometric contraction, a contraction with no change in length! How is this contradiction in terms resolved? Actually, when a muscle undergoes an isometric contraction, the contractile machinery really does shorten, the actin and myosin filaments slide past each other. But other passive parts of the cell attached to the contractile machinery, the tendon and connective tissue, are stretched, so there is no net movement. Those parts of the muscle stretched by the contractile machine are called the series elasticity. The exact identity of the series elasticity is a bit vague, but it is known to include the tendons, connective tissue, and elasticity of the cross bridge hinge regions.
The series elasticity stretches a little even when muscle undergoes isotonice contractions. This follows because at the beginning of a contraction, the series elasticity is slack, and as the contractile machine shortens, this slack is taken up until the series elasticity can support the load that is to be moved. From this point on, the muscle shortens.
Changing the length of a muscle is not the only way to alter the strength of contraction. If a rapid succession of stimulating impulses is delivered to a muscle, the cumulative effect will show a stronger contraction than the contraction resulting from a single impulse; the contractions summate. The contraction of a whole muscle can also be increased simply by stimulating more and more muscle cells, a process called recruitment.
Daryl Conant, M.Ed.