Is it possible to load different areas of the muscle lengthwise?

The answer depends upon our definitions of the terms ‘muscle’ and ‘lengthwise’. If ‘muscle’ refers to a single muscle head, and if ‘lengthwise’ refers to distinct longitudinal points on that head from origin to insertion, then no, we cannot load (differently) different areas of the muscle lengthwise.

Our muscle cells are composed of bunches of myofibrils, and those myofibrils are further composed of functional contractile units called sarcomeres. Myofibrils and sarcomeres run from the origin to the insertion of the muscle head. When an action potential reaches a muscle cell membrane, it triggers the simultaneous contraction of all of the sarcomeres within that muscle cell, developing tension along its length. By definition, that tension, and consequently the ‘load’ that it resists, must be equal across the entire length of the cell.

To understand why this is so, it helps to imagine the muscle as two ropes of different thickness tied tightly between two trees, and knotted to each other at the middle. If the thicker section of rope, for example, were to develop greater tension than the thinner, it would immediately cause the thinner to yield, then begin and continue to accelerate towards it. Force equals mass times acceleration. (In this case, force would be the difference between the greater and lesser tension.) If that does not happen—that is, if the rope does not yield at the thinner end—then we know that the thinner section must be holding the same amount of tension as the thicker. The same thing applies to muscle fibres.

In order to load different parts of our muscles differently lengthwise, we would have to break the laws of physics.

As a caveat, however, we often think of a ‘muscle’ as being the combination of a number of muscle heads, or even as the combined structure of a group of associated muscles. The biceps brachii, for example, as the name suggests, consist of two distinct heads. And the origins of these heads afford them different mechanical properties, allowing them to develop tension more effectively through distinct angles of joint motion. Furthermore, the biceps brachii are assisted by the deep brachialis, whose bulk contributes to the apparent bulk of the biceps, but which again is functionally distinct, albeit overlapping.

Hence, the type of loading that we apply does change the contribution and activation of distinct muscles and muscle heads, based on the efficiency by which their fibres can develop tension in the postures and through the ranges of motion we are employing.

What you are most likely experiencing is changes in degrees of soreness of the brachialis and heads of the biceps brachii, as a consequence of the distinct loading characteristics of the different exercises your are performing.

I hope that explanation is clear.