After reviewing the form of this exercise (Cable Curl), it looks like the person is also doing a deltoid flexion. I would assume that the purpose of the exercise is elbow flexion in isolation without finishing the form with a deltoid flexion. Just my 2 cents.
Thanks for your feedback. The form you are mentioning, where the elbows are kept directly under the shoulders could be considered a somewhat over-simplified cue that most beginners are taught. It keeps things simple without over complicating the biomechanics. However, interpreting this cue literally could be considered a hyper-correction.
Generally speaking, the rationale of maintaining elbow position throughout the arm curl is to prevent the elbows from traveling forward beyond the weight's center of gravity, thereby terminating tension on the biceps before full flexion is achieved.
If you observe anyone with a well developed arm, you will notice they typically do not keep their elbows directly underneath their shoulders throughout the entire range of motion, but instead they move the elbows forward slightly as the weight approaches the top.
To understand why they typically do not adhere to this ‘strict’ form, we first need to understand the basic biomechanics of the arm curl. At the bottom of the movement, there is no torque through the elbow which results in no significant tension through the biceps. At the initiation of flexion the brachialis is highly activated since it's line of pull is more perpendicular to the ulna as compared to the biceps' insertion to the radius. However at this initial range of motion, the tension is still relatively low since the direction of movement is largely forward with little upward motion against gravity. Tension is increased as the elbow angle continues to shorten. As the weight is lifted toward the position where the forearm reaches a horizontal position, the resistance is transferred to more to the Biceps Brachii reaching its maximum tension since this position provides the greatest horizontal distance away from the fulcrum (ie: elbow joint). At this point, the movement becomes entirely upward, directly against gravity. As the weight continues to be lifted beyond the horizontal position, resistance decreases since the horizontal distance from the fulcrum shortens and the resistance begins to move backward and less upward against gravity. Near the top range of motion, the weight comes close to being above the elbow, particularly for a beginner with less muscle mass or arm bulk. As with the lower position, this too is where the barbell is a very short horizontal distance from the elbow and the weight is moved more horizontally and less upward against gravity.
For a beginner with less muscle mass, the arm curl affords a momentary opportunity for a relative decrease of muscular tension at both the top and bottom positions, even with the elbows positioned stationary under the elbow. However, as more muscle mass is added to the arm, the range of motion diminishes nearer the top range of motion due to the greater muscle mass resulting in the weight positioned at a greater horizontal distance from the fulcrum if the elbow continues to be position directly below the shoulder. Therefore the top portion of the movement no longer allows for a relative reduction of muscular tension as it once did or could have with less muscle mass. The exercise now requires more muscular endurance since there is not as much relative decrease of tension at the top of the motion. So, what was an exercise with more basic qualities, providing a bell shaped tension curve, is now is transformed into an exercise with more auxiliary qualities, offering arelatively more peak tension curve. With that sort of intent, one might consider instead performing a drag curl (an auxiliary exercise) where muscular tension is maintained at the top range of motion.
Keep in mind that performing basic exercises, at least for your first movement, offers many benefits over auxiliary exercises. Benefits basic exercises have over auxiliary exercises typically include greater potential for functional strength, muscle mass gains, and metabolic expenditure lasting many hours after intense aerobic exercise.
One could argue that keeping the elbow back reduces a potential slight active insufficiency by keeping the short head of the Biceps Brachii more sufficiently stretched over the shoulder joint allowing for greater tension potential throughout the elbow at the top most range of motion. However, the slight loss of tension potential could be considered inconsequential since it occurs as torque decreases. Furthermore a relative greater peak contraction near the top with the elbows kept directly under the shoulders will likely impede maximal resistance from being used in a consistently strict, full range form, at least in the short term until local muscular endurance increases. After adaptation, greater muscular endurance must be maintained in this top position, to allow for greater workloads to be used which could potentially impend maximal strength gains to some extent, particularly if no other basic biceps exercise is performed.
To maintain the basic qualities of the arm curl, the elbow should shift slightly, just enough to permit the forearm to be positions near vertically at the top most range of motion. It is prudent, however, to prevent the elbow from traveling forward beyond the bar. Doing so would decrease the torque through the elbow to less than zero. This is one of the concerns that the oversimplified form you are suggesting attempts to address.
An alternative form is to keep elbows under shoulders until full flexion is achieved then immediately bringing the elbow forward, positioning the arm vertically thereby mitigating the slight active insufficiency issue mention earlier while allowing for a momentary relative relaxation of the biceps at the top most range of motion.
On a Cable Curl, with the low pulley positioned slightly forward, the resistance is still largely upward yet it angles back slightly, inline with the cable, as opposed to the purely downward force acting upon a free weight, as in a barbell curl. The slightly backward angle of a low pull positioned slightly forward would require the elbows to travel forward slightly more than what would be required during a barbell curl, to achieve the same relative decrease of tension near the top of the movement. This is why the elbows travel slightly more forward in the Cable Curl.
Keeping the elbows directly under the shoulders throughout the full range of motion may slightly increased vascular occlusion (see Pump & Burn), theoretically conducive for sarcoplasmic hypertrophy. In contrast, allowing the elbows to drift forward slightly near the top may allow for a more dynamic Bell Shaped Tension Curve permitting slightly greater resistance to be used, thereby slightly promoting more myofibrial hypertrophy and strength. However it may turn these subtle differences may not be that important in the grand scheme of things, particularly since it appears compound torso work may have a more significant effect on arm development, and presumably strength development, as compared to isolated arm exercises. See Gentil (2013) study summary.
This analysis is intended to more clearly discern acceptable form and to better understanding the benefits of allowing the elbow to travel forward slightly during the curl. For many people's needs, it may be considered over analytical. To that end, it is most important to perform the the arm curl in a reasonably strict fashion, allowing for continued progress for whatever your training goals may be. In fact, most people will make plenty of progress throughout the first several years of your training practically no matter what form they use within reason as long as they are consistent, eat right, avoid overtraining, and follow other common sense guidelines. However as one advances, a more technical approach is required for continued progress. The best techniques will depend largely upon your training goals. However sticking to the basics will yield the greatest benefits for most training goals.
It won't hurt to occasionally perform the exercise with the elbows stationary as an alternative to the basic barbell curl or as an additional auxiliary movement to your biceps program, treating it like any other periodic (ie: monthly) change to your program (see Restimulating Progress by Changing Exercises).
Are there any differences between conventional Preacher Curls, as demonstrated at ExRx.net and so called Spider Curls, performed on the opposite vertical surface of a preacher bench? I am mainly concerned whether this change makes biceps the target muscle, or brachialis remains target muscle as in the conventional preacher curls. Any further comments on Spider Curls (regarding their injury potential, active insufficiency of the short head, etc.) would be highly appreciated as well.
Spider curls were popularized by old time trainer Vince Gironda. According to Larry Scott (first Mr Olympia), Vince Gironda developed a unique specialized spider bench. It included a narrow chest pad on a 15 degree bench at the top end of the bench in the form of a cross. The exerciser lies prone with arms hanging over the top of the cross and the back of the arms supported by the arm rests. Grip is about 8 inches (20 cm) apart with a false grip (thumbs and fingers on the same side of the bar). The bar is curled all the way up to the chin and down again.
Another old design apparently did not have the longer bench for the body to rest upon.
For those without these specialized benches, the exercise had been traditionally performed off the upper end of an elevated bench as depicted in the Prone Incline Curl, also known as a Spider Curl.
As your description depicts, Spider Curls have also been adapted to be performed off the opposite end of a preacher bench, thereby blurring the distinction between this variation and preacher curls.
When the spider curl is performed on a low angled incline bench, the shoulder is flexed even greater than a preacher curl or a spider curl performed off the vertical side of a preacher bench. With greater shoulder flexion, the short (inner) head of the Biceps Brachii experiences is placed in an even higher degree of active insufficiency, thereby emphasis on long (outer) head of the biceps and the underlying Brachialis to a greater extent as compared to preacher curl or even the spider curl performed down the opposite side of a preacher curl. The degree of shoulder flexion is the same when comparing a preacher curl to a spider curl on the opposite side of a preacher bench. The largest difference is in the resistance curve, which is only slightly different. The resistance curve of a preacher curl begins at a slighty greater effort at the beginning (where the brachialis has its mechanical advantage over the Biceps Brachii) and ends with a slightly lower effort at the end due to the sublet varying paths the weights travel against gravity when the arm is placed on an angled surface versus a vertical surface.