Conventional vs Low load Lifting - Risks and Benefits

After 1 year of working with muscle O2 monitoring, low and conventional load weight training, I have come up with a number of observations.  This post will summarize some of them as well as present a warning to those lifters wanting to get back to heavy lifting again.  Like everything in training, each technique has its advantages and disadvantages.
Firstly, many studies have shown that for the most part, lower load lifting can achieve similar muscle mass gains compared to more conventional type loads.  We will see in a bit that some tweaking of your workout may be needed to succeed in this, but it can be done.  However it is pretty universally agreed that low load lifting can not produce the same peak strength seen with heavy lifts.  This makes perfect sense from the specificity standpoint, if you train very heavy, you will excel in peak strength, if you work on "endurance", strength will be less but rep counts will be higher.  As we will see shortly, there may be real medical dangers in resuming heavy loads particularly in the shoulder.  Even though it may be a major letdown to never lift heavy again, you are better off lifting something and maintaining muscle mass, than having repeat shoulder surgery.  I am reminded of a patient who was a power lifter and was unwilling to reduce his weights despite 2 failed rotator cuff repairs.
On the other hand, there may be real benefits (besides more power) from conventional lifting which we will look at from the NIRS perspective.  If some of this can be safely incorporated  into your routine, I would strongly recommend doing so.

Lets first look at the desaturation patterns of a few exercises to better understand the difference between low and high loads.  The first difference is that a high load by virtue of more severe external compression of the vascular compartment, will cause more profound O2 desaturation.

Here is a tracing of the Row, Dip and Pulldown all done at about a 10 RM load:

As noted, the desaturation is impressive, measured on the Lats, with sharp drops in total Hb, indicating excellent compressive force from the muscular contraction.

Compare the above to doing the row with the sensor on the lats but comparing 80 vs 110 lbs(each side).  The 80 is the low load weight, 110 is my 10 RM:

So the first 2 sets have minimal desaturation but an excellent drop in both O2 and THb when lifting heavier.
I had another sensor on a slightly different area of the lats when doing the above 3 sets shown below:

Although the desaturation with the low load is better, it is still way below the heavier load.
However another good example of the heterogeneous nature of muscle O2 kinetics.

Conclusions so far:

• Heavier loads will lead to lower desaturations in general.
• Heavier loads will affect more areas of the muscle intended to be exercised.

What about heavier weights perhaps causing desaturations in muscle groups not typically targeted by a particular exercise.
Here is a good example.  The exercise is a pulldown which typically targets the lats, biceps, forearms.  However, at higher weight, a very good desaturation occurs on the chest (similar to a chest press).  In this instance, the low load dropped O2 very well on the lats on both high and low weight attempts.  But the low load did not have the overlapping effects on the chest seen with the higher weights.

Another example of this is the posterior deltoid/fly exercise.

Typically this targets the rear portion of the deltoid, but as seen above should have some effect on the lats and upper back muscles.
This is a tracing of the above machine using either 30 lbs or 40 lbs, sensors on the deltoid and lats:

Only the heavy weight has a significant effect on the lat muscle desaturation.

In summary, heavier weights will potentially target more areas leading to more training sets per muscle group.

Having said all this, why even bother with lower weights?  
The following paper is an important counterpoint to the attempt to train with heavier weights if you have rotator cuff impingement, partial tears or other pathology.  Two groups were compared, a normal shoulder group and another with typical impingement symptoms.  External rotation and abduction exercises were done with supraspinatus tendon measurements taken before and at times after.  It was shown that in the symptomatic shoulder group, tendon thickness did increase thereby causing possible further subacromial space reduction and more impingement.

 
 This was the conclusion of the paper:

This certainly gives us some concern about pushing heavier weights with already underlying rotator cuff pathology.  The situation is not going to resolve spontaneously and is possibly why it is so difficult to come back from shoulder injuries.  According to other studies, other tendons (Achilles with pathology) may also swell under load.  However, in this case the swelling leads to further structural tearing and damage.


Where does this leave us?
Conventional load lifting will produce lower desaturations than low loads.  More muscles, both in the same and distant groups will be targeted by conventional training.  But, if the lifter has rotator cuff impingement, heavier loads will lead to further space narrowing and tendon damage.
What I have been doing lately is a hybrid approach.  On days when I am lifting light, I will finish with one set of 10 RM weight, except in exercises targeting the rotator cuffs (any previous exercise that caused pain in that area).  So instead of 3 sets of low load (usually a reverse drop set), the last set will be a 10 RM load.  Note, in the reverse drop set method, the last set was at a higher weight anyway, in this case it is just boosted further.

For exercises that load the rotator cuff tendons, only low load variations are done and no attempt at weight progression will be done.