What is safer for your spinal disks: short, high-load sets or longer sets with less weight?

Question

I remember to have read somewhere (and it seems to me reasonable) that you might damage your spinal disks by repetitive excessive stress with not enough recovery time (yes, correct form for the exercises, do not overtrain, reps within the strength / hypertrophy / endurance range, yak yak yak)

I wonder what is safer: more weight but less reps, or a lighter load that happens however more times. Of course, both cases must be somehow equivalent. Let's say that both cases lead to the same theoretical one-rep maximum calculation.

To put it in numbers, let's say you are dealing with Overhead Press. You might equally well do either:

• Up to 15 reps, close to failure, with 30 kg or
• Up to 5 reps, close to failure, with 44 kg.

For this example I have used Brzycki's one-rep max formula (for no special reason).

The question is, what of the two options is safer for your spinal disks?

Answer 1

After a quick research session, I can't find any research that shows that heavy lifting contributes to spinal disc degeneration. So, lift according to your goals, eg. more repetitions for hypertrophy or endurance, less reps for strength.

Previously, heavy physical loading was the main suspected risk factor for disc degeneration. However, results of exposure-discordant monozygotic and classic twin studies suggest that physical loading specific to occupation and sport has a relatively minor role in disc degeneration

Source: Lumbar Disc Degeneration: Epidemiology and Genetic Influences

The study showed that intensive training will increase the [bone mineral content] to an extent that the spine can tolerate extraordinary loads.

Source: The Loads on the Lumbar Spine During Extreme Weight Lifting

In fact, resistance training can actually reduce the need for surgery:

A large number of patients who had been told they needed surgery were able to avoid surgery in the short term by aggressive strengthening exercise.

Source: Can spinal surgery be prevented by aggressive strengthening exercises? A prospective study of cervical and lumbar patients

If you are worried about disc compression, you can use a weight belt:

Results suggest that the use of a lifting belt increases IAP, which may reduce disc compressive force and improve lifting safety

Source: Effects of a Belt on Intra-Abdominal Pressure during Weight Lifting

Answer 2

With regard to spinal disk safety during exercise, the critical factor is amount of impact rather than actual load (within reason). So doing static lifts will usually be less stressful than playing basketball or running for example.

Having said that, maintaining good form during lifts is also critical especially when your back is involved. In my experience, people are much more likely to lose form when doing heavy sets, so longer lighter sets are safer.

Another factor to consider, is wether you already have a damaged disk or not (I do). If you do, stay away from heavy loads (I do). If you are just trying to avoid injury, limit short heavy sets for strength training only.