I've been reading more into the Valsalva maneuver, and I have to say that I am becoming a bit paranoid. Some sites say that it can lead to stroke, passing out and such. This reddit thread gives many contradicting opinion. However, it was six years ago.
What does latest research say? Is the manuveur safe or not?
The Valsalva Maneuver does restrict blood flow to and from the heart. It also does increase the intra-abdominal pressure, which helps spinal stability. Doing it for too long can lead to loosing consciousness and a higher risk of aneurysms.
Therefore, it does have its place, used carefully and for a few moments only, as this systematic review purports:
Strength and conditioning experts acknowledge that a brief VM [Valsalva Maneuver] (no longer than 3 seconds in duration) could assist the experienced resistance trainer by maintaining proper vertebral alignment and support and reducing lower back injury risk.
This is meant when using ">80% maximal voluntary contraction" or "when lifting lighter loads to failure".
When reading the source for that carefully, though, it speaks about that being "unavoidable", ie.:
These observations suggest that the VM is a natural reflex that is evoked during resistance exercise when greater efforts are required.
The "Essentials of strength training and conditioning / National Strength and Conditioning Association ; G. Gregory Haff, N. Travis Triplett, editors. -- Fourth edition." from 2016, a scientific textbook for S&C coaches, has the following to say (pp. 38-39, bolded mine):
When the diaphragm and the deep muscles of the torso contract, pressure is generated within the abdominal cavity. Because the abdomen is composed mainly of fluid and normally contains very little gas, it is virtually incompressible (3). The abdominal fluids and tissue kept under pressure by tensing surrounding muscle (deep abdominal muscles and diaphragm) have been described as a “fluid ball” (figure 2.15) that aids in supporting the vertebral column during resistance training (3). Such support may significantly reduce both the forces required by the erector spinae muscles to perform an exercise and the associated compressive forces on the disks (3, 30). It is important to note that the Valsalva maneuver is not necessary for generation of intra-abdominal pressure. In the Valsalva maneuver, the glottis is closed, thus keeping air from escaping the lungs, and the muscles of the abdomen and rib cage contract, creating rigid compartments of liquid in the lower torso and air in the upper torso. An advantage of the Valsalva maneuver is that it increases the rigidity of the entire torso, making it easier to support heavy loads (15). For example, when lifting heavy loads in the back squat exercise, many athletes use the Valsalva maneuver, particularly when the trunk is most inclined forward, near the transition from the eccentric movement phase to the concentric movement phase. However, pressure in the chest associated with the Valsalva maneuver can have the undesirable side effect of exerting compressive force on the heart, making it more difficult for blood to return to the heart. Also, the Valsalva maneuver can transiently raise blood pressure to slightly elevated levels (15). The diaphragm and the abdominal muscles can contract without the glottis being closed, however, creating the fluid ball in the abdomen without pressurizing the chest compartment. This must be regarded as the safer way, of the two options, to add support to the lower spine without building up pressure in the chest, and is the technique that should be used for most resistance training. One can build up intraabdominal pressure without building up chest pressure by consciously keeping the airway open. During a strenuous repetition, the abdominal muscles and diaphragm contract reflexively, even with the airway open. Athletes, particularly those who compete in Olympic lifting or powerlifting, may choose to use the Valsalva maneuver if they recognize and accept the risks involved and have the experience to avoid increasing pressure to the point of blackout.
Source 15 is: Hackett, DA, and Chow, CM. The Valsalva maneuver: Its effect on intra-abdominal pressure and safety issues during resistance exercise. J Strength Cond Res 27:2338-2345, 2013.
To sum it up: you can use it in the moment when you load your spine, not for longer than 3 seconds, and exhale afterwards but it's playing with your blood pressure and a proper health risk when done for too long. The risks are lowered when you keep the glottis open (which stops making it a Valsalva Maneuver in sensu strictu). If anything, you should take care to consciously open your glottis to normalise thoracic pressure (on the heart) after 3 seconds at most. This does not affect abdominal pressure.
