Throttle said:
how far into your workouts is this happening? when was your last meal/nutrition? are you taking any nutrition during your workouts?
also, pay attention to your breathing. exaggerate it for a while. holding your breath on the concentric portion is tempting as you go up in poundage, but you need a complete breath for each rep. typical advice is to exhale during the concentric phase, and inhale on the eccentric phase, though some powerlifters hold their breath during the concentric phase and breathe between reps. failing to breathe properly raises b.p. and builds up pressure in all the wrong places.
HUGE, HUGE, HUGE for people.
There is a maneuver called the Valslva (it has its place though) and you would be surprised at how many people in the gym do this without even knowing. I am always telling people BREATHE, BREATHE, BREATHE.
Here is what it is......
1. Initial pressure rise: On application of expiratory force, pressure rises inside the chest forcing blood out of the pulmonary circulation into the left atrium. This causes a mild rise in blood pressure.
2. Reduced venous return and compensation: Return of blood to the heart is impeded by the pressure inside the chest. The output of the heart is reduced, the blood pressure falls. This occurs from 5 to about 14 seconds in the illustration. The fall in blood pressure reflexly causes blood vessels to constrict with some rise in pressure (15 to 20 seconds). This compensation can be quite marked with pressure returning to near or even above normal, but the cardiac output and blood flow to the body remains low. During this time the pulse rate increases.
3. Pressure release: The pressure on the chest is released, allowing the pulmonary vessels and the aorta to re-expand causing a further initial slight fall in pressure (20 to 23 seconds) due to decreased left ventricular return and increased aortic volume, respectively. Venous blood can once more enter the chest and the heart, cardiac output begins to increase.
4. Return of cardiac output: Blood return to the heart is enhanced by the effect of entry of blood which had been dammed back, causing a rapid increase in cardiac output and of blood pressure (24 seconds on). The pressure usually rises above normal before returning to a normal level. With return of blood pressure, the pulse rate returns towards normal.
So be careful as to not do this. Sometimes this can cause people to get dizzy and faint if they aren't aware they are doing it.
And if you aren't doing it, at least it is some extra information that they teach you if you specialize or have a degree in an exercise related field.