Thanks for all the feedback! After I wrote that, I remembered that I had actually added 50 lbs to the lift and was trying to maintain the same speed of the lift from before so, as Dr. Clay points out, I probably wasn't prepared for it, i.e., still was too tight. I have the latest Stuart McGill book and I was reading about motor unit recruitment so that's why I was doing faster reps in the first place. I did a quick search just now and found lots of articles on this and they often mention taking care to avoid injury doing this, which I didn't.
Here's an excerpt from one by Waterbury on the topic (
Source):
Now, I've been on my soapbox for years about the importance of fast muscle contractions, but that's not to say that there's no place for anything else. The incorporation of slower eccentrics, pauses, and supramaximal holds definitely have their place within the iron game. What I'm trying to convey is the importance of lifting a load as fast as possible.
The force/motor unit relationship has been effectively demonstrated in the neuroscience community. But, all you really need to understand is there exists a positive correlation between the speed of movement (force production) and motor unit recruitment (i.e. the faster you lift, the more motor units you'll recruit). This is obviously important when you attempt to produce as much force as possible.
Honestly, how many times have you said to yourself, "What can I do to decrease the amount of force in my next set of squats?" Well, you probably haven't ever said that, and in fact you'd probably assume that such a line of thinking is absurd. Guess what? You are decreasing the amount of force your muscles can produce when you lift a load slowly!
Is there ever a reason to lift slowly? Of course there is! It's called rehabilitation. Whenever a joint is damaged, or whenever a movement pattern must be re-programmed, slower concentric contractions are beneficial. But if you're training for the most size and strength, you should strive to lift the load as fast as possible while maintaining perfect control (form).
...and
from Dr. Squat:
The issue of whether one can learn to recruit more fibers (beyond simply practicing the event - which lowers impulse traffic that reaches the motor-neuron) by training at faster velocities is, at best, unresearched. The ability to generate tension and exhibit high power is due to nervous innervation and the myofibrillar activity of the muscle fibers (both genetic). Injuries? I would suggest much carefulness in accelerating with less than maximum training weights.