Last time I tried weightlifting, six months ago, I ended up irritating
muscles in my shoulder joint and had to stop. This was suprising to
me, because I always kept the weight low enough that I could do around
6-10 reps. I was exercising my upper back just as much as my chest, so
there was no imbalance. Perhaps my muscles just got strong too quickly
for the rest of my body to keep up - I went from bench pressing 90
pounds to bench pressing 160 pounds in about 7 weeks. Now I'm starting
over, back at 90 pounds. I don't want to hurt myself again, so I need
to find a new rule for deciding how much weight to lift, other than
"however much you can do for 8 reps". Can you suggest a safer, less
aggressive way of deciding when to add weight?

Hello Gremlin,

Based on what I found, increasing by 70 pounds in 7 weeks is "way too
fast". However there also seems to be a lot of variability based on
the person and the type of outcome you are trying to get. The
following is a list of sites with a summary of the "increase weight"
guidelines to avoid injury. I will also note that many sites also
strongly recommended:
 - work with another person (prefer a trainer)
 - listen to your body (back off if you don't feel well)
 - alternate types of training (e.g, endurance, strength)
and so on. I will provide a few references for that kind of
information as well.

Specific guidelines by age / activity
  http://www.acsm.org/health+fitness/activeaging.htm
for the "active aging" (includes some fitness tests, ages 60+, guide
on starting and gradually increasing the weights). Also has references
to other sites.
  http://www.crucible.net/jkalleyne/wp_adol_str_training.html
for adolescents and children. Has suggestions on adding repetitions
and weight (no more than 1-3 pounds per week!). Also suggests a
medical review if you have had any injury within 12 months.
  http://www.healthgoods.com/Education/Fitness_Information/General_Fitness_Information/exercise_for_health.htm
for general fitness. Recommends no more than +5 pounds per week;
suggests a single set and moving to a second after six weeks. Balanced
between flexibility, strength, and endurance.
  http://ntmain.utb.edu/jloff/3330/chapter_notes/Chapter_11.htm
referring to "strength for sports". Increase weight when you can do 11
repetitions on the third set! Improve strength 1-3% per week. Divides
training into warm up, energy, muscular, and cool down.

Avoiding injury in general
  http://www.fitness2live.com.au/Default.asp?pg=fitnesszone&spg=injury_features
An extensive list of references that suggest a number of ways to avoid
injury.
  http://www.exrx.net/Exercise.html
The "Exercise Prescription". An interesting site due to the extensive
number of references to various reports. For example, should you do
one set, or two to five (with the benefits of each). Has work out
templates as well as specific training guides based on your goals.
  http://www.bodyandfitness.com/Information/Injuries/caring.htm
Caring for injury - what to do if you are injured (and to help avoid
injuries). It also talks about rehab programs (scroll down to about
3/4 of the page).

For additional sites and suggestions, search phrases such as
  rate +"increase weight" avoid injury
or subsets of those can help. The +"increase weight" was important for
your question since a lot of sites have general information but did
not provide guidance on the rate of increase of weight.

Don't hesitate to ask clarification if needed. Good luck on your
strength program.

  --Maniac