Hello.

Most people would agree that the single best way to improve your workouts is to measure them and track the results.  The traditional way of measuring work performed is as follows...

Set 1:  8 reps at 100lbs  = 800lbs of work
Set 2:  6 reps at 100lbs  = 600lbs of work
Set 3:  5 reps at 100lbs  = 500lbs of work
Total = 1900lbs of work

The traditional goal is to keep increasing the amount of work performed for a particular exercise.

As good as this method is (and it is good)...it does have a serious drawback.  This method does not account for rest.  The traditional method would consider the following two workouts equal...

Workout 1:
Set 1:  10 reps at 100lbs = 1000lbs of work

Workout 2:
Set 1:  1 rep @ 100lbs = 100lbs of work  (10 minute rest)
Set 2:  1 rep @ 100lbs = 100lbs of work  (10 minute rest)
Set 3:  1 rep @ 100lbs = 100lbs of work  (10 minute rest)
Set 4:  1 rep @ 100lbs = 100lbs of work  (10 minute rest)
Set 5:  1 rep @ 100lbs = 100lbs of work  (10 minute rest)
Set 6:  1 rep @ 100lbs = 100lbs of work  (10 minute rest)
Set 7:  1 rep @ 100lbs = 100lbs of work  (10 minute rest)
Set 8:  1 rep @ 100lbs = 100lbs of work  (10 minute rest)
Set 9:  1 rep @ 100lbs = 100lbs of work  (10 minute rest)
Set 10:  1 rep @ 100lbs = 100lbs of work
Total = 1000lbs of work.

Anyone trying the above workouts would learn quickly that they are not the same.  So how to measure your progress?

Enter the "Fatigue-Tension Threshold."  This concept, developed (or at least championed) by Health For Life, states that all muscle groups have a "threshold."  If your workout does not cross the threshold, then "Progressive Overload" does not occur and the body is not stimulated to grow.  If your workout exceeds the threshold, then Progressive Overload does occur and your body grows.  

This threshold varies according to the fatigue level in the muscle group.  So lifting 100lbs for 8 reps may not exceed the threshold on the first set but by the 3rd set the fatigue level is such that lifting 100lbs for 8 reps will exceed the threshold.  Also, doing only 50lbs on a leg extension machine may exceed the threshold if the legs have been exhausted by squatting with heavy weights.

The only way I have found to approximate this is to multiply your work performed (i.e., the traditional method) by a ratio of 60-seconds / rest-taken-in-seconds.

So...

Three sets of 8 with 100lbs and 90 seconds rest between sets = 3 * 8 * 100 * 60 / 90 = 1600 Fatigue-Tension achieved.

Three sets of 8 with 100lbs and 30 seconds rest between sets = 3 * 8 * 100 * 60 / 30 = 4800 Fatigue-Tension achieved.

The question becomes how much rest to place between sets?  A good rule of thumb is a minimum of 25 seconds and a maximum of 120 seconds.  Lean towards 25-45 seconds if your goal is ANAEROBIC ENDURANCE (i.e., performing activites at near maximal and above lactate levels).  Lean towards 75-90 seconds (up to 120) if your goal is STRENGTH or MASS.

Vary rest between sets/exercised according to your goals and measure your Fatigue-Tension to ensure that you maintain or increase the level of tension applied to your muscles.

HTH,
James

P.S.  This is adapted from Health for Life's series on exercise performance.  It assumes that strength-training is a means to an end.  If your goals are within the sport of weightlifting, then your rest periods may be different.

Great info...sounds straight...I always(90%)of the time preform my excersises to the point right before or to failure.But I never thought of it in those terms.[:D]