OK, let's start out with the conclusion Sunset Boulevard style and then I'll explain how I got there. The maximum base GURPS ST for a given human body weight is the cube root of the character's body weight times 2.6. The maximum GURPS lifting ST is equal to the cube root of a character's body weight times 3.2. So a 125 lb power lifter who represents the maximum in human muscular development could have a GURPS ST of 13 and a GURPS lifting ST of 16 (representing +3 Lifting ST.) Total cost would be 39 points in 4e - 30 points for ST 13 and another 9 points for +3 Lifting ST. So how did I arrive at these numbers? With the following assumptions:

1. It's been stated by Kromm that to calculate an animal's GURPS ST, take the cube root of weight in pounds and multiply by 2, e.g. a 125 lb creature should have a ST of 10. This suggests that ST is a dervied attribute of mass. As a rule, secondary attributes such as Striking ST or of Lifting ST shouldn't vary more than 30% from base ST. Following that logic, assuming normal terrestial biology a GURPS character's ST shouldn't be more than 30% higher than that predicted by mass. Thus a 125 lb character would have a base ST of 10 (cube root of 125 times 2) and a maximum ST of 13 (30% greater than predicted.) That's where I get the 2.6 mulitiplier.

 2. GURPS ST represents a wide range of ability - carrying capacity, lifting ability, ability to inflict physical damage and structural capacity for physical damage. Weight lifters are likely to be specialized, with Lifting ST on top of normal ST. This goes 30% beyond base ST - thus again our 125lb character representing peak muscular development can have +3 Lifting ST on top of his 13 ST. That's where I get "cube root of a character's body weight times 3.2."

 3. I'm using world records for the clean and jerk and the snatch from the International Weightlifting Federation as a base of comparison. I'm estimating that the basic level for the Snatch is equal to Base Load times 5, and the Clean and Jerk is equal to Base Load time 6. How did I get these numbers? Well, the world records for the Clean & Jerk are about 20% greater than the records for the Snatch, so that gave me the 5:6 ratio. In addition, both Clean and Jerk and Snatch are less than the world records (near as I can tell) for the Bench Press. I estimate the Bench Press as being around BLx7 or BLx8 - about 140 to 160 lbs for a ST 10 character. Putting Snatch at BLx5 and Clean&Jerk at BLx6 seems to line up well with Bench Press.

 4. In a realistic campaign, bonus from Extra Effort and Lifting skill ought to be capped. As written, Lifting skill allows a +5% bonus for every point which you roll under skill - e.g., if you have a skill of 13 and roll a 9 you get a 20% bonus. In the real world, maximum lift doesn't vary that much for a person, so I'd be inclined to cap it at +25% with a small bonus for exceptional effort (say +1% for every additional point the character makes the roll by.)

 OK, so let's take these assumptions and plug them in and measure against reality. Take a 152 lb character. Base on these assumptions, he could have a peak ST of 13.9 and a peak lifting ST of 17.1 (I'm using fractions to aid the reality check.) That gives a BL of 58, a base Snatch of 290 and a Clean & Jerk of 348. Allowing for a +25% boost from extra effort and Lifting skill, that's a Snatch of 362 and a Clean & Jerk of 435. By comparison, the world records for the 152 lb class men's competitors are 363 lbs for the Snatch and 433 for the Clean & Jerk. A reasonably close approximation of reality.

 So where does the 18 come in? Well, as I said, peak ST for a given weight for a realistic human character is equal to the cube root of weight times 2.6. Thus, the minimum weight for a human character of a given ST is equal to (ST/2.6) cubed in lbs, or ST cubed divided by 2.6 cubed. 2.6 cubed is about 17.8 or 18. So based on these assumptions, a realistic human character should weigh at least ((ST cubed)/18) pounds.

 Of course the assumptions could be tweaked - for example, disallow Lifting ST but allow a higher boost from Extra Effort and Lifting skill.  I realize this little discussion isn't really necessary for the game but I think it adds a bit of realism.

As an incidental note, the mass to strength ratio for the world records tracks a 2/3 power relation pretty closely in case anyone ever doubted GURPS Gulliver.

  Addendum: Female strength

 I used the world records for men when calculating these numbers. The records for women are for a little over 3/4 those of men, for the same body weight (i.e., 125 lb man vs 125 lb woman.) However GURPS ST covers a much wider scope of ability than just weight lifting. For female human characters, I suggest peak ST equal to cube root of weight times 2.4 and peak lifting ST equal to cube root of weight times 2.8 for Lifting ST. Using the assumptions above, a 128 lb woman would have a maximum ST of 12.1 and maximum Lifting ST of 14.1. By comparison, Sora from GURPS Basic Characters weighs 130 lbs and has a ST of 12. This gives weight lifting numbers pretty close to the world records. Realistically, a high ST female character would be built like Serena Williams, not Kate Moss.  None of this applies to cinematic campaigns or campaigns with paranormal abilities. Ignore these guidelines if you have characters named Buffy Summers or Diana Princess running around.

- DW