Normal Physical Endurance: Can hold breath for P.E. x melee round.

Extraordinary Physical Endurance: Fatigues at 1/2 the normal rate. Needs only six hours of sleep or mediation per day (minimal sleep is 1-3 hours for 2-6 weeks or no sleep for 72 hours, pushing too hard is 1-3 hours for 7-16 weeks or no sleep for 96 hours, exhausted to the point of collapse is 1-3 hours sleep for 17+ weeks or no sleep for 168 hours). Can hold breath for 2 x P.E. x melee round. Bonuses:
Add 1D6+5 to P.E. attribute.
Add 4D4x10 S.D.C.
Add 3D6 to Hit Points, plus 1D4 per level of experience.

Superhuman Physical Endurance: Fatigues at 1/3 the normal rate. Needs only four hours of sleep or mediation per day (minimal sleep is 1-2 hours for 2-6 weeks or no sleep for 96 hours, pushing too hard is 1-2 hours for 7-16 weeks or no sleep for 168 hours, exhausted to the point of collapse is 1-2 hours sleep for 17+ weeks or no sleep for 336 hours). Heals twice as fast as normal for humans. Can hold breath for 3 x P.E. x melee round. Bonuses:
+3 to save vs poison, drugs and disease.
Add 2D6+5 to P.E. attribute.
Add 4D6x10 S.D.C.
Add 1D4x10 to Hit Points, plus 1D6 per level of experience.

Supernatural Physical Endurance: Fatigues at 1/10 the normal rate. Needs only two hours of sleep or mediation per day (minimal sleep is 1 hour for 2-6 weeks or no sleep for 168 hours, pushing too hard is 1 hour for 7-16 weeks or no sleep for 336 hours, exhausted to the point of collapse is 1 hour sleep for 17+ weeks or no sleep for 504 hours). The character bio-regenerates damage at a rate of 5D6 per 24 hours. Can hold breath for P.E. x minute. Bonuses:
+1 to save vs magic, +5 to save vs drugs, poison & disease
Add 3D6+5 to P.E. attribute.
Add 1D4x100 S.D.C.
Add 1D6x10 to Hit Points, plus 2D4 per level of experience.
Rifts Conversion Note: In Rifts this super power Transforms HP & S.D.C. to M.D.C.

REVISED Movement and Exertion Keep the game simple and quick moving. Light activity, such as walking, jogging, driving, standing guard, doing repairs and similar activity, can be conducted for hours without affecting the character adversely. Both players and Game Master should apply common sense and logic to the duration of an activity. Even intense activity, such as 20 minutes of combat followed by rest or light action, will not significantly impair the character's efficiency. It's all a matter of pacing. Prolonged periods of combat or heavy exertion (an hour or more of intense, continual, physical exertion) will take its toll on the character. Reduce the following once every hour:
Speed -2
Initiative -2
Parry and Dodge -1
Damage -2

Exertion Levels
Light Exertion: walking, jogging, driving, standing guard, SCUBA, doing repairs and similar activity, without carrying or lifting maximum weights.
Medium Exertion: Carrying maximum weight while walking, standing around, or other light activity.
Heavy Exertion: Combat, Swimming, Running (all without carrying or lifting maximum weights).
Strenuous Exertion: Running, Swimming or Combat while carrying maximum weight.
All-Out Exertion: Lifting maximum capacity.* Alternatively, the character could have lifting max capacity considered Strenuous activity, with All-Out Exertion covering Lifting Max Capacity while walking (running, swimming, and combat would all be impossible).

Fatigue Rates
Light Exertion: Negligible fatigue. (P.E.x6 hours)
Medium Exertion: P.E. in hours.
Heavy Exertion: P.E. in minutes.
Strenuous Exertion: P.E. in melee rounds.
All-Out Exertion: P.E. in melee attacks (or seconds).

Exertion penalties should then be applied cumulatively for each time period beyond the fatigue rate for the type of exertion (i.e. a character in combat could fight for his P.E. in minutes, after which, he would suffer the (cumulative) penalties for fatigue for each additional minute of combat).

See the Heroes Unlimited GMs Guide, pages 46-47 for Sleep Deprivation rules.

Additional Revision: Encumbrance: (Since I actually managed to find Palladium's rules for this...along with an expanded ruling for Fatigue which I have no plans to use).
Weight: A character can be slowed down by the weight of load/equipment carried. Encumbrance Modifiers (these are cumulative for each additional Encumbrance Unit):
-2 to Speed
-5% to Prowl
-1 to Dodge
-1 P.E. point with regards to fatigue ratess (i.e. for every unit of encumbrance (q.v.) the character effectively loses one point of P.E. in determining how long it takes for the character to fatigue (as described above).)

Encumbrance Units*:
Every 30 pounds (13 Kg) for characters with Normal or Ordinary Strength.
Every 60 pounds (26 Kg) for characters with "Strong," Giant, or Brute Strength.
Every 100 pounds (45 Kg) for characters with Extraordinary, Beastly, Augmented or Bionic Strength.
Every 300 pounds (136 Kg) for characters with Superhuman Strength.
Every 500 pounds (227Kg) for characters with Deific, Supernatural or Crushing Strength.