DC8 CREW HAD BINDING ELEVATOR CTLS ON DEPT FROM ZZZ.

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FLT DEPARTED BLOCKS LATE AT ZA:14 UTC (XA:14) FOLLOWED BY NORMAL BEFORE TKOF CHKLIST. IN CHKLIST PART OF FLT CTL CHK IS ELEVATOR CTL TAB CHK BY BOTH CAPT AND FO SIMULTANEOUSLY FULL AFT THEN FULL FORWARD WHILE CONFIRMING ASSOCIATED MOVEMENT OF ELEVATOR PITCH GUAGE. THIS WAS ACCOMPLISHED. NO ABNORMALITIES NOTED. AS TKOF WAS INITIATED CAPT CALLED '80 KNOTS.' I (FO) ACCOMPLISHED THE ELEVATOR CHK AND RESPONDED 'ELEVATOR CHKS;' BUT NOTED A SOMEWHAT DIFFERENT FEEL OF YOKE AS IF THERE WAS A RUBBING OR STIFFNESS ON YOKE ELEVATOR ON CTL. I CHKED THE ELEVATOR TWO (2) MORE TIMES FEELING SAME STIFFNESS; BUT HAVING ELEVATOR AUTHORITY I ELECTED TO CONTINUE THE TKOF AS WE WERE APCHING V RAPIDLY. ON CLB OUT I CONTINUED TO FEEL THE ELEVATOR ABNORMALITY TO A POINT WHERE I ASKED THE CAPT TO TAKE THE FLT CTLS AFTER WE HAD BEEN 'CLEANED UP' AND CLRED ON COURSE FOR 2-3 MINS. HE TOOK THE FLT CTLS AND FELT THE SAME RUBBING OR STIFFNESS OF THE ELEVATOR AND IMMEDIATELY SAID WE ARE GOING BACK! FLT CTLS WERE TRANSFERRED BACK TO ME (FO) AND CAPT ASKED ATC (DEP) FOR RETURN TO SDF. WE DID NOT DECLARE AND EMER NOR REQUEST PRIORITY HANDLING ON BASE TO FINAL APCH WHILE STILL ON AUTOPLT IN SPEED MODE OF 200 KTS WE HAD JUST SELECTED APCH FLAPS TO 23 DEG WHEN I FELT A POPPING UNDER THE FLOORBOARD THROUGH MY FEET AND FELT THE SAME IN YOKE. I TOLD THE CAPT AND TOLD HIM I WILL DISCONNECT AUTOPLT AND HAND FLY THE VISUAL APCH (BACKED UP BY THE ILS 17R). NORMAL APP AND LNDG AND TAXI INTO BLOCKS. WE INFORMED MAINT AND HE SAID; 'IT WAS POPPING UNDER YOUR FEET AND YOU FELT IT IN THE YOKE?' I REPLIED 'YES;' HE SAID; 'WELL YOU WONT BE TAKING THIS ACFT CAUSE IT SOUNDS LIKE THE ARTIFICIAL FEEL SPRING UNDER THE FLOOR AND THAT'S AN ALL DAY JOB; ABOUT 6-7 HRS.' WE SWAPPED ACFT. THE NEXT DAY ONE OF OUR ASSISTANT CHIEF PLTS INFORMED ME AND THE CAPT THAT THE CAPTS CROTCH SEATBELT WAS FOUND IN THE FLOOR IN THE SLOT WHERE THE YOKE SITS. 20 MINS LATER HE CAME ON BOARD MY ACFT AND SAID MAINT SAID IT WAS MY CROTCH SEATBELT IN YOKE. WHEN I GOT HOME I RESEARCHED A REVIEW OF THE COMPANY'S FLT OPS MANUAL AND THE 3 APPLICABLE FARS IN FAR 12; REGARDING SEATBELTS. NONE OF THESE REVEALED ANYTHING OTHER THAN SEATBELTS AND SHOULDER HARNESS USE. HAVING NEVER WORN THE CROTCH BELT (BTWN THE LEGS OF THE 5 POINT HARNESS) AND HAVING NUMEROUS CHKRIDES; LINE CHKS; SIM SUPPORTS ON TYPE RIDES AND NEVER BEEN CORRECTED TO THE CONTRARY AND HAVING NEVER WITNESSED OUR OWN CAPTS USING THIS PART OF THE HARNESS I THOUGHT IT WAS OPTIONAL. I MUST CONFESS I HAVE TRIED IT ON BEFORE BUT DID NOT USE IT BECAUSE OF THE DISCOMFORT IN THE REPRODUCTIVE AREA. DUE TO THE HT AND TENSION AT WHICH THE MAIN BUCKLE NORMALLY FALLS ON THE ABDOMEN. A DIFFERENT ASSISTANT CHIEF PLT CALLED ME AND SAID THEY COULD NOT FIND ANYTHING IN THE REGS OR OUR OWN MANUALS SPECIFICALLY ADDRESSING THE CROTCH BUCKLE. HE SUGGESTED MAYBE; I SHOULD START WEARING THE BELT WHICH I DO. I BELIEVE WE THE CREW DID A GOOD JOB USING CRM AND STANDARD SOP TO ACCOMPLISH OUR SAFE RETURN. I BELIEVE THE CONTRIBUTING FACTORS TO THIS OCCURRENCE WAS THE INTERUPTION OF THE NORMAL SLEEP CYCLE AND CIRCADIAN RYTHYMS; AND HAVING JUST COMPLETED A FLT CTL CHK THAT WAS NORMAL TWO MINS PRIOR. I WAS CONTACTED OFF RESERVE STATUS THAT MORNING WITH THE TRIP ASSIGNMENT AT EARLY:47 AM AFTER APPROX 3 HRS UNINTERUPTED SLEEP. (I CAN NEVER GET TO SLEEP TILL AFTER 9 PM LOCAL NO MATTER HOW EARLY I TRY). IN CONCLUSION; I BELIEVE I MADE THE DECISION TO CONTINUE FLT BASED ON THESE FACTORS TO THE BEST OF MY ABILITY. WHILE I BELIEVE WE HAVE EXELLENT MAINT OUR AGING DC8 FLEET ACFT CAN AND DO HAVE A SLIGHTLY DIFFERENT FLT CTL FEEL OR IDIOSYNCRASY FROM ACFT TO ACFT. SINCE I DO NOT FLY THIS ACFT REGULARLY I ALLOWED SOME DIFFERENCE OF FEEL WHICH NOW I CAN NOW KNOW FROM EXPERIENCE WILL BE A REASON TO CALL OUT ABNORMAL ELEVATOR FEEL TO CAPT TO ALLOW HIM TO MAKE DECISION TO REJECT PER OUR STANDARD PROCS. INCIDENTIALLY APRIL 2000 WAS MY PREVIOUS FLT IN THIS ACFT.