I really like your basic design; especially the latex tubing tension part, and the knot release idea. That's brilliant and elegant.
Nevertheless, I humbly offer the following improvement suggestions:
1a) In place of the knotted rope, install a curved, bow-like piece of metal tubing, perhaps like a half-inch or three-quarter-inch conduit. A gentle bowing curve like an inverted U might be best to engage the actuator arm that, in turn, delivers the stored kinetic energy by means of sudden deceleration.
1b) At the front end of the apparatus, constrain side to side movement of the conduit; perhaps with a vertical slot. The front end of the conduit can slide on the floor, or better yet, roll on a small wheel. It will lift off the floor during part of the duty cycle.
1c) In place of a rope knot, use a pin or small bolt through, or bolted to the conduit.
1d) At the back driving end, secure that end of the conduit to the pedal of a bicycle. Discard the front wheel and handle bars, and bolt to a long platform. Use a 10 or 15 speed bike. Attach it so that the rear wheel is above the platform a short distance.
1e) Mount an electric motor with a pulley or friction wheel to contact the bottom of the bike wheel. The motor drives the bike wheel in the opposite direction of the normal rotation of the wheel. This causes the pedals to move counter to the normal direction of a bike. When the motor runs, the conduit lifts, moves forward, grabs the actuator arm, and draws it back in a way that is similar to the knotted rope. Just before the furthest backwards travel of the pedal, the actuator arm slips from the conduit (just like the knotted rope), and releases the actuator arm. Gear controls on the bike give you ten or fifteen rates of delivery of the kinetic energy. Raising or lowering the small front idler wheel on the conduit, can adjust the amount of energy delivered per cycle of the pedal. There is no need to wind a rope, or count knots.
2) OPTIONAL: Arrange the mounting of the actuator arm to shift its position up and down, and/or side to side a few degrees to vary the target area receiving the kinetic energy.
3) To secure the arms of the sacrificial victim, simply have a sheet of plywood or similar material with two slots about a cubit (18 inches) apart from each other, upon which they lay. The arms are free but totally unable to do jack about anything, as long as the lower back is held down. I've heard that this proffering of useless freedom adds a special 'je ne sais quoi' to the futility experienced by the sacrificial victim. A single padded arm with a hinge and timed latch across the small of the back will secure the sacrifice.
4) To secure the lower limbs, use a bar, perhaps with two semicircular places, just above the knee to the front of the victim. A second bar with two sufficiently deep U-shaped curves above the heels prevent any side-to-side or backwards excursions of the lower legs. Once adjusted, no further preparations are needed for these two bars. The sacrifice can approach the altar and step between both bars; but when they lay forwards, arms in the slots and the padded arm lowered and secured across the back, they are certainly in for the duration.
5) OPTIONAL: Although the foregoing can be adapted for SB, two or more inventors might modify the art for use during pleasure outings to remote locations. The HP or HPS leaves the front wheel and handle bars in place, but a few rods raise the rear wheel which is pedaled widdershins. Some thought must be given to means to secure the sacrifice; but once solved, the result should be the same. Fastening an opaque tarp or sheet of poly over the victim and around the waist of the HP/S should provide security from view of anyone who chances to come by. If asked, one can always say that this is a human-powered electric generator for charging a battery, and this is a field test for the patent application; therefore no further questions are allowed until the patent is granted. (Or, simply tell them to %#@! @%%.)