with reference to the drawings , the invention will be described in detail for its various embodiments . in general the invention comprises embodiments of a programmable system and process for monitoring and assisting weight lifting during rehabilitation of training exercise that is utilized with or comprises free weights or a stacked plate weight machine , which shall be referred to herein as the weight apparatus . the system comprises in general a support structure , a cable affixed to a drum pulley operated by a motor , the cable being attached to bar of the free weights or the cable or weight mounting assembly of the weight machine , whereby controlled rotation of the drum pulley results in lifting or lowering assistance to the weights during the concentric or eccentric phases of the exercise . a control panel with input means and display means controls a programmable computer to monitor and control the mechanical operative components . the system determines the exercise parameters for a particular user , controls the assistance program for the particular user , and monitors the exercise to determine when responsive assistance is needed , among other purposes . as shown in the drawings , with particular reference to fig2 , an embodiment of the system 10 of the invention comprises a support structure 11 comprising a base 12 , and vertical post 13 and a boom arm 14 . one or more pulleys 15 are mounted on the boom arm 14 , the pulleys 15 being adapted to receive a cable 16 such that a portion of the cable 16 extends downward adjacent the post 13 and a portion of the cable 16 extends downward from a point farther out on the boom arm 14 . the boom arm 14 may be mounted on the post 13 in a manner that allows the boom arm 14 to be pivoted about the vertical axis of the post 13 , the post 13 may be mounted on the base 12 such that the post 13 and boom arm 14 may be pivoted about the vertical axis of the post 13 , or the base 13 may be provided with wheels , rollers or the like that allow the support structure 11 to be moved or rotated . the elements of the support structure 11 , the cable 16 and other elements of the system are constructed of suitably strong material such that weights totaling hundreds of pounds may be lifted and lowered in a manner described below . the cable 16 is attached to a drum pulley 17 mounted onto the post 13 of the support structure 11 , the drum pulley 17 having a generally horizontal axis of rotation such that rotation of the drum pulley 17 retracts or extends cable 16 . rotation of the drum pulley 17 is achieved by motor 18 , which preferably is an electric servo motor , the motor 18 being operatively connected to the drum pulley 17 using gears 19 and a toothed belt 20 , as shown in fig4 , a direct drive assembly 21 , as shown in fig3 , a common shaft ( not shown ), or other suitable operative mechanisms . a computer processor 22 monitors and controls the operation of the motor 18 , the computer processor 22 being controlled by a control and display panel 23 . the control and display panel 23 allows data input into the computer processor 22 and the display of data sensed , determined or calculated by the system . preferably a housing or cabinet 24 is provided to enclose these components of the system 10 , the cable 16 extending through an opening in the housing 24 . the system 10 as described operates in conjunction with a weight apparatus , either free weights or a stacked plate weight machine 90 of known structure comprising a vertically elongated frame 96 having an upper pulley 91 through which a cable 92 is suspended down to a weight mounting assembly 93 comprising a descending rod member . the weight plates 94 are stacked in flat layers and each plate 94 is provided with a central opening through which the rod member extends in the neutral or non - operative status . the weight plates 94 and the rod member are provided with coordinating apertures through which a removable pin member is horizontally inserted , the location of the pin member determining the total number of weight plates 94 to lifted during the weight lifting exercise . the other end of the cable 92 , which may pass through additional pulleys , is attached to a handle or other gripping device 95 , or is attached to a movable member of a user receiving structure , such as a bench , seat or the like upon which the user sits or lies during the weight lifting operation . in a preferred embodiment , the system 10 is structurally separate from the weight machine 90 and the cable 16 is detachably connected to the cable 92 or weight mounting assembly 93 of the weight machine 90 using a quick release cable attachment member 25 , such as a clamp or other mechanical structure , such that the cable 16 can be easily and quickly attached and detached . alternatively , the system 10 may be connected to the weight machine 90 by permanent attachment of the cable 16 or by structurally connecting the support structure 11 . still further in another embodiment , the operative components of the system 10 may be directly mounted onto the frame 96 of the weight machine 10 . for free weight applications , the cable 16 is attached to the bar of the free weights . in this manner , when motor 18 is activated to turn drum pulley 17 the cable 16 is retracted , thereby providing a lifting force to the weights during the concentric phase . when the weights are lowered in the eccentric phase , the drum pulley is rotated in the opposite direction to extend the cable 16 . the system 10 as described above is adapted to conduct a computer assisted process of monitoring and assisting weight lifting during rehabilitative and training exercise , whereby the beneficial effects of the exercise are increased and desired results are achieved more rapidly and with greater safety . as shown in the fig1 flowchart , the process generally comprises the basic steps of selecting the appropriate weight for the exercise and inputting this value through the control panel , determining and inputting via the control panel 23 ( if not automatically calculated by the processor 22 ) the user range of motion , repetition speed and weight lifting assistance percentage variables , monitoring the exercise operation upon commencement , providing the pre - determined or calculated weight lifting assistance percentage during the concentric exercise phase and / or providing processor controlled weight lifting assistance percentage in response to sensed trigger conditions , and providing no weight lifting assistance during the eccentric exercise phase unless certain trigger conditions are sensed . the weight chosen for a given exercise may be the maximum weight the user can lift unassisted or may be a lesser weight when an increase in repetitions for a given exercise set is desired or during rehabilitation exercise when a muscle is injured . the user range of motion for a given exercise and chosen weight is the distance from start to finish of one exercise stroke and the repetition speed is the rate at which the user lifts and lowers the weights . these parameters determine the start point , the transition point where the concentric phase ( lifting the weights ) changes to the eccentric phase ( lowering the weights ), and the finish point . some or all of these parameters may be sensed , calculated and utilized by the processor 22 based on detection of movement and torsion of the motor 18 and / or drum pulley 17 during the exercise strokes using known sensing devices . alternatively , some or all of these parameters may be manually measured , calculated , noted on the computer control panel and display panel 23 , and / or entered into the processor 22 via the computer control and display panel 23 . in the basic embodiment of the process , the desired assistance percentage for assistance during the concentric phase is input into the processor 22 . the assistance percentage is the amount of lifting force provided by the system 10 which thereby decreases the effective amount of weight being lifted by the user . for example , an assistance percentage of 25 % may be applied to a selected weight of 200 pounds , meaning that the system provides 50 pounds of lifting assistance such that the user is only lifting the equivalent of 150 pounds . once the weight is lifted to the transition point , the assistance is removed and the user lowers the full weight of 200 pounds during the eccentric phase . more commonly the assistance percentage will be selected to vary during the set of repetitions . for example , for a set or twelve repetitions , no assistance may be provided for the first eight repetitions and an assistance percentage of 25 % may be applied to the last four repetitions . alternatively , an assistance percentage of 25 % may be applied to the first four repetitions , an assistance percentage of 37 . 5 % may be applied to the next four repetitions , and an assistance percentage of 50 % may be applied to the last four repetitions . still otherwise , the assistance percentage could be increased for each repetition , could decrease for each repetition , could be alternatingly increased and decreased , etc . the assistance percentage may also vary during a given concentric stroke , such that a lesser percentage may be applied at the beginning and end of the stroke and a greater percentage applied during the middle of the stroke . in most exercise circumstances it is desirable that the effective weight be greater during the eccentric phase than during the concentric phase , meaning in this process no assistance , i . e ., lifting force , is provided during the lowering of the weight . thus , for a 200 pound total weight receiving 25 % assistance during the concentric phase to reduce the effective exercise weight to 150 pounds , the user will be lowering the full 200 pounds during the eccentric phase . thus , once the system 10 senses that the transition point has been reached , i . e ., that the concentric stroke has been completed , the lifting assistance is aborted and the motor 18 is reversed to extend the cable . when the system 10 senses that the finish point of the eccentric stroke has been reached and concentric has begun , the lifting assistance is again commenced . the system 10 and process may also be provided with known sensor mechanisms to sense certain trigger conditions that may occur during either the concentric or eccentric phases . trigger conditions may include a concentric or eccentric stroke that is occurring too fast or too slow , or a stoppage during the concentric or eccentric stroke . upon occurrence of a trigger condition , a visible or audible alarm may be provided . occurrence of a trigger condition may also initiate a response from the processor 22 . for example , if the concentric stroke slows beyond an acceptable range based on the initially determined repetition speed , the assistance percentage may be automatically increased by the processor 22 such that the weights 94 can be raised to the transition point . alternatively if the concentric stroke stops and the user begins to lower the weights 94 , or if the rate during the eccentric stroke increases excessively due to the user being unable to sustain the total weight in a safe manner , lifting assistance may be applied to lighten the effective weight being lowered by the user to prevent injury or the weights 94 crashing onto the weight stack . it is understood that equivalents and substitutions for certain elements set forth above may be obvious to those of skill in the art , and therefore the true scope and definition of the invention is to be as set forth in the following claims .