Weight lifting exercise benches have been known in the art for a number of years.  More recently, as physical fitness has gained greater public interest and awareness, a number of developments have occurred in the design and use of exercise benches. 
The original exercise bench, which is still in use, comprises a flat raised horizontal platform supported by a frame upon which a user lies while performing various arm exercises, such as bench presses and pullovers (which develop the triceps and chest muscle groups.) 
The use of this original bench has limitations, in that an assistant was generally required to hand the weights to and take the weights from the bench user.  Moreover, a bench user, who performed exercises thereon without assistance, could be injured if he became exhausted and was unable to escape from under the weights. 
To improve the above-described original bench design, vertical support members at one end of the bench were extended above the level of the platform and "J"-shaped hook members were added to the upper portions of the extended vertical support members, thereby allowing the barbell weights to be supported above the users head and shoulders.  This improved exercise bench was therefore, more convenient and safer in that a user could install a barbell on the "J"-shaped hook without assistance, and could place the barbell thereon after completion of his exercise so that he could escape from under said barbell. 
Thereafter, exercise benches were provided with a platform having means for slanting the head end thereof upward relative to the horizontal portion of the end of the platform supporting a user's trunk.  Using the aforementioned configuration, a weight lifter, by bench pressing with his upper torso inclined upward from a horizontal position, is required to use certain muscle groups of the upper chest and shoulders, which muscle groups would not otherwise be exercised to the same extent. 
Another prior art improvement to exercise benches was the addition of a leg exercising means.  These prior art leg exercising means are of two types, although many contemporary benches comprise both types in a single embodiment.  The first type of such leg exercise means enables the development of the front thigh muscles, and more specficically, the quadriceps femoris (rectus femoris, vastus intermedius, vastus medials) vastus internua, sartorius, and patella tendon.  In this exercise, the user sits or lies on his back on the platform with his legs hanging downward over one end thereof.  The user's legs are tucked behind a padded member which is connected to the end of a vertical bar extending downard, the vertical bar being pivotally connected at its other end to the frame of the bench near the user's knees, such that the said vertical bar may be rotated about its pivot axis to a position approximately planar with the platform.  Thus, when the user straightens his leg by flexing the quadriceps, he pivots his lower leg forward approximately 90.degree.  so that it is planar with the platform.  A resistance may be added to the pivoting bar such as weights, an elastic or spring means, hydraulic pressure resistance or a pulley system. 
This leg exercise means is deficient in at least two respects.  Firstly, the lower leg which has a pivoting range at the knee of 135.degree.  or more, is not provided with its full range of motion, and therefore, the full potential benefit of the exercise is not achieved.  Generally, the greater the range of angular movement of a joint during an exercise, the more benefit is derived from that exercise.  Secondly, there is a tendency for some bench users performing this leg exercise to lift the whole leg from the hip rather than just using the quadriceps (e.g. to cheat on the exercise).  Therefore, proper isolation of the select muscle groups for which this exercise is designed is not achieved using the prior art device.  The present invention solves the foregoing problems. 
The second type of leg exercise means associated with the use of an exercise bench enables the development of the back thigh and buttocks muscles, and more specifically, the hamstrings (biceps femoris, semimembranosus) and gluteus maximus.  In this exercise, a user lies on the bench platform on his stomach with his leg tucked under a padded portion of a horizontal bar extending approximately planar with the platform, which bar is pivotally connected to the frame in a manner similar to the aforementioned leg extension exercise bar.  Weights or other resistance means are applied to the bar as previously described.  The user curls his lower legs upward and towards his buttocks. 
This prior art exercise bench is deficient in a number of respects in connection with the performance of the back leg and buttocks exercise described above.  In performing this exercise on the prior art flat bench, as the horizontal bar is raised by the user's legs, the hips act as a fulcrum between the leg exercise bar pushing downward and backward on the leg, on the one hand, and the weight of the upper torso of the user, on the other hand.  There is a tendency in this exercise to flex the back muscles to resist this fulcrum effect and to utilize the upper torso to rock or jerk the leg exercise bar upward.  This rocking or jerking, in turn, can cause back muscle strain or more serious back injury.  The present invention minimizes the risks of such strain or injury.  The present invention also allows the user to isolate the muscle groups for which this exercise is intended, by making it difficult to utilize other muscles to cheat on the exercise. 
To facilitate the performance of the leg extension exercise, there is a prior art exercise bench having a platform including head and foot sections divided by a hinge which is fixed in position at the horizontal plane of the platform, and a means for raising the end of the foot section of the platform.  Using this bench, the above-noted deficiencies with regard to the leg extension exercise are avoided.  However, this prior art bench cannot be used or adapted to aid in the performance of the leg curl exercise for which the hinge should be raised above the level of the platform. 
A prior art bench is also available having a platform fixed in the shape of an upside down "V" to overcome the above-described deficiencies of a flat bench for performing the second type of leg exercise.  However, this prior art bench is conveniently usable for performing the leg curl.  Moreover, none of the prior art exercise benches disclose the selective vertical translation of the pivoting means in the middle section of the platform. 
Prior art exercise benches are also known in the art which combine the two types of leg exercise means into a single apparatus.  In such benches, the horizontal bar is attached perpendicularly to a downward extending vertical bar to form a combination leg exercise means, which attaches to the frame of the bench near the intersection between said bars.  The foregoing deficiencies of the two leg exercise means applies equally to this combination system. 
There is a trend in the improvement of these exercise benches toward providing improved safety, increasing the number of different muscle groups which can be developed by its use, and enabling the selective isolation of various muscle groups for exercise.  Thus, a weight lifter can specfically strengthen desired muscle groups.  Similarly, body builders who, unlike weight lifters, exercise to develop the size and appearance of specific muscle groups, rather than strength, can also selectively isolate desired muscle groups.  The present invention enables exercise bench users to achieve their goals by providing safer exercise equipment and improved isolation of the muscle groups.