referring now to fig1 the basic linkage elements of this invention are illustrated in a partially raised position . the frame assembly 1 comprises a pair of side plates 2 and a back plate 3 . channel member 32 see fig2 is coupled to both side plates to provide lateral strength to the frame assembly . the back plate 3 may be fastened to the wall of the loading dock by means of mounting screws or bolts , not illustrated . if the device is to be mounted on the surface of an approach such as a driveway then an angle beam 4 may be added to the front of the frame . the angle beam would then be bolted into the driveway . the side plates may be slightly elevated off the surface so that water , and debris would be swept underneath the device . as illustrated in fig1 and 2 , a pair of arms 5 pivot about a pin 6 , which passes through each of the side plates 2 . plates 30 and 31 join the arms 5 together and provide lateral strength to the restraint unit . a strut member 7 pivots about a pin 8 , which also passes through the side plates 2 . a lever assembly 9 is attached to the arms 5 and the strut 7 by means of pins 12 and 13 . the lever assembly 9 comprises a pair of bars 10 covered by a top plate 11 . the upper end of each of the bars 10 fits between two side plates 16 of the hook assembly 15 . the bars 10 are connected to the hook assembly 15 by means of pin 14 . a secondary hook 17 pivots on pin 14 between the bars 10 and inside the hook 15 , as shown in fig5 . fig1 illustrates the secondary hook 17 in an upward position and fig2 in the dotted line 17a , illustrates the hook 17 pivoted in a downward position . the hook is held up by means of a spring , not shown . the hook assembly 15 is maintained in a horizontal position by a pair of struts 18 . the struts 18 are attached to the hook assembly 15 by means of a pin 19 . the struts 18 are coupled to the arms 5 by means of pin 20 . referring now to fig3 the basic operation of this mechanism is illustrated . fig3 portrays , in line form , the salient elements of fig1 and 2 to demonstrate how the device is raised and lowered . the linkage is a so called 4 - bar type used to translate reciprocating motion into linear movement and the converse thereof . those of working skill will recognize that the geometry is not regular but chosen by member length and pin position to give the necessary movement without any substantial rotational component to the restraining member . from the description so far , the motive force to achieve this raising and lowering has not yet been described , and will be set forth herein . in fig3 the arm 5 pivots about a point a . point a corresponds to the pin 6 . the length of the strut 7 , the lever assembly 9 and the position of point c , the pin 8 , is selected so that the path of point e , pin 14 , is very close to a straight vertical line . that point of travel is illustrated by the chain vertical line in fig3 labelled &# 34 ; locus of point e &# 34 ;. the position of points f and g , which correspond respectively to pins 19 and 20 respectively , are selected so that the hook assembly 15 remains horizontal as the other members rotate . thus , as the device is raised from an inoperative position , shown in fig4 to an operative position , illustrated in fig2 point e moves vertically , and the hook assembly 15 remains in a horizontal position . referring now to fig2 the technique for raising the mechanism is illustrated so that the hook assembly 15 engages the icc bar labelled in that figure &# 34 ; icc &# 34 ;. in the embodiment of fig2 a spring 21 is employed . this is preferably a gas spring having a low spring rate . it is mounted to the frame by means of a pin 22 , which in turn is coupled to a bracket 23 attached to the back plate 3 . the piston end of the spring 21 is attached to one of the arms 5 by means of pin 24 . if needed a second parallel spring may be used attached to the other arm . as illustrated in fig2 the icc bar is attached to the rear of a truck . the restraint of this invention is raised by spring 21 until the hook assembly 15 contacts the icc bar . the bar and , thus , the truck are thus restrained from moving forward , to the right in fig2 by means of the secondary hook 17 . with the hook in the position illustrated in fig2 the load caused on the device should the truck tend to move forward would be borne exclusively by the linkage mechanism and not by the spring assembly 21 . thus , the mechanism isolates the spring and any powered elements from shock loading caused by truck movement . if the icc bar is deformed or otherwise caused to be mounted further forward on the truck , the secondary hook 17 will be forced down to a position 17a . the icc bar will then be restrained by the vertical part of the hook assembly 15 , flat vertical face 15a . fig2 also illustrates the technique by which the icc bar is released from the device . a hydraulic cylinder 25 is employed . the cylinder 25 is attached to one end of the frame by means of pin 26 . the pin 26 passes through side plates 2 . the other end of the hydraulic cylinder is fastened by pin 27 to a pair of arms 28 , which are in turn pivoted on the pin 6 . a stop bar 35 on arm 5 constrains clockwise movement of arm 28 . that is , arms 5 are considered integral to arms 28 when arm 28 abuts bar 35 . as the arms 5 raise as illustrated in fig2 the hydraulic cylinder 25 retracts . however , as the restrained vehicle floats up and down relative movement between arms 5 and 28 takes place . the cylinder is unloaded and does not follow this motion . this reduces friction and wear . to retract the device , the cylinder is actuated . as illustrated in fig4 by extending the piston from cylinder 25 , the piston extension causes pin 27 to move to the left thereby causing arm 28 to abut bar 35 and to move arm 5 for clockwise pivotal movement about pin 6 and cause downward rotation of the hook . at the same time the spring 21 is compressed . while a hydraulic device is illustrated as the technique for causing the device to retract to its inoperative position , one of working skill would recognize that an electric actuator , pneumatic cylinder or other technique of manually lowering the device , held by a latch could also be alternatively employed . additionally , the position of the mechanism and contact of the hook with the icc bar could be sensed by limit switches , not shown , and used to control the actuator and signal lights which are customary in such dock installations . fig4 illustrates a section of the device in its retracted , or inoperative position . the linkage mechanism has been fully retracted and is housed entirely within plates 2 . the front portion of the lever assembly 9 rests on the front plate 29 of the frame assembly 1 . the top plate 11 forms a flat cover which conceals and protects the overall mechanism . plate 30 has a generally horizontal portion to protect that portion of the mechanism between the retracted hook assembly and the end plate . this is illustrated in fig5 . the stored position of the device is illustrated in fig5 a top view of the device . as illustrated , the top plate 11 of the assembly 9 covers the front portion of the mechanism . the hooks 15 and 17 are lowered below the top of the frame assembly 1 . this compact design is thus achieved by nesting the components . as illustrated , all components fit between the side plates 2 of the frame assembly 1 to form a unitary device which may be mounted either on the dock approach or bolted to the dock face . this nesting occurs because the secondary hook 17 fits between the bars 10 of the lever assembly 9 . in turn , the bars 10 fit between the side plates 16 of hook assembly 15 which rests between the arms 5 . thus , as can be appreciated , by this invention a vehicle restraint extends from a low housing , approximately 12 inches high to a height exceeding 30 inches . because the purpose of any vehicle restraint is to prevent a truck from moving away from the loading dock this vertical range represents an important improvement . additionally , by the use of a secondary hook the device can capture an icc bar in a range of locations yet at the same time limit the horizontal travel or running room to half the length of the hook assembly . an additional advantage of this invention is that the assembly itself is not dependent on a particular technique or mode of power . depending on condition and preference manual , pneumatic , hydraulic or electrical actuation may be used as desired . additionally , since the device is always biased upward by means of the spring the device will follow a vehicle of varying height as it is loaded or unloaded . moreover , the device will be in the elevated or operative position and thus act in a fail - safe mode maintaining the locked position of the vehicle in the event of loss of power . it will be appreciated by one of working skill that modifications of this invention may be practiced without departing from the essential scope thereof . for example the arms 5 and 28 can be formed of a rigid member to simplify the construction .