RELATED APPLICATIONS 
       [0001]    This patent claims priority to U.S. Provisional Patent Application No. 62/182,253, filed on Jun. 19, 2015, entitled “Lockable Finger System and Related Methods.” The entirety of U.S. Provisional Patent Application No. 62/182,253 is incorporated herein by reference. 
     
    
     FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0002]    This invention was made with government support under H133G120059 awarded by the National Institute for Disability and Rehabilitation Research (NIDRR). The government has certain rights in the invention. 
     
    
     TECHNICAL FIELD 
       [0003]    The application relates generally to prosthetic devices and in particular, to a lockable finger for a prosthesis. 
       BACKGROUND 
       [0004]    An estimated 41,000 people in the United States have lost use of one or both of their upper limbs through amputation at or above wrist level. See K. Ziegler-Graham, et al., “Estimating the prevalence of limb loss in the United States: 2005 to 2050 ,” Archives of Physical Medicine and Rehabilitation, vol.  89, pp. 422-429, March 2008. Loss of the arm and hand profoundly limits everyday activities such as dressing and eating, affects social interactions and personal relationships, and can threaten basic independence. In particular, difficulty in grasping and holding objects impedes leisure activities and may prevent a return to employment. The most effective treatment for limb loss is replacement of the missing limb with a prosthetic device. Most upper limb amputations are caused by trauma and occur in relatively young, active individuals, who need prostheses that effectively replace functional dexterity of the lost hand. 
         [0005]    Prosthetic devices for individuals with upper limb loss include myoelectric devices (which are controlled by electromyographic (EMG) signals from muscle) and body-powered prostheses (in which movement of a remaining joint, e.g., shoulder flexion, is physically coupled to the prosthetic joint through a harness and Bowden cabling system). A Bowden cabling system, in general, is a flexible cable system which transfers a mechanical force and operates inside a hollow tube. In prosthetics, Bowden cabling systems are used to operate a body-powered device. For example, the user wears a harness across his or her shoulders. The Bowden cable attaches at one end to the harness and the opposite end attaches to the prosthesis (hand or hook or some other device used to manipulate objects in the user&#39;s environment). The user then moves his or her shoulders (typically scapular retraction/protraction) to create tension in the cable which results in movement in the terminal device. A Bowden cable can be useful for this application because the hollow tube surrounding the moving cable protects the user&#39;s skin from the friction of the moving cable. 
         [0006]    The terminal device of a prosthesis is the means by which a user directly interacts with and manipulates their environment. It is the device worn at the end of the prosthesis used to interact with the surrounding environment, and often is considered part of a prosthesis. Examples of terminal devices includes hooks and hands. Terminal devices may alternatively take on many unique and specialized shapes. For example, there are several different special terminal devices for playing sports such as baseball or golf. Thus, the functional utility of the terminal device often plays a role in determining the user&#39;s overall ability to perform necessary or desired activities. 
         [0007]    The human hand is a highly complex mechanism with many joints. The MCP joint is the “Metacarpophalangeal joint”—these joints are what we commonly refer to as our knuckles. The PIP joint is the “proximal interphalangeal joint”—these are the joints we think of when we think of bending our fingers, the middle joint between the knuckles and the small joint at the finger tips. There are different classifications of common grasps that the hand uses when performing activities of daily living. One distinction is the difference between power and precision grasps. One study has shown that daily usage of these two types of grasps is fairly equal. See J. Z. Zheng, et al., “An investigation of grasp type and frequency in daily household and machine shop tasks,” presented at the IEEE International Conference on Robotics and Automation, 2011. Examples of functional hand grasps include 3-jaw chuck, fine-pinch, trigger, and cylindrical grasps. Examples of precision grapes include fine pinch, trigger, and 3-jaw chuck. Examples of power grasps include cylindrical grasp and “power” grasp (like a fist). Users of prosthetics can use power grasps, for example, for tasks like holding heavy objects, holding drinking glasses, or positioning a jar to open. 
         [0008]    Most myoelectric prostheses are electrically powered by batteries and rely on limit switches, potentiometers, force sensitive resistors, or myoelectric sensors for control inputs. Currently available myoelectrically controlled hands are cosmetically appealing and can apply high pinch force, but are quite heavy, are not robust, and are very expensive. Recent myoelectric hands such as the iLimb ultra (Touch Bionics) and the Bebionic 3 hand (SteeperUSA) have achieved a variety of grasps by powering each finger in the hand. These hands can accomplish several grasps, including power grip, key grip, 3-jaw-chuck, and fine pinch; however, in order to accomplish these grasps multiple actuators are required to control each finger individually, which adds weight to the hand and increases its size. 
         [0009]    Body-powered terminal devices include prehensors (e.g., split-hooks and other non-anthropomorphically shaped terminal devices) and hands. Body-powered prehensors and hands are available in one of two modes. Voluntary open (VO) devices are opened by actuation of the Bowden cable and have a default closed position, whereas voluntary close (VC) devices have a default open position and are closed by actuation of the Bowden cable. Both VO and VC devices provide advantages and disadvantages to the user, depending on the task at hand. 
         [0010]    In general, body-powered prehensors are considered more functional than body-powered hands. See C. M. Fryer, et al., “Body-Powered Components,” in  Atlas of Amputations and Limb Deficiencies , D. G. Smith, et al., Eds., 3rd ed Rosemont, Ill.: American Academy of Orthopaedic Surgeons, 2004, pp. 131-143 (hereinafter, “Fryer”). Body-powered hands often do not look natural, and functionally they can be slow, heavy, and awkward, and provide a weak grip force. They do not open very wide, and the user must expend a lot of energy to operate them. 
         [0011]    Both VO and VC hands are commercially available—such as the APRL VC hand; the Becker Lock-Grip hand, and the Sierra VO hand. However, VO hands are seldom used due to their poor pinch forces (as explained in the Fryer reference) and the relaxed position of the VC hands is an open grasp, which is not cosmetically appealing. In addition, these body-powered prosthetic hands are very bulky in the palm section, which further impacts cosmesis. The added weight of a cosmetic shell makes the weight of body-powered hands (300-450 g) heavier than certain prehensors (113-354 g), and similar to certain myoelectric hands (250-440 g). Finally, during pinch grips, substantial user force can be lost due to deformation of the cosmetic glove. These functional and cosmetic issues together result in low user-acceptance rates for available body-powered hands (see Fryer). 
         [0012]    Currently available body-powered hands provide a single degree of movement, actuated through a Bowden cable that drives all of the fingers together. In some devices, movement of the thumb is coupled to movement of the fingers, in others the fourth and fifth fingers remain stationary, or their linkages are compliant, for example they are made of a compliant rubber or are biased by a spring, rather than being rigid (such as hard plastic is rigid). In all cases, only a single grasp can be obtained. 
         [0013]    Body-powered fingers typically have a single axis of rotation, located at the metacarpophalangeal (MCP) joint. Such a design requires a pre-flexed proximal interphalangeal (PIP) joint, which does not result in a cosmetically appealing palm-flat posture. 
         [0014]    However, it is important to note that many grasps may be described as a static posture with movement of a subset of fingers. For example, in cylindrical grip, all of the fingers move together. In three-jaw chuck grip, the fourth and fifth digits are fully flexed, and only the second and third digits move. In trigger-grip, the third, fourth, and fifth digits are fully flexed, and only the second digit moves. 
         [0015]    Fine manipulation tasks usually involve the first finger and thumb, or the first finger, second finger, and thumb. The three-jaw-chuck grip (which uses the thumb, middle, and index fingers) provided by existing prostheses serves as a compromise between power and precision grasps. The fourth and fifth fingers do not contribute functionally to fine manipulation; they are present principally for cosmesis in most available hands, and secondarily used for object stabilization. In body-powered devices, they do not transmit force from the Bowden cable to objects, but they often get in the way of many precision grasps, as the palm and fingers of the hand are typically positioned in the user&#39;s line of sight to the object being grasped. This can prevent visual feedback, unless the user adopts an unnatural posture. 
         [0016]    Thus a grasping device that is able to maintain a static posture while select digits are actuated could achieve a substantial subset of important grasps, without the need for multiple actuators. In addition, a prosthetic grasping device only requires actuation for digit flexion—it is not necessary to actuate digit extension, which can be achieved passively using springs. 
       SUMMARY 
       [0017]    In one embodiment, a prosthetic device may comprise a prosthetic digit, an engagement portion, and a stopping portion. The engagement portion is capable of engaging with the stopping portion to lock the prosthetic digit in a position of flexion in response to a force applied to the prosthetic digit. 
     
    
     
       DESCRIPTION OF THE FIGURES 
         [0018]      FIG. 1  shows a perspective view of a prosthetic hand arranged in a “fine pinch” position. 
           [0019]      FIG. 2  shows a perspective view of a prosthetic hand arranged in a “fine pinch” position with the middle finger, ring finger, and pinky finger locked in a position of flexion. 
           [0020]      FIG. 3  shows a perspective view of a prosthetic hand arranged in a “three-jaw-chuck” position. 
           [0021]      FIG. 4  shows a perspective view of a prosthetic hand arranged in a “three-jaw-chuck” position with the ring finger and the pinky finger locked in a position of flexion. 
           [0022]      FIG. 5  shows a perspective view of a prosthetic hand arranged in a “palm flat” position. 
           [0023]      FIG. 6  shows a perspective view of a prosthetic hand arranged in a “cylindrical grasp” position. 
           [0024]      FIG. 7  shows a side view of one embodiment of a lockable finger. 
           [0025]      FIGS. 8 a , 8 b , and 8 c    show three different configurations of the lockable finger shown in  FIG. 7 . 
           [0026]      FIG. 9  shows a second embodiment of a lockable finger. 
           [0027]      FIGS. 10 a , 10 b , and 10 c    show side view stages of the lockable finger shown in  FIG. 9  as it is locked in a position of flexion. 
           [0028]      FIG. 11  shows a perspective view of the lockable finger shown in  FIG. 9 . 
           [0029]      FIGS. 12 a , 12 b , and 12 c    show side views of the lockable finger shown in  FIG. 9  as it is being unlocked. 
           [0030]      FIGS. 13 a  and 13 b    show one embodiment of a lockable finger with a contoured linkage. 
           [0031]      FIGS. 14 and 15  show one embodiment of a lockable finger in use with a hand, where the hand is configured to operate in voluntary-open (VO) mode. 
           [0032]      FIGS. 16 and 17  show one embodiment of a lockable finger in use with a hand, where the hand is configured to operate in a voluntary-close (VC) mode. 
       
    
    
     DETAILED DESCRIPTION 
       [0033]    The locking aspects of the embodiments described here may be utilized in a variety of prostheses. Examples include those described in U.S. patent application Ser. Nos. 14/030,095, 14/614,187, 14/614,231, and 14/614,256, all of which are incorporated by reference. 
         [0034]    Referring to the drawings, embodiments of the device are illustrated and indicated numerically in the accompanying figures. 
         [0035]      FIG. 1  shows a view of a prosthetic hand  10  having a thumb  15 , an index finger  20 , a middle finger  30 , a ring finger  40 , and a pinky finger  50 . The hand  10  is arranged in a position known as “fine pinch,” where the index finger  20  and the thumb  15  pinch together in an effort to grasp a relatively small object, such as a pen. As shown in  FIG. 1 , the middle finger  30 , the ring finger  40 , and the pinky finger  50  are not used to grasp the object. In certain situations, fingers  30 ,  40 , and  50  can even get in the way of the user&#39;s attempt to use the “fine pinch” grasp to pick up the object. For instance, they can block the view of the object from the user, or physically interfere with the object. 
         [0036]      FIG. 2  shows a view of the same prosthetic hand  10 , but with fingers  30 ,  40 , and  50  each locked in a position of flexion. Locking the fingers  30 ,  40 , and  50  when the hand is in the fine pinch position allows the user to more accurately grasp the pen or other object, including helping the user see the object he or she is trying to grasp. 
         [0037]      FIGS. 3 and 4  show the hand  10  in a “three-jaw-chuck” position, where both the index finger  20  and the middle finger  30  pinch together with the thumb  15 . A user may use the three-jaw-chuck position, for instance, when the user wants to grasp an item like a pen or silverware, which requires precision but some extra force and stability. In  FIG. 3 , none of the digits is locked, which may lead to the same difficulties as described above; namely, that the unlocked digits may get in the way or block a view of the object. In  FIG. 4 , the ring finger  40  and the pinky finger  50  are locked when the hand  10  is in the three-jaw-chuck position, which gives the user a more accurate grip of the object he or she is trying to grasp. 
         [0038]    In various embodiments, each finger (including the thumb) may be locked in a position of flexion, and unlocked from that position, independently of the other fingers. This allows one or more fingers to be locked while the other fingers are unlocked, as shown in  FIGS. 1-4 , while also allowing for all fingers to be unlocked (for example, as shown in  FIG. 5 , where the hand  10  is shown in the “palm flat” position and in  FIG. 6 , where the hand  10  is shown in the “cylindrical grasp” position). 
         [0039]    A lockable finger may be employed on many different kinds of gripping devices, such as prosthetic hands. At least one lockable finger may be used in a gripping device, such as a prosthetic hand, that operates in a voluntary-close (“VC”) mode or a voluntary-open (“VO”) mode. At least one lockable finger may be used in a gripping device, such as a prosthetic hand, that operates in both a VC mode and a VO mode, for instance by switching between the VC mode and the VO mode. Embodiments of a VO/VC device are described in further detail in U.S. patent application Ser. No. 14/030,095 to J. Sensinger, titled Gripping device with Switchable Opening Modes, for instance, which is incorporated by reference. Additionally, a lockable finger may be employed on myoelectric prosthetic hands, where electrical signals generated by the user are used to help control the myoelectric prosthetic hand. Embodiments of a myoelectric prosthetic hand are described in further detail in U.S. patent application Ser. No. 14/614,256 to J. Sensinger and J. Lipsey, titled Modular and Lightweight Myoelectric Prosthesis Components and Related Methods, for instance, which is incorporated by reference. 
         [0040]      FIG. 7  displays a side view of one embodiment of a lockable finger. As shown in  FIG. 7 , finger  100  is positioned adjacent to the latch  108 . The finger  100  may have a surface  109  and notch sides  110 . As shown in  FIG. 7 , the surface  109  may be rounded. The finger  100  may be configured to receive a locking force  134  from an external source. The locking force  134  may be provided by the user, for instance, by pushing on the finger  100  using his or her other hand, by pushing the finger  100  against an object (such as a wall, a table, or some other object in the user&#39;s environment). The locking force  134  may also be provided by other means, such as by another person, or by some other method. 
         [0041]    A latch  108  may be positioned adjacent to the proximal finger linkage  100 . In one embodiment, the latch  108  may comprise a spring-loaded cam. The latch  108  pivots about a pivot  112 . In one embodiment, a spring  111  may provide a torque on the latch  108  in the clockwise direction indicated by arrow  111   t  and a spring  116  may provide a torque on the finger  100  in the counterclockwise direction indicated by arrow  114   t . When a locking force  134  is applied to the finger  100 , the finger  100  engages with the latch  108  to lock the finger  100  in a position of flexion. The force bias introduced by the spring  116  also prevents the finger  100  from flexing or extending independently of actuation, but it may be overpowered by the locking force  134 . In the embodiment shown in  FIG. 7 , when the locking force  134  is applied to the finger  100 , if the torque provided by the locking force  134  is greater the torque  114   t , the finger  100  begins to rotate about pivot  114 . (Spring  116  and the other springs displayed in the figures with a dashed lines may be torsion springs or representations of other springs known in the art.) 
         [0042]    As the finger  100  rotates about the pivot  114 , the surface  109  comes into contact with a portion  107  of the latch  108 . In one embodiment, the portion  107  may protrude from the latch  108 . When the surface  109  comes into contact with the portion  107 , the surface  109  may push the portion  107  in a direction that causes the rotation of the latch  108  about the latch pivot  105 . As the locking force  134  continues to be applied to the finger  100 , the finger  100  continues to rotate in a clockwise direction around pivot  114  until the surface  109  no longer is in contact with the latch  108 . Once the surface  109  is no longer in contact with the latch  108 , a spring  112  may cause the latch  108  to rotate in a clockwise direction, such that the portion  107  or another portion of the latch  108  moves to a position underneath the finger  100 . When the finger  100  is sufficiently flexed by the locking force  134  so that it is in a position of full flexion (defined below), the latch  108  and the finger  100  have become positioned with respect to each other so that it the latch  108  is positioned underneath the notch sides  110  of the finger  100 . When the locking force  134  is no longer applied, the spring  116  causes the finger  100  to rotate counterclockwise around pivot  114  to return to extension. However, the notch sides  110  press against the latch  108 , therefore preventing the finger  100  from extending. In this way, the latch  108  serves as a stopping element, and in the embodiment shown in  FIG. 7 , the engagement of the finger  100  with latch  108  locks the finger  100  in a position of flexion. 
         [0043]    In the embodiment shown in  FIG. 7 , the latch  108  may be disengaged manually from the finger  100 . For instance, the latch  108  may be disengaged by pressing an unlock mechanism  136  that is coupled to the latch  108 . The unlock mechanism  136  may push against the latch  108 , causing the latch  108  to rotate away from the finger  100 , such that the latch  108  is no longer positioned underneath the notch sides  110  of the finger  100 . In one embodiment, the unlock mechanism may be a protruding portion of the latch itself. In another embodiment, the unlock mechanism  136  may comprise a bias spring (not shown) that disengages the latch  108  from the finger  100 . In one embodiment, the bias spring may be balanced against the torsion spring on the latch. As the unlock mechanism  136  is not under high stress, it can be miniaturized. 
         [0044]    Once the latch  108  is disengaged from the finger  100 , the spring  116  actuates the finger  100  to a position that allows it to be actuated by the power source  150 . In a hand where the default position of the finger  100  is one of extension (in other words, a “voluntary-close” hand), the spring  116  returns the finger  100  to a default extended position. In a hand where the default position of the finger  100  is one of flexion (in other words, a “voluntary-open” hand), the finger  100  is released from the locked position by spring  116  but remains in a semi-flexed position. The spring  116  biases the finger  100  towards extension so as to maintain the position of the actuator linkage  104  at the bottom of the actuator slot  102 . 
         [0045]    In an embodiment, the finger  100  may be configured so that it can remain in a locked position while at least one other finger on the same hand is free to move. This feature may be important to the user, for instance, if the finger  100  and the at least one other finger are actuated or otherwise moved using the a common linkage. In this embodiment, having one locked finger does not prevent the other one or more fingers from actuating freely. This feature may also be important for the user to move the finger  100  out of the way during an activity that does not require use of the finger  100 , such as activities involving trigger grip, fine pinch, or 3-jaw-chuck. This feature may also be important for the user if the user desires to lock the finger  100  to assist with a particular type of grasp. For example, the thumb may be locked to assist with the user grasping an object. Alternately, the thumb may be locked to move it out of the way for activities that do not require the thumb. 
         [0046]    For instance, as shown in  FIG. 7 , an actuator slot  102  is provided in the finger  100  and is configured to receive an actuator linkage  104 . The actuator linkage  104  may be coupled to a common link  140  which in turn may be coupled to a power source  150 . The arrow  106  in  FIG. 7  indicates the direction of the actuation force that the power source  150  provides to the actuator linkage  104  through the common link  140 . The power source  150  may be an externally powered motor, a Bowden cable attached to the user&#39;s body, or another powered mechanism. 
         [0047]    One embodiment achieves movement of unlocked fingers through the use of a slot in the proximal finger linkage at the attachment point of the actuator linkage. Forces on the proximal finger linkage only occur when the actuator linkage is at the end of the slot, ensuring larger surface areas and thus acceptable material pressures. Locking one or more fingers does not interfere with movement of the other fingers in either VO or VC modes, since the actuator link is free to move throughout the range of motion of the slot. Similarly, locking the fingers does not interfere in any way with the ability of the user to obtain proprioceptive input during use of the device, since the actuator link still moves throughout the entire range of motion and is not impeded in any way. As shown in  FIG. 7 , an actuation slot  102  in the actuation path enables the digit to move through its range of motion to a flexed position without inhibiting the actuator. Manually flexing the finger to a lockable position allows latch  108  to engage with the notch  110  on the proximal finger linkage  100 . Latch  108  is biased by a spring  112  to close so that once the digit reaches the locked position, latch  108  will automatically engage with notch  110  and statically lock the digit in a flexed position. 
         [0048]      FIGS. 8 a , 8 b , and 8 c    shows three different representations of the finger  100 . Each representation shows the finger  100  in a different configuration during a different stage of use. (It should be noted that in  FIGS. 8 a , 8 b , and 8 c   , spring  116  is represented as a spring attached to a wall, rather than as a torsion spring as shown in  FIG. 7 , so that the force it provides on the finger  100  can be more easily visually compared to the locking force  134 .) 
         [0049]      FIG. 8 a    shows the finger  100  actuated to the maximum extent of the possible range of the motion of flexion. This position is known as “full flexion.” In a preferred embodiment, full flexion occurs at a greater range of motion than can be actuated by the power source  150 . This prevents the power source  150  from accidentally locking the digit. In an embodiment, the finger  100  may be configured to reach full flexion just beyond the point necessary for it to engage with the latch  108 . In order for full flexion to occur at a greater range of motion than can be actuated by the power source  150 , the finger  100  may be constructed so that it can flex beyond the functional range of movement. For instance, if 90 degree flexion is required to accomplish most activities of daily living, the finger  100  may be constructed so that it can flex up to 110 degrees. In this way, the power source  150  is limited to flexing the finger  100  to 90 degrees and then may reach a hard stop. At this point, the user can still manually flex the finger to 110 degrees of flexion, at which point the finger locks. Note that the actuator linkage  104  is not bottomed out in the slot  102 . In this embodiment, it indicates that in order to engage the latch  108 , the finger  100  must be flexed beyond the capabilities of the power source  150 . For the power source  150  to be moving the finger  100  into flexion, the actuator linkage  104  must be bottomed out in the slot  102 . However, since the actuator linkage  104  is not in the bottom of the slot  102 ,  FIG. 8 a    is showing the finger  100  being flexed manually beyond the capabilities of the power source  150 . 
         [0050]      FIG. 8 b    shows the finger  100  locked in a position of flexion. Once the finger  100  has been pushed into the position of full flexion, the latch  108  flips into place, engaging with the notch sides  110  to prevent extension of the finger  100 . This position can be accomplished by the locking force  134 , rather than by the actuator force  106 . 
         [0051]      FIG. 8 c    shows the finger  100  locked in a position of flexion, with the actuator linkage  104  free to move throughout its normal range of motion in the actuation slot  102  in the proximal digit linkage  100 . Locking of the finger  100  does not prevent the at least one other finger (including a thumb) from being actuated through common link  140  and power source  150 . 
         [0052]    In another embodiment, a four-bar linkage may be inserted into the fingers, in order to kinematically couple PIP flexion to MCP flexion. In this manner the fingers remain relatively flat during palm-flat, yet achieve the required PIP flexion angle for chuck-grasp. The four-bar linkage maximizes pinch force while still achieving a kinematically acceptable motion profile, within the constraints of an anthropomorphic hand envelope. Another embodiment of a locking finger is shown in  FIG. 9 . The finger  200  may have a proximal phalanx  201  and a distal phalanx  202 . (A “phalanx” is the part of the finger between two joints, while “proximal” and “distal” are used to describe the relative positions of the phalanxes to the body, with the “proximal” phalanx closer to the body and the “distal” phalanx further from the body.) As shown in  FIG. 9 , in one embodiment, an upper portion of the distal phalanx  202  may be configured in a cam shape, so that a portion  232  of the distal phalanx  202  extends from the surface of the distal phalanx  202  and allows for the receipt of a pin  230  that connects the distal phalanx  202  to the locking linkage  226 . In one embodiment, a support member  270  may be provided that connects to the proximal phalanx  201  by MCP joint  214  and connects to the locking linkage  226  by pin  228 . (MCP joint  214  may be a pin or other suitable fastening mechanism.) Pin  230 , positioned as shown in  FIG. 9  to the left of the PIP joint  220 , may connect the other end of the locking linkage  226  to the distal phalanx  202 . It should be understood that while pins may used to connect different components of the finger, as shown in the various figures herein, other fastening mechanisms known in the art may be used instead. A slot  210  and actuator linkage  204  may be provided, for the finger  200  to be flexed and/or extended independently of other fingers. 
         [0053]    In one embodiment, the user pushes on the distal phalanx  202  to lock the finger  200 . The finger  200  may lock due to engagement between the locking linkage  226  and the stopping element  208 . In one embodiment, the stopping element  208  may be located on the proximal phalanx  201 . 
         [0054]    The locking force  234  applied to the distal phalanx  202  causes the distal phalanx  202  to rotate about PIP joint  220 . As shown in  FIG. 9 , the distal phalanx  202  begins to rotate in a clockwise direction about joint  220 , and continues to rotate until the locking linkage  226  pushes against the stopping element  208 . In doing so, the distal phalanx  202  is flexed beyond the full flexion provided by the actuator. Additionally, the locking linkage  226  passes through the singularity defined by the pin  228 , pin  230 , and PIP joint  220  being collinear. Once the locking linkage  226  passes through this singularity, the finger  200  remains locked (as described in further detail below).  FIGS. 10 a , 10 b , and 10 c    show side views of the stages of the finger  200  as it is locked in a position of flexion. 
         [0055]    A more detailed description of a “singularity” may be found in U.S. patent application Ser. No. 14/030,095 to J. Sensinger, titled Gripping device with Switchable Opening Modes. Briefly, a mechanical singularity is when the position or configuration of the mechanism and its subsequent behavior cannot be predicted. With respect to finger  200 , when the pin  228 , pin  230 , and PIP joint  220  are aligned, it is not possible to determine whether pin  230  will rotate about PIP joint  220  in a clockwise direction (and so working to flex the finger  200 ) or in a counterclockwise direction (and so working to extend the finger  200 ). 
         [0056]      FIG. 11  shows a perspective view of the finger  200  locked in a position of flexion, where the locking linkage  226  is prevented from further flexion by stopping element  208 .  FIG. 11  also shows wall  252 , which provides common support as shown for the components of the finger  200 . Another wall on the opposite side of the stopping element  208  and locking linkage  226  may be provided, but is omitted from  FIG. 11  so that the reader can better see the internal arrangement of the finger  200 . 
         [0057]    In one embodiment, at least one passive spring provides a torque that keeps the finger in a locked position. In one embodiment, shown in  FIG. 11 , spring  224  provides a torque in a counterclockwise direction. When the finger  200  is locked, the torque from spring  224  attempts to extend the proximal phalanx  201 . However, as the proximal phalanx  201  rotates towards extension, the locking linkage  226  creates a clockwise rotation about pin  230 , due to the fixed radius of pin  230  from PIP joint  220 . This clockwise rotation of the distal phalanx  202  moves the locking linkage  226  towards extension, where it collides with the stopping element  208 . In order for the finger  200  to continue into extension, the locking linkage  226  would need to occupy the space of the stopping element  208 . However, since the stopping element  208  is present, the spring  224  at the MCP joint  214  acts to continuously engage the locking linkage  226  and the stopping element  208 . This continued engagement of the locking linkage  226  with the stopping element  208  keeps the finger  200  locked in the position of flexion. 
         [0058]      FIGS. 12 a , 12 b , and 12 c    display three side views of the finger  200  being unlocked. In  FIG. 12 a   , a force  260  may be applied to a top surface of the proximal phalanx  201 . The force  260  may be applied by the user&#39;s other hand, or by an object in the user&#39;s environment. The force  260  causes the finger  200  to flex. In one embodiment, the force  260  causes the proximal phalanx  201  to rotate in a clockwise direction, which rotates the locking linkage  226  to the singularity.  FIG. 12 b    shows the locking linkage  226  at the point of singularity, at which point the PIP spring  222  pulls the distal phalanx  202  outward and MCP spring  224  pushes the proximal phalanx  201  towards extension. The springs  222  and  224  therefore work together, in the directions indicated by the arrows shown around the MCP joint  114  and the PIP joint  120 , respectively, to push the finger  200  back into the unlocked position shown in  FIG. 12   c.    
         [0059]    In an embodiment of the finger, the locking portion may physically configured to avoid catching or otherwise interfering with other portions of the system. For example, as shown in  FIGS. 13 a  and 13 b   , the locking linkage  226  may be contoured so that its movement between extension and flexion of the finger  200  does not interfere with the actuator linkage  204  or the support member  206 . Equivalent support members are shown in the profile views  FIGS. 14-16 . As shown in  FIG. 13 a   , the locking linkage  226  is contoured to receive support member  206  when the finger  200  is extended. As shown in  FIG. 13 b   , the same contour of the locking linkage receives the actuator linkage  204  when the finger  200  is extended. 
         [0060]    In alternative embodiments, a four-bar mechanism is used in each finger, and a fifth bar in each finger is used to transmit torque from the body-powered mechanism, such as a Bowden cable, to the finger. These fifth bars may be rigidly linked across the fingers, and connected by some method to a VO/VC switch mechanism, allowing the hand to function either as a VO or a VC hand. Each of the actuator links is attached to the output of the VO/VC mechanism, such that the output of the VO/VC mechanism is a linkage. The actuator links are attached through a linear bushing to ensure that each of the fingers moves at the same rate. The VO/VC mechanism is in turn attached to the Bowden cable or external motor, which provides the input to the VO/VC mechanism. 
         [0061]      FIGS. 14 and 15  show embodiments of a locking finger in use with a hand  300 , where the hand  300  is configured to operate in voluntary-open (VO) mode. In  FIG. 14 , the hand  300  is shown coupled to an actuation lever  310 . Finger  320  and finger  330  each have a slot  320   s  and  330   s , respectively. In the embodiment shown in  FIG. 14 , the actuation lever  310  is coupled to the fingers  320  and  330  by a transmission. Actuation of the actuation lever  310  causes rotation of the actuation lever  310  about a central pivot, causing switch  360  to rotate in a counter clockwise direction. In doing so, switch  360  acts on pin  362  and link  364  to transfer an upward force through pin  366  to the coupler linkage  368 . The coupler linkage  368  is connected by pin  370  to driver linkages  340 , which actuate the fingers  320  and  330 . The driver linkage  340  is coupled to finger  320  by a pin  320   p  and is coupled to finger  330  by a pin  330   p . Pin  320   p  is positioned in the slot  320   s  and pin  330   p  is positioned in the slot  330   s . As the driver linkage  340  moves in the upward direction, each pin  320   p  and  330   p  allows its respective finger  320  and  330  to move by releasing it from VO mode and allowing it to extend. In one embodiment, extension may be provided by one or more springs coupled to each finger. 
         [0062]    However, a finger, for example finger  330 , may be locked, so that actuation of the actuation lever  310  does not cause the finger to extend. For example,  FIG. 15  shows finger  330  locked in a position of flexion even as the actuation lever  310  has been actuated by a force  350 , such as being pulled on by a Bowden cable. As the hand  300  is actuated, the driver linkage  340  moves in an upward direction. Spring  320   sp  pulls finger  320  into a position of extension. Finger  330  has a similar spring around its joint, but because the finger  330  is locked, instead of that spring acting on finger  330  to cause extension, pin  330   s  instead slides through slot  330   p , and comes to rest at the top of slot  330   p  as shown in  FIG. 15 . 
         [0063]      FIGS. 16 and 17  show an embodiment of a locking finger in use with a hand  300 , where the hand is configured to operate in a voluntary-close (VC) mode. As shown in  FIG. 16 , the finger  330  is locked in a position of flexion, and the pin  330   p  is at the top of the slot  330   s . The finger  320  is in a position of extension due to torque provided by a spring about main pivot  345 . In  FIG. 17 , with the hand  300  in VC mode, a force  350  is applied to flex the finger  320 . The pin  320   p  moves to the bottom of slot  320   s  to flex the finger  320 , while the pin  330   p  slides through the bottom of the slot  330   s  as the finger  330  is already locked in a position of flexion
A prosthetic device comprising a first prosthetic digit, a first engagement portion, and a first stopping portion, wherein the first engagement portion is capable of engaging with the first stopping portion to lock the first prosthetic digit in a position of flexion in response to a force applied to the first prosthetic digit.