Abstract:
There is provided a fall detection and/or prevention system, comprising one or more sensors for detecting characteristics of movement of a user of the fall detection and/or prevention system and for generating corresponding signals; processing means for analyzing the signals from the one or more sensors using a fall detection algorithm to determine if a fall has taken place or is likely to take place; wherein the processing means is further adapted to update said fall detection algorithm based on the result of the analysis of the signals and an indication whether a fall has actually taken place from the user or a third party.

Description:
TECHNICAL FIELD OF THE INVENTION 
     The invention relates to fall detection and/or prevention systems, and in particular to a fall detection and/or prevention system having a fall detection algorithm that can be adapted to the characteristics of a particular user. 
     BACKGROUND TO THE INVENTION 
     Falling is a significant problem in the care of the elderly that can lead to morbidity and mortality. From a physical perspective, falls causes injuries, while from the mental perspective, falls causes fear-of-falling, which in turn leads to social isolation and depression. 
     In terms of intervention, there are two aspects where electronic devices can assist. One is to provide an automated and reliable fall detection system, and the other is to provide a fall prevention system that provides early feedback to the user or the user&#39;s care provider if the user engages in a (more) risky situation. The first assures adequate measures will be taken in case of a fall incident, which also provides a level of reassurance to the user, and the second assists the user in staying healthy, which provides a further level of reassurance. Fall detection systems are becoming widely available, and fall prevention systems are expected to appear shortly. 
     Commonly, automated fall detection systems are centered around an accelerometer which is to be attached to the user&#39;s body. The detector tracks the signals from the accelerometer and determines that a fall has taken place if a characteristic pattern is identified. A typical pattern is a combination of a high impact value in which the acceleration signal exceeds a preconfigured threshold, followed by a period of relatively constant acceleration, for example gravity only, since the user is lying motionless on the ground. The pattern may continue by revealing activity, deviating from the relatively constant period, when the user stands up again. 
     Several refinements and extensions exist to this simple system. For example, gyroscopes and/or magnetometers can be used to measure the body&#39;s orientation to check for a sustained non-vertical position in evaluating whether a fall has occurred. 
     Current automatic fall detection systems are typically equipped with an “alarm-reset” button that the user can press to suppress false alarms (false positives—FP) before they reach a care provider, so that further intervention by the care provider is aborted. Often, the alarm-reset button, or alternatively an “alarm” button, is used to enable the user to request assistance, which, in a way, indicates a missed alarm (i.e. a false negative—FN). These two functions can appear as two separate buttons for the user to press. They can also be integrated in one physical button, in which case the function switches with the current state of the detection algorithm (no-fall versus fall detected). It should be noted however that the buttons are not required to be part of the device attached to the user&#39;s body. They could also be part of a base station, located in the home of the user, to which the sensor communicates and which further transmits an alarm to the care provider&#39;s call centre. It makes most sense to mount the button for the reset function on the base station and to have the alarm function with the sensor. 
     One problem with automatic fall detection systems is the reliable classification of falls and non-falls, characterized through sensitivity and specificity. Clearly, for reliable classification, false positives and false negatives should be suppressed as much as possible. Full reliability (i.e. no FP or FN) is only achievable if the characteristics of the signal feature set can be distinguished completely in two separate sets, one characterizing a fall incident, the other a non-fall incident. Obviously, in fall prevention, the system cannot make use of the high acceleration events in the signal, since they will not (yet) be present, and the problem of correct identification of increased risk situations is even more difficult. 
     Many techniques to arrive at correct classifications are known. They are collectively referred to as machine learning [T. M. Mitchell,  Machine Learning , McGraw-Hill, 1997]. In these methods, an algorithm is designed that classifies value combinations of features from the sensor signals as characterizing a fall or a non-fall. Using feature sets that are known to correspond to a fall or non-fall, the algorithm&#39;s parameters are adapted to provide a correct response to this training data. The amount of adaptation is usually derived from a statistical analysis of the algorithm, so that the update process converges to a situation that matches an optimality criterion. Of course, in order to be perfectly successful, it is required that the signals, i.e. their observed features, are distinguishable in the ideal, i.e. noise-free, situation. If this is not the case, errors (FP and FN) will fundamentally remain, and the task is to find an optimal setting trading these FP and FN. For an effective training of the algorithm, a sufficient amount of data samples are needed, so that the classification boundaries can be optimized for the variance in the feature set. 
     A problem that remains concerns the acquisition of the reference data so that it is of sufficient size and sufficiently represents the classes to be distinguished. Since people move in different ways, and hence will generate different signals and patterns, it is hard to provide a “one-size-fits-all” set of reference data. 
     Therefore, it is an object of the invention to provide a fall detection and/or prevention system that can be adapted to a particular user&#39;s fall or activity characteristics in order to improve the reliability of the fall detection algorithm, without requiring the user to spend a dedicated period of time training the detector. It is a further object of the invention to provide a fall detection and/or prevention system that can adapt to changes in the user&#39;s activity characteristics (for example, due to ageing). 
     SUMMARY OF THE INVENTION 
     In accordance with a first aspect of the invention, there is provided a fall detection and/or prevention system, comprising one or more sensors for detecting characteristics of movement of a user of the fall detection and/or prevention system and for generating corresponding signals; processing means for analyzing the signals from the one or more sensors using a fall detection algorithm to determine if a fall has taken place or is likely to take place; wherein the processing means is further adapted to update said fall detection algorithm based on the result of the analysis of the signals and an indication whether a fall has actually taken place from the user or a third party. 
     Therefore, as an indication of whether a fall has actually taken place is compared with the result of the fall detection algorithm, the fall detection algorithm can be updated in order to reduce the incidence of false positives and false negatives. 
     Preferably, the processing means is adapted to generate an alarm signal in the event that a fall has taken place or is likely to take place. 
     In a preferred embodiment, the system further comprises a memory for storing the signals, an indication from the fall detection algorithm of whether the fall detection algorithm has determined that a fall has taken place or is likely to take place and the indication whether a fall has actually taken place. 
     In a further embodiment, the system further comprises means for generating a trace for a plurality of signals from the one or more sensors, and the memory is adapted to store the trace of the signals. 
     Preferably, the indication whether a fall has actually taken place comprises a reset signal. 
     Preferably, the processing means determines that the fall detection algorithm has provided a false positive in the event that the fall detection algorithm detects that a fall has taken place and the reset signal is present, and the processing means is adapted to update the fall detection algorithm accordingly. 
     Preferably, the processing means determines that the fall detection algorithm has provided a true positive in the event that the fall detection algorithm detects that a fall has taken place and the reset signal is not present, and the processing means is adapted to update the fall detection algorithm accordingly. 
     In a further embodiment, the system further comprises means for receiving the reset signal from a third party. 
     In a further embodiment, the system further comprises a first user operable component for allowing a user to selectively generate the reset signal. 
     In a further preferred embodiment, the system further comprises a second user-operable component for generating an alarm signal. 
     Preferably, the processing means determines that the fall detection algorithm has provided a false negative in the event that the fall detection algorithm does not detect that a fall has taken place and the alarm signal is present, and the processing means is adapted to update the fall detection algorithm accordingly. 
     Preferably, the processing means determines that the fall detection algorithm has provided a true positive in the event that the fall detection algorithm detects that a fall has taken place and the alarm signal is present, and the processing means is adapted to update the fall detection algorithm accordingly. 
     Preferably, the fall detection algorithm comprises one or more feature sets representing signals from the one or more sensors. 
     Preferably, the processing means is adapted to monitor the frequency with which the fall detection algorithm is updated, and if the frequency exceeds a threshold, the processing means is adapted to remove one or more feature sets from the fall detection algorithm. 
     Preferably, the processing means determines if a fall has taken place or is likely to take place by comparing the one or more feature sets with the corresponding signals generated by the one or more sensors. 
     Preferably, the processing means is adapted to update the fall detection algorithm in order to selectively optimize false positives, where the algorithm incorrectly detects a fall, false negatives, where the algorithm incorrectly detects that no fall has taken place, or to obtain a stable ratio between false positives and false negatives. 
     A second aspect of the invention provides a method of training a fall detection and/or prevention algorithm for use in a fall detection and/or prevention system, the method comprising obtaining measurements of characteristics of movement of a user of the fall detection and/or prevention system; analyzing the measurements using a fall detection algorithm to determine if a fall has taken place or is likely to take place; and updating the fall detection algorithm based on the result of the step of analyzing and an indication whether a fall has actually taken place from the user or a third party. 
     A third aspect of the invention provides a computer program product comprising executable code that, when executed on a suitable computer or processor, performs the steps of receiving signals indicating characteristics of movement of a user of a fall detection and/or prevention system; analyzing the signals using a fall detection algorithm to determine if a fall has taken place or is likely to take place; and updating said fall detection algorithm based on the result of the analysis of the signals and an indication whether a fall has actually taken place received from the user or a third party. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be described, by way of example only, with reference to the following drawings, in which: 
         FIG. 1  shows a fall detection system attached to a user; 
         FIG. 2  is a block diagram of the fall detection system; 
         FIG. 3  is a flow chart illustrating a first method in accordance with the invention; 
         FIG. 4  is a flow chart illustrating a second method in accordance with the invention; and 
         FIG. 5  is a flow chart illustrating a method of training a fall detection algorithm in accordance with the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  shows a fall detection system  2  attached to a user  4  via a band or other attachment means  6 . The fall detection system  2  is preferably attached at the upper part of the user&#39;s body  4 , such as around the waist, at the wrist, or as a pendant around the neck. 
     In this embodiment, the fall detection system  2  includes an alarm reset button  8  that the user  4  can operate to prevent or stop an alarm signal being sent to a call-centre or other assistance unit. Thus, if the fall detection system  2  detects a fall by the user  4 , an alarm signal will be sent to a call-centre or other assistance unit, unless the user  4  indicates that a fall has not taken place by pressing the alarm reset button  8 . This is considered to be a false positive (FP). 
     In this case, the fall detection algorithm executing in the system  2  is considered to have incorrectly identified a fall from the signals received from the sensors. It may be that the criteria or parameters used to identify falls from the received signals are not set at an appropriate level for the particular user  4  of the system  2 , so it is desirable to train or adapt the fall detection algorithm to the particular characteristics (for example gait and balance) of the user  4 . In addition, it is desirable for the fall detection algorithm to learn the types of situations or falls for which the user does or does not require assistance. Near falls for which the user  4  does not require call-centre intervention can be used to train the algorithm to classify them as non-falls. 
     In addition, if the user  4  does fall but stands up again, user  4  may want to decide him/herself whether assistance is needed and the fall detection system  2  should not alarm autonomously. For example, system  2  may observe the duration of the time period of relative constant acceleration when user  4  is lying down after a fall and before they stand up. If this period exceeds a threshold, the final decision on a fall is made and an alarm is sent to the call centre. 
     Suppressing this alarm, possibly even before the period reaches the threshold indicates that this time-out period should be extended for user  4 . Also, the other way around, calling for help, i.e. pressing the alarm button (if present) before the period reaches the threshold, indicates the threshold of the time-out period needs to be shortened. 
       FIG. 2  is a block diagram of a fall detection system  2  in accordance with the invention. 
     The system  2  comprises one or more sensors  10  that detect characteristics of movement of the user  4  and that generate corresponding signals. The one or more sensors  10  can comprise an accelerometer, magnetometer, gyroscope and/or other sensors. 
     The signals from the sensor(s)  10  form a feature set, possibly after some processing. Exemplary features include magnitude, spectral content, directional distribution, mean, variance, etc., but alternatively the signals themselves, i.e. the time series of sample values, can serve as feature set. The features are provided to decision logic  14  that executes the fall detection algorithm. In particular, the decision logic  14  determines whether a fall has taken place by comparing the feature set to a set of parameters that are used to classify whether a fall has taken place or not. These parameters can include, or be based on, feature sets from known falls, or risky situations. 
     At least a subset of the signals, or the extracted features, from the sensor(s)  10  are also provided to a FIFO buffer  16  that temporarily stores them for a predetermined time period. The duration of this time period can be different for different parts of the stored signals and features. For example, sub-sampling may be applied after passing a first time period. The stored signals and features are provided from the FIFO buffer  16  to a trace generating unit  18  that generates a trace for the signals that can be selectively stored in a memory  20 . A trace is generated in case a fall is detected by the decision logic  14  or in case the alarm reset button  8  has been pressed. The state of the decision logic  14  (fall/no-fall) as well as of the button  8  (pressed/not-pressed) is labeled with the trace. 
     If the decision logic  14  determines that a fall has taken place, an alarm signal is generated and sent to a time-out unit  22 . The time-out unit  22  is connected to the alarm reset button  8 , and, if the time-out unit  8  receives an alarm reset signal from the button  8  within a predetermined time-out period (which may be zero), the alarm signal is stopped. Otherwise, if no alarm reset signal is received within the time-out period, the alarm signal is transmitted to a call-centre or other assistance point. Alternatively, the alarm may be issued immediately to the call-centre, and a reset signal sends a revocation to the call-centre. 
     It should be noted that, in alternative embodiments, the fall detection system  2  can comprise a sensor unit for attachment to a user and a separate base station that receives the signals from the sensor unit and hosts the processing required to detect falls and generate alarm signals. In further alternative embodiments, the processing can be located at the call centre or at an intermediate location between the system  2  and call centre. 
     The complete trace, i.e. the signal and/or features from the FIFO  16  and states of decision logic  14  and button  8 , are provided to the memory  20 . 
     In some embodiments, as suggested above, the alarm reset button  8  can also be used by the user  4  to indicate that a fall has taken place, in the event that a fall is not detected by the system  2 . In this case, if the decision logic  14  does not detect a fall from the feature set, but the alarm reset button  8  is pressed, an alarm signal can be transmitted. In addition, the signal from the decision logic  14  indicating that no fall has been detected is provided to the memory  20 , along with the signal from the alarm reset button  8 , where they are stored with the relevant signal trace. 
     If no alert is generated by the decision logic  14  and the alarm-reset button  8  is not pressed, the relevant feature sets that led to this decision can be discarded from the FIFO buffer  16 . In these cases, the decision logic  14  has correctly identified from the features sets that no fall has taken place, or that no fall is likely to take place. 
     In alternative embodiments, an alarm button can be provided in addition to the alarm reset button  8  for allowing the user  4  to explicitly indicate that a fall has taken place (whether or not the algorithm has detected a fall), or that assistance is required. In this embodiment, if the decision logic  14  does not detect a fall from the feature set, but the alarm button is pressed, an alarm signal can be transmitted. The signal from the alarm button is provided to the memory  20  where it is stored with the trace of the signals from the sensor(s)  10 . 
     Thus, the fall detection system  2 , which can comprise a single accelerometer, is extended with a storage system  16 ,  18 ,  20  that is dedicated to store a feature set of the signals from the accelerometer. Raw sensor signals from the accelerometer can also be stored in cases where this is more efficient, for example when the decision logic  14  is based on direct signal characteristics, such as a threshold of the magnitude or frequency of the signal. In addition to the signal and/or its feature set, other data can be stored, such as time stamp data. It should be appreciated that, although not shown in the illustrated embodiment, the storage system can be physically remote from the accelerometer (i.e. remote from the device attached to the user  4 ). Timing data can be relative, indicating the progression within one trace of subsequent feature sets. 
     During operation, feature sets are stored in the memory  20  and are analyzed by the decision logic  14  for characterizing a fall, in case of fall detection, or an increased risk for falling, in case of fall prevention. Clearly, the algorithm can be used in both fall detection and fall prevention. The algorithm can use the stored data directly, i.e. compare current signal/features with those in memory  20 . It can also use the stored data indirectly, in which case the algorithm maintains internal settings and thresholds which are regularly adapted during an update process based on the (new) data stored in memory  20 . An update can be triggered upon each change in memory  20  (trace added or trace removed), or after a certain number of changes, possibly combined with a time out. An (additional) update can also be triggered if the rate at which memory  20  is updated changes. 
     As described above, if the alarm-reset button  8  is pressed, the trace of feature sets in the buffer is copied into the memory  20 , where it will be kept for a possibly indefinite length of time. Next to the trace data, the decision value is stored. Thus, in the case that the decision logic  14  has raised an alert, but the alarm reset button  8  is pressed, the trace data is labeled to represent a FP. In the case of no alert, but there is an indication from the user  4  that there was a fall, the trace data is labeled to represent a FN. Trace data raising an alert and for which no button press has been received can be stored as a TP (true positive). Optionally, signals and feature sets that do not raise a fall detection by the decision logic  14  and which are neither labeled with an (alarm) button press can be stored as well, labeled as TN (true negative). This may help the training of the adaptive algorithm. 
     In order to adapt to possible changes in the user&#39;s characteristics, e.g. related to ageing, traces in memory  20  may expire. Expiration can be triggered by similar mechanisms as the updating of the decision algorithm  14 . Expiration itself can trigger such an update. 
     At first use of the fall detection system  2 , the memory  20  and the algorithm  14  can be loaded with values that represent the characteristics of the population in general. These values, or part of them, can be labeled to expire in any case, or to expire in a shorter time period, e.g. as soon as a sufficient amount of user specific data has been collected. 
     In the alternative embodiment where separate buttons may be present for performing an alarm reset and for activating the alarm, traces representing TP can be selected based on the explicit alert presses (together with a generated alert). 
     In accordance with the invention, the stored information is used to adapt or train the algorithm used in the decision logic  14  to reduce the rates of false positives and false negatives. Thus, the decision logic  14  is trained using the trace data and the associated button press status (i.e. was a reset button  8  pressed?) or the trace data and associated status, FP, FN or TP. 
     The algorithm used in decision logic  14  can be updated each time that a button  8  is pressed, or can be updated every five button presses, say. Alternatively, the algorithm can be updated after a given period of time has passed, or any combination of the above. 
     In this way, the algorithm used in the decision logic  14  will become personalized to moving patterns of the user  4 . In addition, in the case of fall prevention, the algorithm will learn what situations the user  4  considers risky. In preferred embodiments, by obtaining data from multiple sensors  10  and sensor types, the measurable space of these risky situations will be expanded. 
     In particular, in the case of fall prevention, physiological data is of interest, such as characteristics indicating dizziness, and including quantities like blood pressure, oxygen level (SPO2), heart rate (ECG), muscle activity and fatigue (EMG and MMG), temperature, lung sounds, etc. 
     If the fall detection system  2  correctly classifies a non-risk situation (i.e. correctly in terms of the trained algorithm with its reference data and user feedback), but is succeeded closely by a fall, the system  2  can revisit its risk and non-risk categories and classify the traces therein with reference to earlier data (from other people, or from initial or factory settings). In this way, it is possible to identify those traces in the training set that are labelled as non-risky but are classified as risky in the earlier reference data. These traces can be refracted from the personalized training set, after which the decision algorithm can be trained again. 
     A refinement for the updating algorithm is to check the update rate, i.e. the time interval between subsequent button presses. If the intervals are small, this can indicate that the algorithm has a suboptimal adaptation state (i.e. the algorithm is frequently generating false positives or false negatives), whereas long intervals, or saturation in getting longer, indicates that optimality is reached. In particular, if the update rate increases (i.e. the intervals get shorter), this may indicate the algorithm is becoming “over fitted”, or too specific/narrow. To prevent this, samples (i.e. traces) can be removed from the training set. However, this process should also take into account that there may be changes in the user&#39;s moving patterns (gait &amp; balance). For example, the user&#39;s ability to maintain balance can decline over time. This latter information can, for example, be entered on the basis of the regular examination by the user&#39;s general practitioner. 
     The computation for determining the time interval between updates can also be adapted to the user&#39;s activities. For example, if the user takes off or switches off the fall detection system  2 , this time should not be counted towards an update interval. Similarly, if the user tends to sit steadily or stay in bed for long time periods, the update time interval computation can take this into account. In some embodiments, the time intervals can be computed relative to the average duration between instants where, say, the measured acceleration exceeds a or some reference thresholds. 
     In some embodiments, another measure that can be used to estimate the optimality of the algorithm is a stable ratio between FP and FN rates, or between TP and FP rates. This indicates that further improvement of the algorithm (in terms of reducing FP and FN) is not possible without the addition of additional or other types of sensor signals. In some embodiments, the user  4  can be informed of the ratio. It is also possible for the user  4  to be provided with the ability to set the ratio is considered optimal. For example, “no FN” can be a setting, and the ratio can be used to train and tune the algorithm accordingly. 
     In further embodiments of the invention, instead of solely labeling the traces on the basis of the alarm-reset button  8  (or an alarm button) being pressed, other interventions can trigger the described storing and training process as well. For example, a care provider may observe a near-fall or a risky situation and trigger the system  2  to use the data for training. This trigger may comprise the care provider pressing the button  8  on the system  2 , or the care provider remotely sending a signal to the system  2 . 
     The stored patterns or traces can also be set apart for inspection by the care provider or general practitioner. In particular, if they have been labeled as false positive by the user  4 , the care provider can use the data as a report to form an expert opinion on the well-being (and the trend therein) of the user  4 . Possibly, the care provider can decide to override the user&#39;s label to consider the incident a false positive. 
     In addition to the user  4  initiating the training of the algorithm when an alarm reset/alarm button is pressed, a care provider or care centre can also initiate the training update. For example, if an alert reaches the call centre and the user does not issue an alert-reset, while the care centre finds out if it was a false alarm, the care centre can send a training command to the system  2 . 
     Referring now to  FIG. 3 , the method of operating a fall detection system  2  that has an alarm reset button  8  is presented. In step  101 , a feature set is received from the sensor(s)  10  and is analyzed using the fall detection algorithm in the decision logic  14 . If a fall is not detected (step  103 ), the process returns to step  101  where a subsequent feature set is analyzed. 
     If a fall is detected (step  103 ), the process moves to step  105  where the fall detection system  2  waits for a predetermined period for the alarm reset button  8  to be pressed. 
     If the reset button  8  is pressed (step  107 ), the feature set or a trace of the feature set is stored in a memory  20  (step  109 ). This feature set or trace is stored along with the alarm reset indication, which means that it is stored as a false positive (step  111 ). The process then returns to step  101 . 
     If the reset button  8  is not pressed (step  107 ), an alarm signal is transmitted (step  113 ). In alternative embodiments, step  113  can also be triggered directly by a ‘yes’ at step  103 , in which case ‘yes’ by step  107  can raise a revocation. 
     Optionally (as indicated by the dashed arrows and boxes), the feature set or a trace of the feature set is stored in a memory  20  (step  115 ) along with an indication that the alarm reset button  8  was not pressed, which means that it is stored as a true positive (step  117 ). In either case, the process then returns to step  101 . 
     A method of operating a fall detection system  2  that has both an alarm reset button  8  and an alarm button is shown in  FIG. 4 . In step  131 , a feature set is received from the sensor(s)  10  and is analyzed using the fall detection algorithm in the decision logic  14 . 
     If a fall is detected (step  133 ), the process moves to step  135  where the fall detection system  2  waits for a predetermined period for the alarm reset button  8  to be pressed. 
     If the reset button  8  is pressed (step  137 ), the feature set or a trace of the feature set is stored in a memory  20  (step  139 ). This feature set or trace is stored along with the alarm reset indication, which means that it is stored as a false positive (step  141 ). The process returns to step  131  where a subsequent feature set is analyzed. 
     If the reset button  8  is not pressed (step  137 ), an alarm signal is transmitted (step  143 ). Step  143  can also be triggered directly by a ‘yes’ at step  133 , in which case ‘yes’ by step  137  can raise a revocation. 
     Optionally (as indicated by the dashed arrows and boxes), the feature set or a trace of the feature set is stored in a memory  20  (step  145 ) along with an indication that the alarm reset button  8  was not pressed, which means that it is stored as a true positive (step  147 ). Alternatively, or in addition, if the alarm button was pressed, the feature set can be stored in the memory  20  along with an indication that this alarm button was pressed. The process can then return to step  101 . 
     If a fall is not detected at step  133 , it is determined whether the alarm button has been pressed (step  149 ). If the alarm button is not pressed, then no fall has occurred, and the process returns to step  131 . 
     If the alarm button is pressed, an alarm signal is transmitted (step  151 ), and the feature set or a trace of the feature set is stored in the memory  20 , along with an indication that the alarm button was pressed (step  153 ). Thus, this is stored as a false negative (step  155 ). 
     The process then returns to step  131 . 
       FIG. 5  is a flow chart illustrating the steps in the method of training or adapting the fall detection algorithm in accordance with the invention. In step  161 , suitable training data is acquired. This training data, which comprises feature sets or traces of feature sets, along with indications of whether the feature sets were false positives, false negatives and/or true positives, can be acquired as described above with reference to  FIGS. 3 and 4 . 
     Then, in step  163 , this training data is used to update the fall detection algorithm. In particular, if the fall detection algorithm includes a category or categories of feature sets or traces that indicate falls or non-falls, the newly acquired training data can be used to further populate those categories and/or be used to remove existing feature sets or traces, if it has now been found that those existing feature sets or traces are not appropriate for that category. 
     Thus, there is provided a fall detection system that can be adapted to a particular user&#39;s fall or activity characteristics in order to improve the reliability of the fall detection algorithm. In particular, the training data for the algorithm is generated from sensor measurements and on whether an alarm reset button is pressed by a user or care provider. In this way, the algorithm used for detecting falls or near falls can be trained as the detection system  2  is in use, so realistic data can be obtained and used in the training, rather than being artificially created by a user mimicking a fall or non-fall in a specific training phase, as in conventional systems. In addition, by training the algorithm for a particular user, the algorithm will be adapted to the particular physical characteristics of that user, such as gait and posture, that user&#39;s movement patterns and that user&#39;s opinion on the severity of falls that require assistance from a call centre. 
     While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments. 
     Variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. A single processor or other unit may fulfill the functions of several items recited in the claims. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage. A computer program may be stored/distributed on a suitable medium, such as an optical storage medium or a solid-state medium supplied together with or as part of other hardware, but may also be distributed in other forms, such as via the Internet or other wired or wireless telecommunication systems. Any reference signs in the claims should not be construed as limiting the scope.