CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. Provisional Application No. 62/219,246, filed on Sep. 16, 2015, which is incorporated herein by reference. 
     
    
     TECHNICAL FIELD 
       [0002]    The present disclosure relates generally to measurements performed using a wearable device. 
       BACKGROUND 
       [0003]    A wearable device can be worn by a person riding a bicycle. A cyclist may wish to receive a measurement or estimate of their power output and/or calories burned while they ride. One way by which power output can be measured is the installation of a power meter onto a bicycle. However, such meters require installation and may be expensive. Another method for estimating power output is to measure the cyclist&#39;s heart rate and generate an estimate of power output based on the heart rate. While inexpensive, heart rate measurements and estimation of cyclist power output based on heart rate can require significant power to operate the sensor. Another approach is to estimate power output based on distance traveled and/or speed at which that distance is traveled. However, estimation based on these quantities can be inaccurate. For example, calculating an estimate of power output based on distance traveled and/or speed at which that distance is traveled will not account for dynamically experienced resistance such as that provided by wind, or by changes in elevation during the ride. 
         [0004]    When riding a bicycle, three main factors contribute to total energy expenditure: rolling resistance, contributed by friction of the bicycle tires against the ground; grade, contributed by the force of gravity pulling against the mass of the cyclist and bicycle; and wind resistance or drag, contributed by the force of air drag against the cyclist and bicycle moving through the atmosphere. When riding a bicycle at a constant speed, the total of these three main factors represents the major power output of the cyclist. 
       SUMMARY 
       [0005]    The present disclosure relates generally to determining estimates for the resistances experienced by a cyclist and thus the total power output of the cyclist. In particular, estimates of the wind resistance experienced by a cyclist can be obtained through calibration of the effective resistance a cyclist experiences as a function of heading, and thereby separated from the effective resistance due to rolling resistance and grade. Once such estimates are obtained, the user&#39;s total power output can be more accurately tracked throughout an activity. 
         [0006]    In order to improve estimates of the contribution of each of the three portions of total power output, and thus of total power output, a cyclist may wear a wearable device. The wearable device can include a heart rate sensor to provide a series of measurements of cyclist heart rate. The wearable device can also include motion sensors to collect data about the wearable device&#39;s position and orientation in space and to track changes to the wearable device&#39;s position and orientation over time. Accelerometers in the device may track acceleration, including high frequency variation in acceleration, and use this to detect surface type. Because a cyclist can wear the wearable device, the orientation of the device can provide information about the cyclist&#39;s body position. For example, when cycling, the cyclist&#39;s arms may be in a variety of positions, depending on the cyclist&#39;s style of riding and the type of handlebars on the bicycle. If the cyclist wears the wearable device on the cyclist&#39;s wrist, the wearable device may be able to infer the cyclist&#39;s hand position, and based on this hand position may be able to infer the cyclist&#39;s riding position and thereby provide an estimate of drag contributed by the cyclist&#39;s body (e.g., less drag when riding in a tuck or more drag when riding sitting upright.) 
         [0007]    Combining these measurements of heart rate, position and orientation, velocity, altitude, and riding position, the cyclist&#39;s total power output and the contribution of each component of that power output may be estimated. 
         [0008]    By estimating the relative contribution of each component to total power output, a less expensive and more power-efficient technique for providing accurate estimates of cyclist power output can be created. 
         [0009]    Embodiments of the present disclosure include a wearable device and techniques for estimating total power output and/or the contribution of wind resistance to the total power output of a cyclist wearing the wearable device. The wearable device may be worn on a wrist, such as a watch, and it may include one or more microprocessors, a display, and a variety of sensors, such as a heart rate sensor and one or more motion sensors. 
         [0010]    In some embodiments, the motion sensors may include, for example, an accelerometer, a gyroscope, a barometer or altimeter, a magnetometer or compass, etc. The wearable device may also include a motion coprocessor, which may be optimized for low-power, continuous motion sensing and processing. 
         [0011]    In some embodiments, the wearable device may be capable of communicating with a companion device. The wearable device may communicate with a companion device wirelessly, e.g., via a Bluetooth connection or similar wireless communication method. The companion device may be a second mobile device, such as a phone, which may include additional sensors. The additional sensors in the companion device may include a Global Positioning System (GPS) sensor, accelerometer, gyroscope, barometer or altimeter, motion coprocessor, etc. The companion device may, for example, communicate location information based on data from the GPS sensor to the wearable device. 
         [0012]    In some embodiments, the (first) wearable device may be capable of communicating with other wearable devices. The first wearable device may communicate with other devices wirelessly, e.g., via a Bluetooth connection or similar wireless communication method. In some embodiments, some of the other wearable devices may include different hardware or firmware and may communicate using a common inter-device protocol and implement a given application programming interface (API). The first wearable device may, for example, communicate motion data or other information to the other wearable devices. The first wearable device may also be configured to receive information in kind from the other wearable devices. 
         [0013]    Other features and advantages will become apparent from the following detailed description and drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    In order to facilitate a fuller understanding of the present disclosure, reference is now made to the accompanying drawings, in which like elements are referenced with like numerals. These drawings should not be construed as limiting the present disclosure, but are intended to be illustrative only. 
           [0015]      FIG. 1  shows a wearable device in accordance with an embodiment of the present disclosure. 
           [0016]      FIG. 2  depicts a block diagram of a wearable device in accordance with an embodiment of the present disclosure. 
           [0017]      FIG. 3  shows a companion device in accordance with an embodiment of the present disclosure. 
           [0018]      FIG. 4  shows a schematic representation of a user with a wearable device riding a bicycle in accordance with an embodiment of the present disclosure. 
           [0019]      FIG. 5  shows the various relevant forces acting on a cyclist during a ride and the relevant velocities of the cyclist. 
           [0020]      FIG. 6  shows a method for determining an estimate of wind resistance in accordance with an embodiment of the present disclosure. 
           [0021]      FIG. 7  illustrates the variation of effective resistance or total cyclist power output with heading in the presence of wind. 
           [0022]      FIG. 8A  illustrates the hand position termed “tops” and associated riding position. 
           [0023]      FIG. 8B  illustrates the hand position termed “hoods” and associated riding position. 
           [0024]      FIG. 8C  illustrates the hand position termed “drops” and associated riding position. 
           [0025]      FIG. 9  shows a method for using an accelerometer to improve estimation of C rr  according to some embodiments of the present disclosure. 
           [0026]      FIG. 10  shows a method for calculating power output in accordance with an embodiment of the present disclosure. 
       
    
    
     DESCRIPTION 
       [0027]    The present disclosure describes a wearable device that may be configured to estimate total cyclist power output and/or the contribution of wind resistance to total cyclist power output while the wearer is riding a bicycle. 
         [0028]    As described in the Background, three main factors contribute to total energy expenditure made by a cyclist moving at a constant speed: rolling resistance, grade, and wind resistance or drag. The total of these three main factors represents the total power output of the cyclist. 
         [0029]    Grade resistance is a function which varies linearly with vertical ground speed, with a constant scalar that may be determined based purely on the mass of the combined cyclist and bicycle system. 
         [0030]    Rolling resistance is also a function which varies linearly with ground speed. While calculating rolling resistance requires estimating the rolling resistance coefficient, it is a single unknown that does not vary significantly based on time or speed. The rolling resistance coefficient is a function of the tires and road conditions and is consistent for a given bicycle on a given terrain type. 
         [0031]    However, wind resistance varies not only linearly with ground speed but also with the square of the relative velocity between the cyclist and the air around the cyclist, as well as depending on the exposed surface area and drag coefficient of the cyclist, which varies based on factors such as cyclist body size, equipment, and the cyclist riding position. Because wind resistance includes a significant number of unknowns, calculating wind resistance is difficult. In addition, wind resistance depends on the square of the relative velocity between the cyclist and the air around the cyclist, and the air around the cyclist may move in different directions and at different speeds depending on the speed and direction of the wind in the cyclist&#39;s current location. Even if local weather data including wind velocity is available (e.g., by downloading from a remote weather service), that velocity may be inaccurate for the cyclist&#39;s current position, particularly in locations like cities where streets form channels for winds and wind direction and speed may not correlate to the downloaded data. 
         [0032]    Using a wearable device, the estimate of power output and the individual contributions of each of the three components of total power output can be improved by incorporating sensor information from the wearable device. Using this sensor information, an estimate of the contribution of each of these three components to the total power output of the cyclist can be provided, and in particular an estimate of wind resistance. 
         [0033]      FIG. 1  shows an example of a wearable device  100  in accordance with an embodiment of the present disclosure. In some embodiments, the wearable device  100  may be any suitable wearable device, such as a watch configured to be worn around an individual&#39;s wrist. As described in more detail below, the wearable device  100  may be calibrated according to physical attributes of the individual and physical activity by the individual user who is wearing the wearable device  100 , including, for example, activity participation statistics. 
         [0034]      FIG. 2  depicts a block diagram of example components that may be found within the wearable device  100  in accordance with an embodiment of the present disclosure. These components may include a heart rate sensing module  210 , a motion sensing module  220 , a display module  230 , and an interface module  240 . 
         [0035]    The heart rate sensing module  210  may include or may be in communication with a heart rate sensor as previously described. The wearable device  100  can measure an individual&#39;s current heart rate from the heart rate sensor. The heart rate sensor may also be configured to determine a confidence level indicating a relative likelihood of an accuracy of a given heart rate measurement. In other embodiments, a traditional heart rate monitor may be used and may communicate with the wearable device  100  through a near field communication method (e.g., Bluetooth). 
         [0036]    The wearable device  100  may also include the motion sensing module  220 . The motion sensing module  220  may include one or more motion sensors, such as an accelerometer or a gyroscope. In some embodiments, the accelerometer may be a three-axis, microelectromechanical system (MEMS) accelerometer, and the gyroscope may be a three-axis MEMS gyroscope. A microprocessor (not shown) or motion coprocessor (not shown) of the wearable device  100  may receive motion information from the motion sensors of the motion sensing module  220  to track acceleration, rotation, position, or orientation information of the wearable device  100  in six degrees of freedom through three-dimensional space. 
         [0037]    In some embodiments, the motion sensing module  220  may include other types of sensors in addition to accelerometers and gyroscopes. For example, the motion sensing module  220  may include an altimeter or barometer, or other types of location sensors, such as a GPS sensor. 
         [0038]    The wearable device  100  may also include the display module  230 . Display module  230  may be a screen, such as a crystalline (e.g., sapphire) or glass touchscreen, configured to provide output to the user as well as receive input from the user via touch. For example, display  230  may be configured to display a current heart rate or a daily average energy expenditure. Display module  230  may receive input from the user to select, for example, which information should be displayed, or whether the user is beginning a physical activity (e.g., starting a session) or ending a physical activity (e.g., ending a session), such as a running session or a cycling session. In some embodiments, the wearable device  100  may present output to the user in other ways, such as by producing sound with a speaker (not shown), and the wearable device  100  may receive input from the user in other ways, such as by receiving voice commands via a microphone (not shown). 
         [0039]    In some embodiments, the wearable device  100  may communicate with external devices via interface module  240 , including a configuration to present output to a user or receive input from a user. Interface module  240  may be a wireless interface. The wireless interface may be a standard Bluetooth (IEEE 802.15) interface, such as Bluetooth v4.0, also known as “Bluetooth low energy.” In other embodiments, the interface may operate according to a cellphone network protocol such as LTE or a Wi-Fi (IEEE 802.11) protocol. In other embodiments, interface module  240  may include wired interfaces, such as a headphone jack or bus connector (e.g., Lightning, Thunderbolt, USB, etc.). 
         [0040]    The wearable device  100  may be configured to communicate with a companion device  300  ( FIG. 3 ), such as a smartphone, as described in more detail herein. In some embodiments, the wearable device  100  may be configured to communicate with other external devices, such as a notebook or desktop computer, tablet, headphones, Bluetooth headset, etc. 
         [0041]    The modules described above are examples, and embodiments of the wearable device  100  may include other modules not shown. For example, the wearable device  100  may include one or more microprocessors (not shown) for processing heart rate data, motion data, other information in the wearable device  100 , or executing instructions for firmware or apps stored in a non-transitory processor-readable medium such as a memory module (not shown). Additionally, some embodiments of the wearable device  100  may include a rechargeable battery (e.g., a lithium-ion battery), a microphone or a microphone array, one or more cameras, one or more speakers, a watchband, a crystalline (e.g., sapphire) or glass-covered scratch-resistant display, water-resistant casing or coating, etc. 
         [0042]      FIG. 3  shows an example of a companion device  300  in accordance with an embodiment of the present disclosure. The wearable device  100  may be configured to communicate with the companion device  300  via a wired or wireless communication channel (e.g., Bluetooth, Wi-Fi, etc.). In some embodiments, the companion device  300  may be a smartphone, tablet computer, or similar portable computing device. The companion device  300  may be carried by the user, stored in the user&#39;s pocket, strapped to the user&#39;s arm with an armband or similar device, placed in a mounting device, or otherwise positioned within communicable range of the wearable device  100 . 
         [0043]    The companion device  300  may include a variety of sensors, such as location and motion sensors (not shown). When the companion device  300  may be optionally available for communication with the wearable device  100 , the wearable device  100  may receive additional data from the companion device  300  to improve or supplement its calibration or calorimetry processes. For example, in some embodiments, the wearable device  100  may not include a GPS sensor as opposed to an alternative embodiment in which the wearable device  100  may include a GPS sensor. In the case where the wearable device  100  may not include a GPS sensor, a GPS sensor of the companion device  300  may collect GPS location information, and the wearable device  100  may receive the GPS location information via interface module  240  ( FIG. 2 ) from the companion device  300 . 
         [0044]    In another example, the wearable device  100  may not include an altimeter or barometer, as opposed to an alternative embodiment in which the wearable device  100  may include an altimeter or barometer. In the case where the wearable device  100  may not include an altimeter or barometer, an altimeter or barometer of the companion device  300  may collect altitude or relative altitude information, and the wearable device  100  may receive the altitude or relative altitude information via interface module  240  ( FIG. 2 ) from the companion device  300 . 
         [0045]    In another example, the wearable device  100  may receive motion data from the companion device  300 . The wearable device  100  may compare the motion data from the companion device  300  with motion data from the motion sensing module  220  of the wearable device  100 . 
         [0046]    The wearable device may use motion data to predict a user&#39;s activity. Examples of activities may include, but are not limited to, walking, running, cycling, swimming, etc. The wearable device may also be able to predict or otherwise detect when a user is sedentary (e.g., sleeping, sitting, standing still, driving or otherwise controlling a vehicle, etc.). The wearable device may use a variety of motion data, including, in some embodiments, motion data from a companion device. 
         [0047]    The wearable device may use a variety of heuristics, algorithms, or other techniques to predict the user&#39;s activity. The wearable device may also estimate a confidence level (e.g., percentage likelihood, degree of accuracy, etc.) associated with a particular prediction (e.g., 90% likelihood that the user is running) or predictions (e.g., 60% likelihood that the user is running and 40% likelihood that the user is walking). 
         [0048]      FIG. 4  shows a schematic representation of a user with a wearable device riding a bicycle in accordance with an embodiment of the present disclosure. In the example of  FIG. 4 , a user  410  is wearing a wearable device  420  (e.g., the wearable device  100 ) on the user&#39;s wrist. The user may be holding the handlebars  430  of the bicycle in a variety of different grips, as shown in  FIGS. 8A-C . In this illustration, the user is holding the handlebars in the “tops” position. In some embodiments, the wearable device  420  may be worn on other portions of the user&#39;s body, such as the arm, finger, leg, or foot, so long as the portion of the user&#39;s body experiences motion related to the motion of the vehicle. 
         [0049]      FIG. 5  depicts the velocities and forces acting on a user  500  riding a bicycle  510 . This includes vectors  520 ,  530 ,  540 ,  550 ,  560 ,  570 , and  580 . Vector  520  represents the velocity of the bicycle with respect to the ground. Vector  530  represents the relative velocity between the cyclist and the air. Vector  540  represents the force exerted by air drag on the cyclist, while vector  550  represents the force exerted by friction on the bicycle. Vector  560  represents the force exerted by gravity on the bicycle and cyclist and vector  570  represents the force exerted by the ground on the bicycle and cyclist. Finally, vector  580  represents the force exerted by the cyclist by pedaling the bicycle. In the example of  FIG. 5 , the bicycle is on relatively level terrain (relatively little or no pitch), and the direction of gravity acting on the vehicle is shown as “down,” approximately perpendicular to the road or other terrain. 
         [0050]    As described above, the total power output of the bike can be described as the sum of three components, the rolling resistance, the grade, and the drag, as shown in Equation 1: 
         [0000]        P   bike   =f   rr   +f   grade   +f   drag    (Eq. 1)
 
         [0051]    P bike  is represented in  FIG. 5  by vector  580 , f rr  is represented by vector  550 , f grade  is a function of vector  560 , and f drag  is represented by vector  540 . All three of f rr , f grade , and f drag  depend on the ground velocity of the bicycle shown by vector  520 , but only f drag  exhibits dependence on the relative air velocity shown by vector  530 . 
         [0052]    f rr  represents the rolling resistance experienced by the cyclist. This portion of total power depends on the combined mass of the bike and the rider, m c , the force of gravity g, the rolling resistance coefficient C rr , and the ground velocity V g , as shown in Equation 2: 
         [0000]        f   rr   =m   c   ·g·C   rr   ·V   g    (Eq. 2)
 
         [0053]    f grade  represents the contribution of gravity to the forces experienced by the cyclist. This portion of total power depends on m c , g, V g , and the grade of the cycling surface S, as shown in Equation 3: 
         [0000]        f   grade   =m   c   ·g·S·V   g    (Eq. 3)
 
         [0054]    Finally, f drag  represents the wind resistance experienced by the cyclist. This portion of total power depends on V g , the exposed surface area A and drag coefficient C d , the air density ρ, and the relative velocity through air V rel , as shown in Equation 4: 
         [0000]        f   drag =1/2 ·ρ·A·C   d   ·V   2   rel   ·V   g    (Eq. 4)
 
         [0055]    Some of these parameters may be measured directly or estimated directly from measured data. For example, P bike  may be measured using a power meter or estimated using known techniques for calculating energy expenditure based on heart rate. Vg, S, and ρ may be determined using a GPS sensor or other positional sensor that provides altitude data and/or a barometer. Other parameters may be input by the user. m c  is an example of this type of parameter. Finally, some parameters can be estimated. These parameters include C rr , A, C d , and V rel . 
         [0056]      FIG. 6  illustrates a method for determining an estimate of wind resistance in accordance with an embodiment of the present disclosure. Upon detection in step  600  that a user is engaged in the activity where wind resistance is to be determined (for example, cycling), the method begins. In steps  610 ,  620 , and  630 , position, speed, heading, altitude, and heart rate measurements are collected, for example by using GPS measurements and a heart rate sensor. These measurements are collected periodically time-stamped with the time of collection. The sampling period between collections of each of the three data points may be regularly spaced or irregularly spaced and steps  610 ,  620 , and  630  need not be performed at the same time or intervals. For example, as heart rate determination may require a larger amount of energy to perform, heart rate determination may occur less frequently than collection of speed, heading, and altitude. 
         [0057]    In step  640 , the time-stamped position, speed, and heading information may be split into straight line segments representing travel by the cyclist in an approximately straight line during the time period the segment represents. Each segment represents a distance of known length and heading traveled from one known position to a second known position between two known points in time. In some embodiments, the straight line segments are split by determining whether a cyclist&#39;s path has deviated from a straight line subsequent to the second known position in a set of candidate known positions by comparing the subsequent known position to the line defined by the first and second known positions and beginning a new straight line segment if the subsequent known position deviates from the line defined by the first and second known positions by more than a predetermined threshold. In step  650 , the position, speed, heading, and altitude measurements are used to determine the grade at that moment in time. For example, change in altitude over time with a known speed, or change in altitude between two positions on a straight line segment can be used to estimate grade. In step  660 , based on heart rate or a power meter, total power output can be determined. 
         [0058]    In step  670 , a wind speed and direction are determined, either based on the outputs of steps  640 - 660  or by reference to an external source of information such as an online weather service. For example, wind data for the current location may be retrieved from an online weather service using a data network and the current location of the wearable device. 
         [0059]    Alternatively, if wind data is unknown or cannot be retrieved, then wind direction and speed may be estimated. In step  670 , when estimating wind speed and direction according to embodiments of the present disclosure, total power output is determined at a variety of directional headings of the cyclist. For example, the straight line segments determined in step  640  may be used to determine the cyclist&#39;s heading during a particular period of time and total power outputs determined during that period of time are correlated to the cyclist&#39;s heading, providing a total power output at the directional heading of the straight line segment. By using multiple straight line segments with multiple directional headings that are different from one another, total power output for each of a variety of headings may be determined. 
         [0060]    Once total power output has been determined across a variety of headings, the direction of the wind may be identified by examining the total power output data with respect to heading. Maximum power output is required when the cyclist is heading directly into the wind, while minimum power output occurs when the cyclist is riding with the wind. Because power output contributions from rolling resistance and grade are independent of wind speed, resistive forces contributed by these terms are effectively constant with respect to whether a cyclist is heading into or out of the wind once any systematic variations in grade (for example, cycling up or down a slope) according to heading are taken into account. The heading and speed of the wind can thus be estimated by fitting the total power output data at a variety of headings to a curve and determining the estimated V rel  for that curve. 
         [0061]    In step  680 , based on the outputs of steps  640 - 660 , and based on the knowledge of wind speed and direction determined in step  670  (whether estimated or directly obtained), estimates of various unknown parameters (for example, V rel , A, and C d ) and components of total resistance (rolling resistance, grade, and drag) may be calculated and may be used to replace, refine, or otherwise supplement existing methods of measuring total power output such as power meters or heart rate-based determinations. 
         [0062]    According to embodiments of the present disclosure, when step  670  determines that no wind is present, C rr , A, and C d  can be estimated based on a set of P bike  measurements at various ground speeds V g  (measurements with speed diversity) by solving sets of equations of the form shown in Equations 1-4. By comparing an adequate number of P bike  measurements at various V g  and incorporating direct measurements of other parameters, the system can solve for estimates of the unknown parameters C rr , A, and C d . 
         [0063]    According to embodiments of the present disclosure, when step  670  determines that a wind of known speed and direction is present (e.g., if accurate wind data is available from a weather service, or via estimation of the wind), C rr , A, and C d  can be estimated without any requirement for speed diversity. A measurement of P bike  combined with direct measurements of other parameters allows the estimation of C rr , A, and C d . 
         [0064]    According to embodiments of the present disclosure, once wind direction is determined (whether by estimation or directly) in step  670 , V rel , A, and C d  can be estimated in step  680  by solving sets of equations of the form shown in Equations 1-4. If the drag coefficients A and C d  have been previously calibrated (e.g., in a no wind or known wind situation), the estimate of wind speed can also be refined to a higher precision. Once wind speed has been estimated via this method, the known wind speed and direction method can be used to determine an estimate of rolling resistance C rr . 
         [0065]      FIG. 7  illustrates the ideal model of total power resistance versus heading into or out of a wind out of the northwest, as well as an estimated model generated based on observed data from actual cyclists. The solid line  710  represents the ideal model, while the dotted line  720  represents the estimated model. Dots  730  represent measured power output at various headings. By fitting a curve to the measured power outputs, the estimated model may be generated and used to determine points of predicted minimum and maximum power output. These points of predicted minimum and maximum power output can be used to derive V rel . 
         [0066]      FIGS. 8A-8C  illustrate various hand positions riders may take. These positions may be termed “tops”, as shown in  FIG. 8A , “hoods”, as shown in  FIG. 8B , and “drops”, as shown in  FIG. 8C . Each of these positions influences the body positioning of the cyclist, as shown in  FIGS. 8A-8C , and thus influences the air drag by changing the exposed surface area A (and potentially by changing the drag coefficient C d ). In some embodiments of the present disclosure, a wearable device worn on the cyclist&#39;s wrist or hand may detect the cyclist&#39;s hand position. For example, a direction of gravity as experienced by the wearable device, combined with knowledge of the wrist on which the device is worn, may be used to estimate the hand position of the rider. Based on hand position, the body position of the cyclist may be estimated, and this estimate may be used to refine A and C d  and/or to adjust these parameters in real-time as the cyclist&#39;s body position changes. By adjusting the air drag parameters with the cyclist&#39;s changes in body position, estimates of total power output may be made more accurate. 
         [0067]      FIG. 9  illustrates a method for using an accelerometer to improve estimation of C rr  according to some embodiments of the present disclosure. In step  900 , accelerometer data is collected at a high sampling rate. In step  910 , high frequency accelerometer data is separated from low frequency accelerometer data. In step  920 , high frequency accelerometer data is used to obtain an estimate of roughness of the riding surface and thereby provide or refine an estimate of C rr . For example, a surface generating relatively little high frequency accelerometer output is likely a smooth surface such as an asphalt road, with a lower C rr , while a surface generating more high frequency accelerometer output may be a rougher surface such as a dirt road, with a higher C rr . If the cyclist is riding on a mountain trail or similar very rough surface, significant high frequency accelerometer output may be present, signaling a very high C rr . In step  930 , the estimate of surface C rr  is used to refine or replace the existing estimate of C rr . 
         [0068]      FIG. 10  illustrates a method for calculating power output in accordance with an embodiment of the present disclosure. In step  1000 , the parameters described above with respect to  FIG. 6  (e.g., C rr , V rel , A, and C d ) are obtained, for example by direct measurement or estimation, as described with respect to the steps of  FIG. 6 . After obtaining these parameters, in step  1010 , heart rate measurements to estimate total power output are reduced in frequency, in order to save battery life of the wearable device. In some embodiments, heart rate measurements are reduced in frequency after the parameters have been obtained. In other embodiments, heart rate measurements are discontinued after the parameters have been obtained. In step  1020 , an estimate of total power output by the cyclist is obtained by applying the obtained parameters to Equations 1-4 without use of a heart rate measurement. By providing power output estimates without using heart rate measurements or while using them less frequently, the wearable device can accurately estimate cyclist power output without consuming as much battery power. 
         [0069]    The present disclosure is not to be limited in scope by the specific embodiments described herein. Indeed, other various embodiments of and modifications to the present disclosure, in addition to those described herein, will be apparent to those of ordinary skill in the art from the foregoing description and accompanying drawings. For example, while the present embodiments are described with respect to a cyclist, a runner would also experience frictional forces, forces due to gravity, and wind resistance, and the embodiments could also be applied to a runner by one of ordinary skill in the art. Thus, such other embodiments and modifications are intended to fall within the scope of the present disclosure. Further, although the present disclosure has been described herein in the context of at least one particular implementation in at least one particular environment for at least one particular purpose, those of ordinary skill in the art will recognize that its usefulness is not limited thereto and that the present disclosure may be beneficially implemented in any number of environments for any number of purposes.
Improved techniques and systems are disclosed for determining the components of resistance experienced by a wearer of a wearable device engaged in an activity such as bicycling or running. By monitoring data using the wearable device, improved estimates can be derived for various factors contributing to the resistance experienced by the user in the course of the activity. Using these improved estimates, data sampling rates may be reduced for some or all of the monitored data.