The effects of tandem skiing on posture and heart rate in children with profound intellectual and multiple disabilities

F. Degache, A. Bonjour, D. Michaud, L. Mondada & CJ. Newman

To cite this article: F. Degache, A. Bonjour, D. Michaud, L. Mondada & CJ. Newman (2018): The effects of tandem skiing on posture and heart rate in children with profound intellectual and multiple disabilities, Developmental Neurorehabilitation, DOI: 10.1080/17518423.2018.1462268
To link to this article: https://doi.org/10.1080/17518423.2018.1462268

Published online: 16 Apr 2018.

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DEVELOPMENTAL NEUROREHABILITATION

https://doi.org/10.1080/17518423.2018.1462268

The effects of tandem skiing on posture and heart rate in children with profound intellectual and multiple disabilities
F. Degache a,b, A. Bonjoura,b, D. Michauda,b, L. Mondadab, and CJ. Newman c
aUniversity of Health Sciences, University of Applied Sciences and Arts Western, Lausanne, Switzerland; bInstitute of Sport Sciences University of Lausanne (ISSUL), University of Lausanne, Lausanne, Switzerland; cPediatric Neurology and Neurorehabilitation Unit, Lausanne University Hospital, Lausanne, Switzerland

ABSTRACT
Purpose: The objective of study was to determine the effect of tandem ski (TS) activity on postural control and cardiac activity in children with profound intellectual and multiple disabilities (PIMDs).
Method: Twenty children with PIMD and 20 age-matched controls (typically developed (TD) children)
participated. Body segment movements were measured with inertial sensors (Physilog®) placed on the head, C7, trunk (including ECG) and pelvis with a seat reference. Each participant was measured during a 12-turn slalom pattern.
Results: In each group, significant differences were observed between the head vs. trunk and head vs.
pelvis angular speeds (p<0.001). In both groups, heart rate differed significantly during rest (PIMD 99 bpm, TD 97 bpm), exercise (PIMD 140 bpm, TD 139 bpm; rest vs. exercise p<0.001) and recovery (PIMD 101 bpm, TD 107 bpm; exercise vs. recovery p<0.001).
Conclusions: In children with PIMD, TS elicits active postural control associated with cardiac activities similar to that of the controls.
ARTICLE HISTORY
Received 1 September 2017
Revised 27 February 2018
Accepted 4 April 2018
KEYWORDS
Heart rate; postural adjustment; profound intellectual and multiple disabilities; tandem skiing

Introduction
Profound intellectual and multiple disabilities (PIMDs) are defined as a combination of severe intellectual and motor disabilities. Individuals with PIMD typically exhibit a number of additional primary or secondary disabilities.1 PIMD has variable causes; typically severe neurological diseases can be acquired or congenital and static or progressive.
Motor disabilities can include paresis or plegia, abnormal muscle tone (hypotonia, spasticity, and dystonia), involuntary movements, and ataxia, and these disabilities strongly impact voluntary movements in isolation or combination. Sensory impairments occur quite frequently with intellectual and motor disabilities. Many people with PIMD experience visual impairment, between a quarter and a third have auditory impairments, and for many, tactile and cutaneous senses are believed to be impaired to some degree.2,3 In addition to intellectual, motor, and sensory disabilities, most children with PIMD also have disorders such as epilepsy, gastrointest- inal disorders, sleeping difficulties, and issues related to gen- eral fitness and feeding/drinking.4,5
Consequently, physical activities are strongly limited in this population due to the associated major cognitive and motor issues together with a lack of independence in the activities of daily life. Furthermore, those with PIMD have limited oppor- tunities for compensation because they might lack the internal drive to move by themselves due to severe intellectual dis- abilities or have limited awareness of external cues because of
sensory impairment. Thus, persons with PIMD are usually strongly dependent on support caretakers for a majority of their motor activities. Therefore, activities offered to children with PIMD should be aimed at overcoming their limited autonomy and increasing their opportunities for movement.6 The importance of increasing physical activity and reducing sedentariness in children with and without disabilities has grown in both medical and social circles.7,8 The activity guidelines for typical children and teenagers indicate that school-age children and adolescents should participate in at least 60 min/day of moderate-to vigorous-intensity physical activity, including vig- orous-intensity activities at least 3 days/week and activities that strengthen muscle and bone at least 3 days/week.9 Moderate- intensity physical activity is defined as an activity that increases the person’s heart rate to 50–70% of the maximum. Vigorous- intensity physical activity is defined as an activity that increases
the person’s heart rate to 70–85% of the maximum.10
For persons with PIMD, physical activity is critical because these people engage in lower levels of physical activity than their typical counterparts and tend to have comparatively more serious health concerns.11–13 Persons with PIMD have been previously demonstrated to use relatively small fractions, only 20 to 30%, of their heart rate reserves.14
Certain physical activity interventions and/or sport-like activities can be offered to children with PIMD to improve physical activity levels and for therapeutic purposes. A recent systematic review of physical activity interventions across all levels of motor disability in cerebral palsy (CP) showed

CONTACT F. Degache [email protected] Institution: University of Health Sciences, University of Applied Sciences and Arts Western, Av. De Beaumont 21, Lausanne 1011, Switzerland
Color versions of one or more of the figures in the article can be found online at www.tandfonline.com/ipdr.
© 2018 Taylor & Francis

evidence of efficacy for postural control during hippotherapy.- 15 Hippotherapy has also been demonstrated to increase heart rate in children with severe CP to a higher degree than that for ambulatory children with CP.16 Active video gaming17 and wheelchair dance18 have also shown promising results in individuals with severely limiting CP.
Data examining snow sports in children and youth with physical disabilities are sparse, but a review of this research showed that the risks of engaging in snow sports appear similar to those of the general population and that participat- ing in snow sports might positively influence self-esteem, standing balance, and gross motor function.18 A tandem ski (TS) allows persons with even highly limited mobility to access snow sports and descend ski slopes. In tandem skiing, the passenger rides in an adapted bucket seat with a high-end shock absorber, while the pilot rides the articulated skates (Figure 1). Within a national trend toward improved inclu- sion and the strong support of charitable foundations and associations, access to regular tandem skiing has steadily increased over the last decade for children with disabilities in Switzerland in both private and educational settings. Furthermore, ski resorts are progressively adapting their infrastructure and snow sport accessibility to accommodate skiers with special needs.
Wearable microsensors are increasingly used during var-
ious sport activities to track physiological parameters such as heart rate and motion during exercise.19 Specifically, inertial measurement units that combine 3D accelerometers and 3D gyroscopes have recently allowed body movement quantifica- tion during snow sports like downhill skiing20 and ski mountaineering.21
The aim of this study was to evaluate the effects of tandem skiing on the sitting posture and heart rate of children and youth with PIMD compared to those of a population of children with typical development (TD) who have no issues with muscular function or postural control. We hypothesized that children with PIMD would exhibit heart rate changes and larger postural alterations during TS practice than those in the control group.

Figure 1. Tandemski (Tandem-Flex, Tessier, France).
Methods
Participants
We included two groups in this work. The first group contained 20 children with PIMD (14.0 ± 2.8 years old) and the second group consisted of 20 TD children (13.7 ± 3.0 years old).
The PIMD group inclusion criteria were as follows: (1) age 8–18 years old (based on the usual threshold ages for the TS activity) and (2) a diagnosis of PIMD based on assessments by a pediatric neurologist and/or pediatric physiatrist associating with severe-to-profound intellectual disability (IQ estimated or measured below 35) with a motor disability that limited walk- ing in the community (i.e., for children with CP, Gross Motor Function Classification System (GMFCS) levels III to V22). The PIMD group exclusion criteria included as follows: (1) any acute health issue precluding study participation (e.g., infec- tious disease, especially respiratory infection), (2) recent frac- ture or orthopedic intervention (<6 months), (3) skin allergy to surgical tape (inertial sensors), and (4) any other contraindica- tions linked to the TS activity (e.g., cardiopathy, chronic respiratory disorder, and known intolerance to high altitude). For children with PIMD, the following data were recorded from their medical files: etiology of the PIMD (CP, syndromic encephalopathy, neurodegenerative disease, and other encepha- lopathies) and important information related to axial motor dysfunction such as scoliosis and sensory disorders.
The TD group inclusion criteria were as follows: (1) age 8–18 years old and (2) good health. The TD group exclusion criteria included the following: (1) any acute health issue precluding participation in the study (e.g., infectious disease, especially respiratory infection), (2) recent fracture or ortho- pedic intervention (<6 months), (3) skin allergy to surgical tape (inertial sensors), and (4) any other contraindications linked to the TS activity (e.g., cardiopathy, chronic respiratory disorder, and known intolerance to high altitude).
The regional ethics committee approved the study proto- col, and the parents and participants when possible (control group) approved and signed the written consent form.

Study design
We performed a single-center descriptive study. Each partici- pant was measured during a standardized slalom activity consisting of six left turns and six right turns. All the selected slopes were in similar condition (steepness, speed, and width) for the PIMD and TD groups. All the subjects in the two groups performed the activity on one test slope and were previously warmed up because they were tandem skiing for at least 5 to 10 mins prior to the analysis.
Each subject was set up according to the practice guidelines on the use of TSs (Teissier, St Rémy de Maurienne, France). The professional pilot, placed at the back of the TS, guides the movement via braking or carving with his/her feet, which were situated at the back of the skis in articulated pallets. The parti- cipant was placed in a high bucket seat that supplied trunk support and protection against high-amplitude head movements during skiing and was secured with a four-point harness and a homologated and size-adapted ski helmet (Figure 1). To access

the chairlift, a special system was included in the TS that allows the passenger to remain seated during the entire chairlift ride.

Materials
Five inertial sensors (Physilog 4®, Gait Up SA, CH, USA) were placed on the participants to analyze their movements and adjustment of their body segments and to monitor the speed and trajectory of the TS. These inertial sensors include a triaxial accelerometer (500 Hz sampling frequency), a triaxial gyroscope (50 Hz sampling frequency), and a triaxial magnet- ometer (50 Hz sampling frequency). One of these sensors also contained an electrocardiographic measurement system (ECG) with two skin sensors (200 Hz sampling frequency).
The sensors were placed on the following locations with rigorous alignment in the sagittal plane for all subjects: (1) a “head” sensor was placed on the top (vertex) of each subject’s head; (2) a “trunk” sensor was placed on the mid-sternum and fixed on the skin with surgical tape (MEFIX, Mölnlycke Health Care, France) and connected to an electrode on the mid-clavicular line and an electrode on the right para-sternal line (Vermed society, Devon, UK). This sensor collected both heart rate (at rest, during tandem skiing, and during recovery) and inertial data; (3) a “pelvis” sensor placed in front of the pelvis was fixed onto the anterior section of a belt with a Velcro strap, and the belt was placed by ensuring that it rested across the anterosuperior ilial tuberosities and sacrum and was tightened securely; (4) a “seat” sensor was placed in a protected box between the horizontal and vertical cushions of the TS; and (5) a “GPS” sensor was located in the pocket of the pilot. The latter supplied information on the speed and displacement of the TS. All the sensors were synchronized with proprietary embedded software. The seat sensor was considered the reference sensor.
An automatic turn-detection algorithm was developed in MATLAB (MathWorks Inc., MA, USA). The algorithm detected the positive and negative peaks of the previously filtered magnetometer standard with an adaptive low-pass filter based on the pelvis sensor because the metal structure of the TS distorted the signals of the magnetometer in the seat reference sensor. Once the start and the end of each turn were detected, the metrics were extracted for the individual signals of each axis of each module and of each sensor. The 3D Euclidean norms of angular speeds, supplied by the gyro- scopes, were also calculated.
To analyze the results, the mean angular speed over 12 turns (°.s−1) for each body segment; the average speed of the TS (m.s−1); and the mean heart rate at rest (immediately before slalom during 5 mins at rest), during practice (during slalom), and during recovery (immediately after slalom during 5 mins at rest) (bpm) were retained.

Statistical analysis
Statistical analyses were performed with SigmaPlot 12.5. Data are expressed as the mean ± SD for all variables. The normality of the samples was tested using the Kolmogorov–Smirnov test. A t test was used to assess differences between the two groups. For heart rate data, a two-way repeated-measure analysis of
variance (ANOVA) test (phase X groups) was used to compare values during different periods (rest, practice, and recovery) and between groups. The level of significance was set at p ≤ 0.05.

Results
Clinical characteristics of the participants
The demographic and anthropometric characteristics of the study population are shown in Table 1. The children in the PIMD group had lower body weight and shorter stature than their TD peers. In the PIMD group, 9 children had bilateral spastic CP and 11 children had other severe encephalopathies such as genetic and/or polymalformative syndromes or epileptic encephalopa- thies. None of the children could walk independently in the community, seven had additional severe sensory disorders (i.e., blindness and/or deafness), and three had scoliosis.

Tandem ski speed
No significant differences were observed for average TS speed between the two groups for the entire run (PIMD 11.81 ± 3.11 m.s−1; TD 12.16 ± 4.56 m.s−1) and for each of the 12 turns (Figure 2). Therefore, the tandem skiing conditions were considered comparable between the groups.

Body segment angular speeds
The differences in body segment angular speeds between both groups are shown in Table 2. These results show significantly lower angular speeds for the trunk (p = 0.002) and pelvis (p = 0.01) segments in the PIMD group. No significant differ- ences were found between groups for the head segment.
In each group (PIMD and TD), significant differences were observed between the head and trunk angular speeds (p < 0.001) and between the head and pelvis angular speeds (p < 0.001) (Figure 3).

Heart rate
Changes in the mean heart rate during rest (before physical activity), TS practice, and recovery (5 mins after the end of physical activity) are shown in Figure 4. For both groups, the active periods show a significantly higher heart rate compared with the rest and recovery periods. For the PIMD group, the heart rate during the active period was 42% higher than that during the rest period (p < 0.001), and the heart rate during

Table 1. Demographic and anthropometric characteristics of participants in the profound intellectual and multiple disabilities (PIMD) and typically developing (TD) groups.

PIMD (n = 20) TD (n = 20)
Male 10 (50%) 8 (40%)
Age (years) 14.0 ± 2.8 13.7 ± 3.0
Weight (kg) 32.6 ± 9.8 47.7 ± 15.4
Height (m) 1.4 ± 0.2 1.6 ± 0.2
BMI (kg/m2) 16.4 ± 3.6 18.7 ± 3.2
Values are expressed as mean ± standard deviation, categorical variables as frequencies and percentages.
TD: typically developing children.
PIMD: Children with profound intellectual and multiple disabilities.

Values are expressed as mean ± standard deviation.
PIMD: children with Profound Intellectual Multiple Disabilities. TD: typically developed children.
T : turns

Figure 2. Tandemski velocity evolution (m.s−1) for two groups.

Table 2. Comparison of angular speeds of body segments between the pro- found intellectual and multiple disabilities (PIMD) and typically developing (TD) groups.

Segments PIMD (n = 20) TD (n = 20) p Value
Head 92.3 ± 50.3 79.8 ± 66.0 NS
Trunk 8.4 ± 23.6 30.0 ± 18.3 0.002
Pelvis 1.6 ± 21.5 17.0 ± 12.5 0.01
Values are expressed as mean ± standard deviation.
PIMD: children with profound intellectual and multiple disabilities. TD: typically developed children.

Figure 3. Comparison of angular velocity between seat and all other segments in PIMD and TD groups.

the recovery period was 28% lower than that during the active period (p < 0.001). For the TD group, the heart rate during the practice period was 44% higher than that during rest (p < 0.001) and 23% lower during recovery than that during the active period (p < 0.001). No significant differences were found for rest, active, and recovery periods between the PIMD and TD groups (at rest: 99 vs. 97 bpm; during practice: 140 vs. 139 bpm; during recovery: 101 vs. 107 bpm, respectively). The repeated-measure ANOVA showed no significant phase X group interaction.o

Discussion
In this study, we explored adjustments to posture and heart rate in adolescents with PIMD and their TD peers during tandem skiing. Children with PIMD, such as those with severe CP (GMFCS levels IV and V), typically exhibit poor trunk control and difficulties aligning and stabilizing the center of mass of the head over the base of support during sitting.23 Segmental trunk control plays a major role in the extent of motor disability, explaining up to 40% of the variation in

Figure 4. Comparison of the heart rate (bpm) between PIMD and TD groups during rest, practice, and recovery period.

motor function in children with CP.24 Therefore, offering physical activities that stimulates postural adaptations at the

head and trunk level in children with PIMD could potentially improve their motor abilities.
During the high-speed turns of tandem skiing, lateral sway and centrifugal forces generate pelvic obliquity, upper-body imbalance and head movements that if ignored can strain the neck and trunk. Because poor control of the head and trunk, which increases the risk of cervical microinjury, is typically observed in children with PIMD, we expected to record sig- nificantly higher head angular speeds in these children com- pared to those in the children in the TD group. Contrary to this hypothesis, we observed no significant difference between the groups for head angular speeds. It is possible that during tandem skiing, the aforementioned perturbations during turn- ing induce postural adaptations that are present throughout highly varying levels of motor ability. Upper body and head control in response to transient rapid perturbations has been previously demonstrated to rely on rapid-acting stretch reflexes and intrinsic biomechanical mechanisms (i.e., passive stiffness and damping from joints, spinal ligaments, and mus- cles/tendons) as primary contributors as well as on medium- to long-latency sensory integration from the vestibular, pro- prioceptive, and (to a lesser extent) visual systems.25 In chil- dren and youth with PIMD, even partial integrity of these lower-level neural and musculoskeletal systems might allow them to adapt their postures to the external perturbations induced by tandem skiing, thus eliciting motor activity in this severely physically limited population.
The mean angular speeds at the trunk and pelvis level were
significantly lower in the PIMD group. Passive external con- straints, such as the tension placed on the four-point harness to secure the riders in the TSs, might have played a role in decreas- ing trunk and pelvis movements for the PIMD participants. Even if the pilots were instructed to respect identical security measures in both groups, one cannot exclude the possibility that the participants with disabilities were more tightly secured into the TSs due to both physical and behavioral issues. In addition, voluntary movements, such as the optimization of sitting posture and comfort during the activity, might have been higher in the TD group and might have contributed to the observed difference. The increase in heart rate observed during the activity was similar in both groups and is an additional indication of the significant and most likely positive physiological effects of tan- dem skiing for children with PIMD. Previous research has shown that in habitual daily activity, persons with PIMD use only small fractions of their theoretical heart rate reserve, most likely due to their sedentariness and low exposure to physical activity.14 The elevation of heart rate to an average of 140 bpm in children with PIMD brings them to above 70% of the average maximal heart rate of the group based on the Tanaka equation (208–0.7×age)26, which is well within the moderate-to-vigorous physical activity range and the recommended heart rate targets for aerobic fitness in children.27 Surprisingly, heart rate adapta- tion upon stopping the activity was also analogous in the PIMD and TD groups, indicating that children with PIMD might be appropriate targets for aerobic training when provided with a
correctly tailored and adapted physical activity.
One of the main limitations of our study was that we only measured proxy markers of postural control and increased phy- sical activity in our subjects. We were limited to measuring the
3D angular speeds of body segments during each turn because orientation estimation was not possible in our particular setup. Indeed, we were not able to perform the preliminary active and/ or passive calibration movements that would have been neces- sary for adequate segment orientation in children with PIMD because of (1) their lack of voluntary movements and (2) their orthopedic limitations, particularly spinal misalignments, at rest. For postural control, future studies should include surface elec- tromyography recordings of the main neck and trunk muscles to directly measure postural and phasic muscle activity in children with PIMD and to confirm that active postural adaptations occur during tandem skiing. For physical activity, energy expenditure should ideally be measured by portable indirect calorimetry, but tolerance of the face mask necessary to measure oxygen con- sumption might prove an issue in children with severe cognitive and possible behavioral issues.
In conclusion, tandem skiing appears to elicit active postural adaptations and increased heart rate in children and youth with PIMD to a similar extent to those of their TD peers. Therefore, tandem skiing can be considered a rare genuine sport activity for this category of children with special needs, and in our opinion, its practice should be encouraged on a regular basis. Tandem skiing can be offered at scale, as demonstrated by charitable foundations in Switzerland, and its implementation in educa- tional or rehabilitation settings with appropriate access to ski resorts is therefore conceivable. Further research aimed at mea- suring the effects of repeated and regular practice of tandem skiing in children with PIMD could supply further evidence of the usefulness of this approach.

Acknowledgments
F. Degache, A. Bonjour, D. Michaud, L. Mondada and C.J. Newman thank all the participating special-needs schools and teachers for their help, the Terrévent foundation (Geneva, Switzerland) for their generous sponsorship of our project, and the Just for Smiles foundation (Villeneuve, Switzerland) for providing tandem skiing sessions and logistic support. Finally, F. Degache, A. Bonjour, D. Michaud, L. Mondada and C.J. Newman are indebted to the participants and their families for their participation and exceptional commitment.

Declaration of Interest
The authors report no conflicts of interest. The authors alone are responsible for the content and writing of the paper.

ORCID
F. Degache http://orcid.org/0000-0002-1620-9330 CJ. Newman http://orcid.org/0000-0002-9874-6681

References
⦁ Nakken H, Vlaskamp C. A need for a taxonomy for profound intellectual and multiple disabilities. J Policy and Practice Intellect Disabil. 2007;4:83–87.
⦁ Vlaskamp C, Cuppen-Fonteine H. Reliability of assessing the sensory perception of children with profound intellectual and multiple disabilities: a case study. Child Care Health Dev. 2007;33:547–51.
⦁ Janicki MP, Dalton AJ. Sensory impairments among older adults with intellectual disability. J Intellect Dev Disabil. 1998;23:3–11.

⦁ Holenweg-Gross C, Newman CJ, Faouzi M, Poirot-Hodgkinson I, Berard C, Roulet-Perez E. Undernutrition in children with profound intellectual and multiple disabilities (PIMD): its prevalence and influence on quality of life. Child Care Health Dev. 2014;40:525–32.
⦁ Lancioni GE, O’Reilly MF, Basili G. Review of strategies for treating sleep problems in persons with severe or profound mental retarda- tion or multiple handicaps. Am J Ment Retard. 1999;104:170–86.
⦁ Nakken H, Vlaskamp C. Joining forces: supporting individuals with profound multiple learning disabilities. Tizard Learn Disabil Rev. 2002;7:10–15.
⦁ Floriani V, Kennedy C. Promotion of physical activity in children. Curr Opin Pediatr. 2008;20:90–95.
⦁ Keeton VF, Kennedy C. Update on physical activity including special needs populations. Curr Opin Pediatr. 2009;21:262–68.
⦁ Lipnowski S, Leblanc CM. Canadian Paediatric Society HAL, Sports Medicine C. Healthy active living: physical activity guidelines for children and adolescents. Paediatr Child Health. 2012;17:209–12.
⦁ Gellish RL, Goslin BR, Olson RE, McDonald A, Russi GD, Moudgil VK. Longitudinal modeling of the relationship between age and maximal heart rate. Med Sci Sports Exerc. 2007;39:822–29.
⦁ Lauruschkus K, Westbom L, Hallstrom I, Wagner P, Nordmark E. Physical activity in a total population of children and adolescents with cerebral palsy. Res Dev Disabil. 2013;34:157–67.
⦁ Sato H, Iwasaki T, Yokoyama M, Inoue T. Monitoring of body position and motion in children with severe cerebral palsy for 24 hours. Disabil Rehabil. 2014;36:1156–60.
⦁ Van Der Heide DC, Van Der Putten AA, Van Den Berg PB, Taxis K, Vlaskamp C. The documentation of health problems in relation to prescribed medication in people with profound intellectual and multiple disabilities. J Intellect Disabil Res. 2009;53:161–68.
⦁ Waninge A, Van Der Putten AAJ, Stewart RE, Steenbergen B, Van Wijck R, Van Der Schans CP. Heart rate and physical activity patterns in persons with profound intellectual and multiple dis- abilities. J Strength Conditioning Res. 2013;27:3150–58.
⦁ Dewar R, Love S, Johnston LM. Exercise interventions improve postural control in children with cerebral palsy: a systematic review. Dev Med Child Neurol. 2015;57:504–20.

⦁ Dirienzo LN, Dirienzo LT, Baceski DA. Heart rate response to therapeutic riding in children with cerebral palsy: an exploratory study. Pediatr Phys Ther. 2007;19:160–65.
⦁ Chung PJ, Vanderbilt DL, Schrager SM, Nguyen E, Fowler E. Active videogaming for individuals with severe movement disor- ders: results from a community study. Games Health J. 2015;4:190–94.
⦁ Terada K, Satonaka A, Terada Y, Suzuki N. Training effects of wheelchair dance on aerobic fitness in bedridden individuals with severe athetospastic cerebral palsy rated to GMFCS level V. Eur J Phys Rehabil Med. 2017;53(5):744–50.
⦁ Trost SG, O’Neil M. Clinical use of objective measures of physical activity. Br J Sports Med. 2014;48:178–81.
⦁ Yu G, Jang YJ, Kim J, Kim JH, Kim HY, Kim K, Panday SB. Potential of IMU Sensors in Performance Analysis of Professional Alpine Skiers. Sensors. 2016;16(4):463.
⦁ Fasel B, Praz C, Kayser B, Aminian K. Measuring spatio-temporal parameters of uphill ski-mountaineering with ski-fixed inertial sensors. J Biomech. 2016;49:3052–55.
⦁ Palisano RJ, Rosenbaum P, Bartlett D, Livingston MH. Content validity of the expanded and revised gross motor function classi- fication system. Dev Med Child Neurol. 2008;50:744–50.
⦁ Saavedra SL, Woollacott MH. Segmental contributions to trunk control in children with moderate-to-severe cerebral palsy. Arch Phys Med Rehabil. 2015;96:1088–97.
⦁ Curtis DJ, Butler P, Saavedra S, Bencke J, Kallemose T, Sonne- Holm S, Woollacott M. The central role of trunk control in the gross motor function of children with cerebral palsy: a retro- spective cross-sectional study. Dev Med Child Neurol. 2015;57:351–57.
⦁ Goodworth AD, Peterka RJ. Contribution of sensorimotor inte- gration to spinal stabilization in humans. J Neurophysiol. 2009;102:496–512.
⦁ Tanaka H, Monahan KD, Seals DR. Age-predicted maximal heart rate revisited. J Am Coll Cardiol. 2001;37:153–56.
⦁ Siegel J. Children’s target heart rate range. Am J Health Educ. 1988;59:78–79. TD-139