The study aims to identify which Syde®-derived digital outcomes are reliable in FGFR3-related Skeletal Dysplasia. This requires to set-up a natural history study to measure limb movements in patients with ACH or HCH.

Inclusion Criteria:

1. Aged 3 years old or older at the time of consent.

2. Written informed consent obtained:

   1. From the parent(s) or legal guardian(s) for participants under 18 years old.
   2. From participants 18 years old and older.

3. Affiliated to, or beneficiary of a social security category

4. Able to walk unassisted for at least 10 meters.

5. Genetically confirmed diagnosis with one pathogenic variant (ACMG class IV & V) of achondroplasia or hypochondroplasia.

6. Participant (and caregivers for participants under 18 years old) willing and able to comply with all study procedures including: questionnaires, Syde® related procedures

Exclusion Criteria:

1. Subjects who have short stature condition other than ACH/HCH.
2. Presence of cognitive disorders that limit their understanding of the data collection process (training of device use and 4-week recording periods every 6 months, device return at the end of the study), the implication of the study and consent.
3. Presence or history of any concurrent disease or condition that could interfere with study participation, impact pediatric growth, affect motor or balance or gait function (such as neurological, endocrine, infectious, allergic, osteoarthritis, or inflammatory), assessed by the investigator.
4. Females who are pregnant, or planning to become pregnant during the study duration.
5. Body Mass Index = 35 kg/m2.
6. Recent upper and/or lower limbs injury (trauma/fracture or surgery) in the 6 months preceding inclusion.
7. Prior limb lengthening surgery or planned or expected to have limb lengthening surgery while enrolled in the study.
8. Presence of guided growth hardware (such as 8-plates) or planned orthopedic surgeries during the study.
9. Vulnerable patient (guardianship, curatorship or safeguarding of justice), unable to provide informed consent or who is unable to express their consent.

Participants will be recruited from the patient registry of the Centre de Référence des Maladies Osseuses Constitutionnelles located at NeckerHospital / IMAGINE Institute. The APPT association (Association des Personnes de Petite Taille) will also help with the recruitment through flyers distribution. All visits will take place at Necker Hospital.