Background:
Toe-walking is a phenomenon present in 20-30% of ASD subjects. Using a qualitative structured assessment, we previously described three mutually exclusive clinical functional classes of tip toe behavior (TTB) of increasing severity: during running(TTB1); during walking and running (TTB2); during standing, walking and running(TTB3). In another study, we found a positive relationship between the presence and severity of TTB and the Achilles’ tendon shortening. It is not clear if ASD subjects who present TTB manifest the Achilles’ tendon shortening at a young age and if this deficit is related to the age of onset.

Objectives:
This cross-sectional study aimed to evaluate: 1) if ASD subjects who present moderate-high TTB severity manifest a deficit in gastrocnemius and soleus muscles(GM and SM) length values (the two muscles that coalesce into Achilles’ tendon) at a young age (i.e., before 8 years); 2) if the SM and GM length values decrease with agein moderate-high TTB severity subgroup.

Methods:
The study group consisted in 106 subjects (mean age: 11.27 years – 4.95 SD; 83males) with autism diagnosed according DSM-5 or ICD-10 criteria. A therapist assessed the presence of TTB during standing, walking, and running using direct observation and an interview of the main caregiver living with the children. Two therapists assessed both the SM and GM length values using a manual goniometer following the methodology previously described. We considered moderate-highseverity TTB subjects those belonging to TTB2 and TTB3 subgroups. Therefore, to assess the presence of muscle length deficit at a young age (0-8 years), we compared the GM and SM length values between NON-TTB (n=28) and TTB2+TTB3 (n=9)subgroups using a Mann-Whitney test. To assess if the SM and GM length values decrease with age in moderate-high TTB severity subgroup we performed aSpearman correlation test between age and GM and SM length values in theTTB2+TTB3 subgroup (n=25).

Results: At age of 0-8 years, the mean length value of the left GM of NON-TTB subgroup (n = 28) was 18.61° (6.37°SD) vs a value of 9.22° (4.09°SD) TTB2+TTB3subgroup (n = 9) (p<0.001). The mean length value of the right GM of NON-TTB subgroup was 18.32° (4.97°SD) vs a value of 11.33°(4.36°SD) TTB2+TTB3 subgroup(p<0.001). The mean length value of the left SM of NON-TTB subgroup was 27.39°(7.57°SD) vs a value of 15.56° (4.5° SD) TTB2+TTB3 subgroup (p<0.001). The mean length value of the right SM of NON-TTB subgroup was 26.39° (5.83°SD) vs a value of 18° (2.97° SD) TTB2+TTB3 subgroup (p<0.001). Moreover, we found a significative inverse correlation between age and left GM (r=-0.702; p<0.001), right GM (r=-0.623;p=0.001), and right SM (r=-0.412; p=0.04) of the TTB2+TTB3 subgroup. We found a not significative correlation between age and left SM (r=-0.253; p=0.221).

Conclusions: ASD subjects who present moderate-high TTB severity manifest a significant decrease of Achilles’ tendon length values at a young age (i.e., before 8years). We also found the existence of a significative inverse relationship between age and the Achilles’ tendon length values.