Abstract: Background 
Toe-walking is a phenomenon present in 20-30% of ASD subjects. Persistent tip-toe behavior (TTB) can produce a shortening of the Achilles’s tendon (made up by the soleus muscle – SM, and gastrocnemius muscle - GM) in ASD subjects. It is currently unclear why some ASD subjects develop this tendon shortening while others do not. One possible contributing factor could be the amount of time subjects spend in TTB during the day. In a previous study, we described three mutually exclusive clinical functional TTB classes of increasing severity: TTB present only in running (TTBclass1), in walking and running (TTBclass2), in standing, walking and running (TTBclass3). We also found the existence of a positive correlation between the severity of TTB presence and the Achilles’s tendon shortening using a qualitative testing approach. 
Objectives 
The aim of this cross-sectional study is to evaluate the relationship between the quantity of TTB expressed during both static and dynamic tests and the Achilles tendon shortening in a cohort of TTB and NON-TTB ASD subjects. 
Material and methods 
The cross-sectional study included 51 consecutive ASD subjects (44 males, 7 females, mean age = 13.9 years – 3.67 SD) diagnosed according to the DSM V criteria and under observation at our Institute. The ASD severity was established through ADOS (2nd version). A therapist assessed the amount of TTB during both standardized static and dynamic tests previously described. The intensity of TTB expression during the static and dynamic tests was quantified as percentage of time spent on the tip-toes and as the percentage of toe steps, respectively. Two therapists, blind to the TTB quantitative testing, assessed both the soleus and gastrocnemius muscle lengths using a manual goniometer.  
Results 
The overall ADOS calibrated severity score (CSS) of all the subjects was 7.61 (1.65 SD). Overall 21/51 subjects presented TTB. Two subjects were in TTBclass1, 10 in TTBclass2 and 9 in TTBclass3.The mean percentage of time spent in TTB during the static test was 5.96% (15.68 SD) (range: 0%-96%). The mean percentage of toe steps during the dynamic test was 15.39% (26.51 SD) (range: 0%-100%). The mean length of the left and right GM of the sample were 6.84° (6.2 SD) and 7.9° (6.07 SD) respectively. The mean length of the left and right SM of the sample were 16.1° (6.89 SD) and 15.96° (6.36 SD) respectively. Using a Pearson’s correlation test we found a significant inverse correlation between the percentage of time spent on the tip-toes and both the GM and SM lengths (left GM: r=-0.703, p<0.001; right GM: r=-0.678, p<0.001; left SM: r=-0.541, p<0.001; right SM: r=-0.518, p<0.001 respectively). We also found an inverse correlation between the percentage toe steps and both the GM and SM lengths (left GM: r=-0.484, p<0.001; right GM: r=-0.425, p=0.002; left SM: r=-0.386, p=0.005; right SM: r=-0.363, p=0.009 respectively). 
Conclusions 
The quantitative data show the existence of an inverse relationship between TTB intensity during both the static and the dynamic tests and the Achilles’s tendon shortening in ASD subjects. Further research is required to confirm the results. 

Notes:

(1) Autism Research Unit, Villa Santa Maria Foundation, Tavernerio, Italy,
(2) School of Medicine and Surgery, University of Milan-Bicocca, Milano, Italy