Ankle-foot orthosis (AFO) is commonly prescribed for stroke survivors with foot drop to aid in foot clearance while walking and reduce fall risk. However, its impact on the contraction patterns of paretic ankle muscles remains inconclusive. This pilot study investigated the contraction of paretic tibialis anterior (TA) and medial gastrocnemius (MG) muscles in twenty sub-acute stroke patients wearing AFO during walking using a wearable dynamic ultrasound imaging and motion sensors. Results showed an increase in TA muscle thickness throughout a gait cycle (p > 0.05) and a significant increase in TA muscle surface mechanomyography (sMMG) signal during pre- and initial swing phases (p < 0.05) when using AFO. MG muscle thickness generally decreased with AFO (p > 0.05), aligning more closely with healthy adults' trends throughout a gait cycle. MG sEMG signal significantly decreased during the initial and mid-swing phases when wearing AFO (p < 0.05). The TA-MG co-contraction index notably decreased during initial and mid-swing phases with AFO (p < 0.05). These findings suggest that AFO can promptly influence the contraction patterns of paretic ankle muscles during walking in stroke patients, but further research is needed to understand its long-term effects.