**Haoyu Li 1,\*, Stéphane Derrode <sup>1</sup> and Wojciech Pieczynski <sup>2</sup>**


Received: 16 July 2019; Accepted: 26 September 2019; Published: 29 September 2019

**Abstract:** Lower limb locomotion activity is of great interest in the field of human activity recognition. In this work, a triplet semi-Markov model-based method is proposed to recognize the locomotion activities of healthy individuals when lower limbs move periodically. In the proposed algorithm, the gait phases (or leg phases) are introduced into the hidden states, and Gaussian mixture density is introduced to represent the complex conditioned observation density. The introduced sojourn state forms the semi-Markov structure, which naturally replicates the real transition of activity and gait during motion. Then, batch mode and on-line Expectation-Maximization (EM) algorithms are proposed, respectively, for model training and adaptive on-line recognition. The algorithm is tested on two datasets collected from wearable inertial sensors. The batch mode recognition accuracy reaches up to 95.16%, whereas the adaptive on-line recognition gradually obtains high accuracy after the time required for model updating. Experimental results show an improvement in performance compared to the other competitive algorithms.

**Keywords:** gait analysis; lower limb locomotion activity; triplet Markov model; semi-Markov model; on-line EM algorithm
