Background and aims : Sarcopenia, defined as a decline in both muscle mass and function, has been recognized as a major determinant of poor outcome in hemodialysis patients. It is generally assumed that sarcopenia is driven by muscle atrophy related to protein energy wasting. However, dynapenia, defined as weakness without atrophy, has been characterized by a different disease phenotype from sarcopenia. The aim of this study was to compare the characteristics and prognosis of sarcopenic and dynapenic patients among a prospective cohort of chronic haemodialysis (CHD) patients. Methods : Two hundred and thirty two chronic haemodialysis patients were enrolled from January to July 2016 and then followed prospectively until December 2018. At inclusion, weakness and atrophy were respectively evaluated by maximal voluntary force (MVF) and creatinine index (CI). Sarcopenia was defined as the association of weakness and atrophy (MVF and CI below the median) while dynapenia was defined as weakness not related to atrophy (MVF below the median, and CI above the median). Results : From a total of 187 prevalent CHD patients (65% of men, age 65,3 (49,7-82,0) years), 44 died during the follow-up period of 23.7 (12,4-34,9) months. Sarcopenia and dynapenia were observed in 33.7% and 16% of patients respectively. Compared to patients with sarcopenia, patients with dynapenia were younger and with a lower Charlson score. By contrast, mortality rate was similar in both groups (38 and 27% respectively). After adjustment for age, sex, LTI, serum albumin, hs-CRP, haemoglobin, nPCR, dialysis vintage and Charlson score, only patients with dynapenia were at increased risk of death (HR = 2.99 CI (1.18-7.61) p = 0.02) . Conclusions : Screening for muscle functionality is highly warranted to identify patients with muscle functional impairment without muscle atrophy. By contrast to sarcopenia, dynapenia should appear as a phenotype induced by uremic milieu, characterized by young patients with low Charlson score and poor prognosis outcome independently of serum albumin, hs-CRP, haemoglobin, nPCR and dialysis vintage.