A total of 41 older patients participated in this prospective observational study, which was undertaken between September 2017 and November 2018 at our geriatric acute care hospital department. A detailed description of the methods has been reported elsewhere . Briefly, in order to investigate the effect of immobility on mid-thigh cross-sectional muscle area, patients were recruited based on their mobility status at hospital admission. Mobility status was evaluated according to walking ability as described by the respective item of the Barthel-Index (BI)  and patients were grouped into two categories as mobile group (walking ability score of 15 or 10) and immobile group (walking ability score of 5 or 0).
It is worth noting that mobile and immobile patients were selected from the geriatric day clinic and the geriatric hospital department, respectively. Geriatric day clinic is a facility in between ambulatory and in-patient treatment, i.e. patients stay for 7-8 hour every day for several days and get their diagnostic procedures and treatment. The remaining time they stay at home, which implies they are more or less mobile and able to take care for themselves. Indeed, patients from the geriatric day clinic had better functional and nutritional status compared to those from the geriatric hospital department. In general, the patients who are admitted to the geriatric hospital department are really ill and frail and sometime stay even more than 16 days in hospital based on their clinical situation.
The inclusion criteria for participation of both groups were patients of 65 years or older who were expected to be hospitalized for at least 14 days, ability to cooperate and written informed consent. Exclusion criteria were immobility for more than 3 weeks prior to admission, edema, leg amputation, decompensated heart failure and expected change in diuretic dose during hospitalization and pacemaker implants. Functional status, body weight and mid-thigh MRI measurements were conducted within 24 hours after hospital admission (baseline) and before discharge (follow-up). In addition, geriatric assessment was performed at hospital admission except the Barthel-Index and the muscle strength measurement, which were evaluated on admission and at discharge. C-reactive Protein (CRP) was analyzed according to standard clinical procedures at hospital admission. The study protocol had been approved by the ethical committee of Ruhr-University Bochum (17-6048, approved on 08.08.17).
Nutritional status was evaluated using the Mini Nutritional Assessment Short Form (MNA-SF)  and self-caring activities were determined using Barthel-Index (BI) . The point’s range of the German version of the BI is 0-100 pts., with 100 pts. indicating independency in all activities of daily living. Evaluation of frailty was based on the FRAIL simple scale  and the risk of sarcopenia was investigated with the use of SARC-F questionnaire . Medical comorbidities were evaluated using Charlson Comorbidity Index (CCI) .
Food intake was determined using the semi-quantitative plate diagram method . Irrespective of mobility status, similar nutrition was provided to all patients except oral nutritional supplements which were only provided for malnourished patients. Physical therapy for at least 30 minutes twice a day was offered to all patients as a routine rehabilitation program. However, immobile patients who were more or less bedridden participated less. Furthermore, all patients had an individualized training program according to the deficiencies in activities of daily living.
Body weight was assessed in light clothing with an accuracy of 0.1 kg and height was measured to the nearest 0.5 cm with a stadiometer during hospitalization. The degree of unintentional weight loss prior to admission was obtained either by interviewing the patients, if competent, or asking their proxy, where necessary.
The protocol described by Gandevia  and a Jamar dynamometer (Lafayette Instrument Company, Lafayette, IN) were used to assess isometric knee extension strength (Strength measuring device FK, Sauter GmbH, Balingen, Germany) and hand grip strength (HGS) respectively. Knee strength was measured with the patient in a seated position with a strap around the leg 10 cm above the ankle joint whereas the hip and knee joint angles positioned at 90 degrees (Figure 1a). Functional status was measured three times at the dominant or unaffected side of hand/leg and the maximum score was recorded.
Mid-thigh MRI cross sectional area
The MRI scan was performed with a Siemens Magnetom Sonata, 1,5 Tesla (Siemens Medical Solutions, Erlangen, Germany) to assess the mid-thigh muscle, subcutaneous and intermuscular fat cross-sectional area (CSA). Briefly, middle length of the non-affected, preferably dominant thigh was measured and marked with a semi-circumferential line, drawn with a permanent marker for replication at the time of follow-up. Directly before the MRI measurement, two MRI-detectable capsules were fixed at both ends of the line (Figure 1b). Five T1-weighted transversal scans with slices thickness of 4 mm were obtained and the single-slice with both landmarks was selected for segmentation. Single slice CSA of muscle, subcutaneous and intermuscular fat at mid-thigh were manually segmented (Figure 1b) using the SliceOmatic software (version 5.0; Tomovision, Montreal, Canada). The segmentation of MRI images was blinded for group assignment of subjects.
The statistical analysis was performed using SPSS statistical software (SPSS Statistics for Windows, IBM Corp, Version 24.0, Armonk, NY, USA). Continuous variables are expressed by their means and standard deviations (SDs) or median values with interquartile ranges (IQR), as appropriate. Categorical variables are expressed as n (%). Differences between variables and between baseline and follow-up within immobile and mobile groups were analyzed by using paired samples t test for normally distributed values. Differences in variables at baseline and in magnitude of changes at follow-up between groups were analyzed by using an unpaired t test in normally distributed variables and the Mann-Whitney U test for continuous variables with non-normal distribution. Chi square test was used to compare Categorical variables. A stepwise multiple regression analysis was used to examine the impact of risk factors such as mobility status, age, BMI, malnutrition and inflammation (as independent variables) on changes in mid-thigh muscle CSA as a percentage of initial muscle area as (dependent variable). P < 0.05 was determined as the limit of significance.