This study showed a dramatic reduction of the MVCQ assessed by the dynamometer in the stable patients with CF in comparison to the general population [3]. The 1STST was also decreased in the stable patients with CF in comparison to the general population (79%PV). Overall, we found similar values assessing the 1STST in patients with CF when as percentage of predicted values (79%PV) than in other studies (71%PV and 75%PV, respectively) [21, 22]. Of note, our sample was the only one including children and was larger. In patients with COPD, 1STST is estimated around 50%PV [18]. Obviously, COPD patients are older than CF patients and a decline in performance in absolute values across age groups has been described, like in CF [26].

Most importantly, the 1STST seems to be a good alternative to the dynamometer in order to assess the quadriceps force in patients with CF. Indeed, we showed a positive correlation between both tests. This is supported by the fact that our correlation coefficients are either similar to previous studies conducted in CF (r = 0.520; p = 0.008), or better than in COPD (r = 0.424, p = 0.03) [22, 29]. In addition, the 1STST has the advantage over the dynamometer, to assess functional exercise capacity of the patients. Moreover, the 1STST showed moderate correlation values with the disease severity, expressed by the FEV1. This was also previously described in CF and in COPD patients [21, 30].

We showed a reduced 1STST and MVCQ assessed by the dynamometer at the beginning of the IV antibiotherapy in comparison to the general population (69%PV and 50%PV respectively). Similarly, Wiebolt et al. showed an 8% reduction of MVCQ assessed by the dynamometer for the same patient before and 1 month after the IV antibiotherapy [10]. In contrast, Burtin et al. did not show any significant differences at the admission, using both a voluntary and an involuntary method to assess the quadriceps force (femoral nerve stimulation) [12]. Nevertheless, by the fact that at the beginning of an exacerbation patients are often more symptomatic, a voluntary test could underestimate the muscular force compared to an involuntary test. In parallel, COPD patients had a higher reduction of the quadriceps force when registered at the admission of an exacerbation (22%) [31].

After the IV antibiotherapy, we assessed for the first time in CF a significant gain in the 1STST, irrespective of the localisation or of the indication of the antibiotherapy. This could partly due to the rehabilitation program in hospital as we observed that the gain of 1STST was higher when the antibiotherapy was performed in the hospital compared to home (Fig. 3A) [20, 21]. A learning effect cannot be excluded totally, even if the subject trained partly before. Furthermore, 13 patients (43%) showed a clinically relevant improvement in the number of repetitions as it was higher than the minimal clinically important difference (5 repetitions) [21]. In COPD, it has been previously shown that the 1STST was sensitive to a rehabilitation program but did not seem correlated to the exacerbation rate [18, 30]. Regarding to the MVCQ, we showed only a trend toward an improvement after the IV antibiotherapy irrespective of the localisation or the indication of the IV antibiotherapy. Wiebolt et al. described a significant enhancement after the IV antibiotherapy given for an exacerbation in patients with CF and conducted at the hospital without any specific rehabilitation program; Selvadurai et al. showed also a significant improvement of the quadriceps force, in children, but here, only with resistance exercise program [10, 11]. In COPD, Spruit et al. described a 5% decline of quadriceps force after an hospitalization for an exacerbation without pulmonary rehabilitation [31].

As hypothesized, the gain in 1STST was positively and moderately correlated to the improvement of the MVCQ. However, the enhancement of the 1STST and MVCQ was not correlated to physical activity at the beginning of the IV antibiotherapy. Indeed, no significant difference was found regarding to the gain in the 1STST depending on the number of steps. We could have expected that the more active patients could have a better 1STST and quadriceps force at the admission than the sedentary patients, and would thus less ameliorate their tests after the IV antibiotherapy, as previously suggested by Burtin and Trooster [3, 12]. Thus, this discrepancy may be explained by the relatively active patients included in our study (with a mean > 8000 steps/day). A longer time of accelerometer’s wearing could have maybe be proposed [3, 10].

As Burtin, we did not see any correlation between the enhancement of the 1STST and the quadriceps force and inflammation [12]. The enhancement of the 1STST was significantly better when the IV antibiotherapy was realized at hospital, despite a significant lower level of physical activity (number of steps/24 h) due to the patient’s confinement into their room. This supports that specific rehabilitation program during the IV antibiotherapy is necessary and efficient to counteract isolation measures.

The limitations of our study are the absence of individual values (at steady state) for each patient treated by antibiotics, the lack of the evaluation of physical activity intensity and the short duration of the actimeter measurement. Only 2 studies have calculated reference values for the 1STST in the general population: Strassman et al. for people aged from 20 to 79 years old and Reychler et al. for children aged from 6 to 12 years old [32]. Between those ages (1–5 and 13–19 years old), we do not have reference values. Reference values form Strassmann et al., were used in this study. So, we extrapolated these reference values for 13 patients (aged from 9 to 19 years) which could have led to a small overestimation of their reference values. Finally, no practice 1STST was realized in our study. However, to reduce the learning effect, the test was first demonstrated by the operator, and subjects had the opportunity to perform a few practice cycles to ensure correct realization [21]. Accelerometers were used during only 3 days to avoid extra visits to the centre and the influence of the different behaviour related to the weekend although this duration could be debated.

In conclusion, as muscular strength and exercise capacity are negative prognosis factors for CF, they should be part of the routine evaluation through 1STST. The 1STST is useful to detect and follow muscular weakness during an IV antibiotherapy, in parallel with a specific rehabilitation program.