All participants were grouped based on the score of Fried’s Frailty phenotype model [4] into the groups “Non-frail” (n = 36, frailty score = 0), “Pre-frail” (n = 14, frailty score = 1 or 2), and “Frail” (n = 11, frailty score = 3, 4 or 5) (see Table 1 for details).

Included in the study were participants able to walk without walking aids. Exclusion criteria were: cognitive impairment (< 24 points in the Mini-Mental-Status-Test (MMSE) according to Folstein et al. [34] or a severely limited mobility that precludes independent care (e.g., bedridden). The latter was determined during a screening interview and was considered fulfilled if the participant was largely able to move independently within the home. Participants with severe visual impairments, uncontrolled cardiovascular disorders, uncontrolled Parkinson’s syndrome, acute chronic obstructive bronchitis, or acute states of confusion (e.g., delirium) were also not eligible to participate in this study.

All participants gave their written and oral consent. The study was approved by the independent medical Ethics Committee at the RWTH Aachen Faculty of Medicine (ethics committee number 142/18).

The study was performed in the motion analysis laboratory of the department of geriatric medicine of the university hospital RWTH Aachen, Germany. A three-dimensional optical motion capture system (Qualisys AB, 5+ series, Göteburg, Sweden) with 10 cameras tracked the marker trajectories at 120 Hz. In total, 52 reflective markers were placed at anatomical landmarks on participants’ bodies following a prescribed marker set protocol [35]. The calibrated anatomical system technique (CAST) was used to place and determine the movement of segments. The measurements were done using Qualisys Track Manager (Version 19.1, Qualisys AB, Gothenburg, Sweden). After markers labeling at the Qualisys Track Manager software, raw data were exported to .c3d for further analysis with the software Visual 3D (Version 6.0, C-Motion. Inc., Germantown, MD, USA). Force data were recorded by two force plates (Bertec Corporation, Columbus, Ohio, USA), which were embedded in the surface in the middle of a 10-m walkway. The movement and force data were filtered using a fourth-order low-pass Butterworth filter with a cut-off frequency of 5 Hz.

In all participants, the five criteria of Fried’s phenotype of frailty [4] were assessed before GI data collection: unintentional weight loss, subjectively perceived fatigue, low physical activity, slow walking speed, and muscle weakness. For this purpose, questions were first asked about unintentional weight loss of more than 5 kg within the last year and about subjectively perceived fatigue. Last-mentioned was done by the “Fatigue assessment according to Fried”, which takes up two questions of the Center for Epidemiologic Studies Depression Scale [36]. Using a short version of the Minnesota Leisure Time Physical Activity Questionnaire [37], physical activity was assessed by asking about various leisure time activities within the last 4 weeks. Walking speed was measured over a 4.57 m walking distance, and finally, to detect possible muscle weakness, strength measurement of the dominant hand was performed three times with calculation of the mean value. We used Fried’s cut-off values to assess the grip strength. One point was awarded for each deficit in one of the five categories. If one to two categories are fulfilled, the classification as “pre-frail” is made, from three as “frail”. Moreover, to evaluate the fear of falling and the balance ability of the old participants, the questionnaires of the Falls Efficacy Scale-International (FES-I) [38, 39] and the Activities-Specific Balance Confidence Scale (ABC) [40, 41] were collected before starting the GI trials.

For the measurement process, each participant was initially asked to stand quietly on a force platform in a relaxed posture on both legs. Both feet were then placed in a parallel position on the first force plate with the toes close to the second one. The width was not dictated and should correspond to their natural stance. Acquisition of force and motion data was triggered, just before the participants received a verbal cue, to begin walking. In response to the cue, they initiated gait with their leading leg at their usual walking pace until the end of the movement lab which corresponded to a walking distance of about 4 m. To become familiar with the experimental protocol, each participant first performed a practice trial. The practice trial was then immediately followed by five data collection trials. Each participant had the opportunity to take a break after a trial to avoid any exhaustion effects. For the study, every participant wore comfortable clothing, including a t-shirt, shorts, and anti-slip socks.

To describe GI, it was subdivided into the four sub-phases described above. Parameters, including the duration of the release phase (s), unloading phase (s), single support phase (s), and double support phase (s), were calculated. We used specific events to automatically identify the distinct phases by using the analysis software Visual 3D. The start of GI, and therefore also of the release phase, was defined to be 0.15 s before the minimum velocity of the CoP in the walking direction. The furthest point of posterolateral CoP displacement then marked the beginning of the unloading phase. The following single support phase started as soon as the toes of the swinging leg lost contact with the ground. The last sub-phase, the double support phase, was defined by the recontact of the heel of the swinging leg with the ground and ended with the lift-off of the toes of the initial stance leg.

The total duration of GI and the percentages of each sub-phase on the total duration of a respective study participant were calculated separately.

The length of the first step (m) was calculated between the first toe off-event of the swing phase and the initial contact of the foot with the force plate.

The maximum foot clearance (max. FC) during the first step (m) was calculated by the maximum value of displacement of a marker at the midfoot. The marker is a virtually created marker, whose position was determined centrally, i.e. at a 50% distance between the real markers at the toe and heel of the feet.

Descriptive statistics (mean and standard deviation (SD)) were calculated for demographic data (age, height, weight, and BMI) and each determined parameter from the arithmetic mean values of the five trials per person. Between-group differences in the total duration of GI, the durations of the four sub-phases, the step length of the first step, and the max. FC were tested with a one-way analysis of variance (ANOVA) for continuous variables. Bonferroni-adjusted post-hoc analysis was used during follow-up testing.

Since the duration of the sub-phases is also affected by a change in the total duration of the GI, we found that the relative durations of the sub-phases to the total duration of the GI is another interesting aspect to illuminate. Therefore, we additionally calculated the relative proportion (%) of the phase durations of the sub-phases in relation to the total duration of the GI. Therefore, group differences in the percentages of the different phase durations of GI were tested statistically with a one-way multivariate analysis of variance (MANOVA). Follow-up tests on separate univariate ANOVAs were conducted when appropriate. The level of significance was 0.05.

In addition to comparing the max. FC during the first step between groups, we also investigated the results of the FES-I and ABC for a possible correlation with this parameter. This was done for all participants (n = 61) to determine if there is an correlation between max. FC and fear of falling and/or confidence in balance, regardless of the participant’s frailty score. For this purpose, Spearman’s rank correlation was computed for each case. The interpretation of the effect strength was based on the classification according to Cohen [33]. Accordingly, the effect limits are 0.10–0.29 (weak), 0.30–0.49 (moderate), and greater or equal to 0.5 (strong).

All statistical tests were performed using IBM SPSS Statistics Version 27 (Armonk, New York, USA).