The latest systematic reviews and consensus report conclude that evidence-based treatment of patellofemoral pain (PFP) includes patient education combined with hip and quadriceps strengthening (1–3). One of the biggest challenges in managing adolescents with PFP is that they do not follow their exercise prescription (4). They perform the exercises too fast, resulting in too short a contraction time, and with too few repetitions (5). This may impede successful recovery as a low exercise dose is associated with lower odds of recovery in individuals with PFP (4,6).
Several exercise parameters influence adaptations to exercises such as load, range of motion, and the number of repetitions and sets performed (7). One of the important parameters is contraction time, also referred to as time under tension (8). Evidence has highlighted the importance of adequate contraction time with longer contraction times found to increase muscle protein synthesis more than shorter contraction times (9). Elastic bands are often used to gain these adaptations, but without feedback, the patient does not easily know if they have stretched the band sufficiently to ensure a resistance that will evoke optimal adaptations.
Technologies such as metronomes (10,11) and exercise games (12) may improve exercise quality in both healthy and patient populations. These are not without shortcomings; a metronome does not give feedback on performance, and exercise games often require a lot of space to capture the movements by camera. One technology to provide real-time feedback on elastic band exercises is a sensor called BandCizer™ (BandCizer Aps, Denmark). It is a valid tool that can quantify contraction time, the number of repetitions performed, and the force used to stretch the elastic band by measuring the thickness of the band (13). BandCizer™ transmits data to an iPad (Apple Inc., Cupertino, CA), and the BandCizer™ app provides the user with real-time feedback on exercises (13,14). In a previous study of adolescents with PFP, BandCizer™ was found feasible for measuring contraction time and exercise dose during a 6-wk exercise intervention (5).
The purpose of this trial was to investigate if real-time feedback on contraction time during exercises would improve the ability to perform the exercises with the prescribed contraction time per repetition compared with no feedback on contraction time among adolescents with PFP during a 6-wk intervention. We hypothesized that adolescents who received real-time feedback on contraction time (feedback group) would deviate significantly less from the prescribed contraction time compared with the group not receiving feedback on contraction time (control group).
This is the primary trial report of a randomized, controlled, participant-blinded, superiority trial (NCT02674841) with a two-group parallel design conducted in Aalborg, Denmark. Reporting follows CONSORT guidelines for reporting parallel group randomized trials using the extension for nonpharmacological treatments (15). The reporting of the intervention follows the Template for Intervention Description and Replication (16). The trial protocol was published in an open access journal (17) and is referred to as recommended by the CONSORT Statement (18).
Deviations from trial protocol
The pulling force exerted during exercise was a predefined secondary outcome (17); however, some adolescents progressed to using silver and gold elastic bands, for which BandCizer™ was not calibrated. This did not influence the feedback on contraction time of either group, but repetitions performed with silver and gold elastic bands showed less pulling force than that of elastic bands with less resistance such as red, blue, and black elastic bands. Because of the uncertainty of the validity, pulling force data were not included.
Mean contraction time was added as a secondary, nonpredefined outcome to explore if deviations from the prescribed contraction time were due to a contraction time too long or too short or equally distributed, as the primary outcome did not differentiate between under- or overdosing.
The trial was approved by the Ethics committee of North Denmark Region (project ID no. N-20150070) before the inclusion of participants. Adolescents with PFP were recruited from a local GP clinic, through Facebook, or by other referral. A telephone screening was performed, and adolescents who fulfilled the criteria were invited to attend a clinical examination together with their legal guardian at Aalborg University Hospital. The assessor obtained written informed consent from the participants before any examination. Those under 18 yr of age required additional consent from a legal guardian (17).
Adolescents had to meet the following eligibility criteria, which were in line with a previous study (4). Inclusion criteria were as follows: 15 to 19 yr of age; anterior knee pain of nontraumatic origin, which is provoked by at least two of the following activities—prolonged sitting with bent knees or kneeling, squatting, running, jumping, or ascending or descending stairs; tenderness on palpation of the peripatellar borders; pain of more than 6 wk duration; and self-reported worst pain during the previous week ≥30 mm on a 100-mm visual analog scale (VAS). Exclusion criteria were as follows: concomitant pain from other structures in the knee (e.g., ligament, tendon, or cartilage), the hip, or the lumbar spine; previous knee surgery; and self-reported patellofemoral joint instability (17).
Adolescents were block randomized in block sizes of 2 to 8 (1:1) into two parallel groups of 20 adolescents using a random number generator on www.random.org. A researcher not involved in the data collection or analysis generated the allocation sequence and was the only person who knew the block sizes. After all baseline measurements were made, the assessor took a sequentially numbered opaque sealed envelope in which allocation was indicated (17).
Adolescents received different settings of real-time feedback from the BandCizer™ app on the iPad. The feedback group was provided with visual and auditory feedback on contraction time and pulling force, whereas the control group was only provided with real-time feedback on pulling force. Additional details on the app and the feedback can be found in the study protocol (17).
Adolescents were instructed to perform three elastic band exercises: seated knee extension and freestanding hip abduction and extension. These exercises have previously been found effective in patients with PFP (1,19,20). The exercise descriptors, adopted from Toigo and Boutellier (8), are described in Table 1.
Adolescents received an elastic band, a BandCizer™, and an iPad with the BandCizer™ app. They were instructed to perform exercises twice a week at home and once a week during a supervised group training session over 6 wk. To ensure that the adolescents stayed blinded toward the different types of feedback, they only attended group training sessions together with others randomized to the same type of feedback. Each adolescent’s 10–12 repetition maximum was determined by shortening the elastic band to a length where the adolescents felt that they would not be able to perform more than 10 repetitions. When the assessor considered the exercise to be performed correctly, the pulling force was measured at the end position when the pulling force was at its highest by the BandCizer™ and used as the recommended initial minimum pulling force in the app. If the adolescents were able to perform more than 10 repetitions per set, they were instructed to increase the load by shortening the band or changing to a different color of band.
A registered physiotherapist (the assessor) with 4 yr of clinical experience was responsible for participant inclusion and exercise instruction.
Adolescents were advised to continue participating in physical activity if (a) their pain was no higher than 30 mm on a 100-mm VAS during the activity, (b) their knee pain did not outlast the physical activity, and (c) there was no strong increase in symptoms postactivity. They were asked not to do any other type of strengthening exercises for the lower extremities during the trial period or seek other treatment for their knee pain.
All adolescents were told throughout the trial that compliance to exercises was important and would improve their odds of recovery. If they were unable to attend a group training session, they were asked to contact the assessor (17).
The primary outcome was mean deviation from the prescribed contraction time per repetition in seconds during the intervention. Mean deviation was calculated as the difference between actual contraction time and prescribed contraction time per repetition (8 s). For example, if the actual contraction time was 6.5, the deviation was −1.5 s, and if contraction time was 9.5 s, the deviation was +1.5 s. Contraction time was chosen as the primary outcome as it plays a large role in the total training dose (8,21–23), and adolescents have previously shown difficulties performing exercises with the prescribed contraction time (5). Deviations were presented in absolute values because both durations too long or too short were indicative of exercise not performed as prescribed, which was the aim of this study.
Secondary outcomes included (i) the number of repetitions performed during the intervention period; (ii) isometric strength (presented as N·kg−1) of knee extension, hip extension, and hip abduction; (iii) pain; (iv) Kujala Patellofemoral Scale (KPS); and (v) Global Rating of Change (GROC). Isometric strength was collected at baseline and follow-up using a dynamometer (Commander PowerTrack; JTECH Medical, Midvale, UT). Knee extension was tested in a seated position, hip abduction was tested in a side-lying position with a long lever, and hip extension was tested in prone with a short lever (17). The adolescents entered their rating of pain intensity into the app before and after each exercise. Pain was measured on a 100-mm VAS, where 0 mm was no pain and 100 mm was worst pain imaginable. A post hoc analysis of pain flare-up (defined as an increase of the clinically relevant 20 mm (24) or more on a 100-mm VAS) during exercise was performed. KPS was collected at baseline and at follow-up. KPS is a frequently used validated outcome measure in PFP (19,25,26), with scores from 0 to 100 where lower scores indicate greater pain and/or disability. GROC was recorded at follow-up on a 7-point Likert scale (1 = much worse to 7 = much improved). Adolescents were categorized as improved if they rated themselves as “improved” or “much improved” (categories 6 and 7) and not improved if they rated themselves from “much worse” to “slightly improved” (categories 1–5). This scale has previously been used in trials resembling this trial to assess self-reported change from baseline (4,19).
To blind the adolescents to the trial hypothesis, they were told that the trial concerned adolescents with PFP and exercising with a new sensor. They were told there would be two groups that used the BandCizer™ app in different ways, but they did not receive any information about the primary outcome measure. The adolescents attended separate group training sessions based on their randomization and had no contact with the participants of the opposite group (17).
Sample size estimation
It was expected that the feedback group would not deviate from the 8-s contraction time per repetition whereas the control group would have a deviation of 1.5 s, which is close to previous results of contraction time in this patient group (5). On the basis of the standard deviation of 1.22 found in the aforementioned study, a two-sided 5% significance level and a power of 80%, a sample size of 15 adolescents per group, was necessary. Taking into consideration possible dropouts, 40 adolescents were included.
The primary intention-to-treat analysis tested the between-group difference of mean deviation from the prescribed contraction time per repetition using an independent t-test. Any missing sets or repetitions from the prescribed were interpreted as noncompliance. A priori, we defined that if an adolescent had failed to perform any repetitions, the group mean value was imputed (17). If any adolescent failed to complete follow-up, the group mean value was imputed for the missing variable. In addition to the predefined secondary analyses (17), an independent t-test of mean contraction time and a z-test of the between-group difference of proportions of repetitions performed with the prescribed contraction time ±1 s were conducted. The explorative analysis of the association between pain and compliance originally described in the trial protocol was further defined as pain from before to after exercise during the first three training sessions. Correlations were labeled low (r = 0–0.35), moderate (r = 0.36–0.67), high (r = 0.68–0.89), or very high (r = 0.9–1) (27). The percentage of exercise dose performed and thereby overall compliance was calculated as a percentage of the prescribed total contraction time, which was 25,920 s (8 s per repetition × 10 repetitions per set × 3 sets per leg × 3 exercises per session × 3 weekly sessions × 6 wk of intervention). STATA version 14 was used as statistical software.
Adolescents were enrolled from February 2016 to September 2016. Final follow-up was conducted in October 2016. Of the 59 adolescents who contacted the primary investigator or were contacted through the GP, 41 were eligible to be examined at the hospital. One did not meet the criteria for inclusion (Fig. 1). Baseline characteristics were similar in the groups (Table 2). Data from one adolescent could not be extracted, so the group mean was used for this adolescent. Two adolescents failed to participate in the follow-up. One of these did answer KPS and GROC. Group means were imputed for the missing outcomes of these two participants.
The feedback group performed the exercises with a mean deviation of 1.5 ± 0.5 s and were 2.7 s (95% confidence interval = 2.2–3.2, P < 0.001; Fig. 2) closer to the prescribed contraction time of 8 s than the control group who performed the exercises with a mean deviation of 4.3 ± 1.0 s.
The mean contraction time of the feedback group was 6.5 ± 0.5 s, whereas the mean contraction time of the control group was 3.9 ± 1.1 s (mean difference = 2.7 s, 95% confidence interval = 2.1–3.2, P < 0.001). On the basis of total contraction time throughout the intervention, the feedback group received 35.4% of the prescribed total contraction time, whereas the control group received 20.3%. The feedback group increased isometric hip and knee strength from 7.71 ± 2.65 to 9.55 ± 3.06 N·kg−1, whereas the control group increased isometric strength from 8.37 ± 2.3 to 9.28 ± 2.02 N·kg−1, which was significantly less (Table 3). This corresponded to relative changes of 25% versus 7%, respectively. There was a moderate correlation between total contraction time and the difference in isometric strength from baseline to follow-up (r = 0.38, P = 0.016), with a higher total contraction time associated with larger increases in isometric strength.
Pain and exercise
There was no association between the difference in pain from before to after the exercises during the first three training sessions and total contraction time (r = 0.09, P = 0.611). Among the control group, 17% of the exercises caused a pain flare-up (≥20/100 mm), whereas the feedback group had 13% flare-ups. Differences in pain before and after exercises and flare-ups by training session are seen in Figure 3.
No adverse events were observed.
This is the first trial investigating the efficacy of real-time feedback on contraction time during hip and knee exercises among patients with PFP. The trial shows that real-time feedback on contraction time improves the ability of adolescents with PFP to follow the prescribed contraction time, which was reflected in greater strength gains.
Longer contraction times are associated with improvements in strength
PFP is one of the most common knee conditions in adolescents with a population prevalence of 6%–7% (1,28). Although the condition has traditionally been considered self-limiting, more than 55% will continue to experience pain after 2 yr (29). This emphasizes the need for effective treatments. One of the largest barriers to successful intervention is compliance (4–6). The real-time feedback tested in this trial significantly improved the adolescents’ ability to follow the prescribed contraction time and resulted in a total exercise dose almost twice as high in the feedback group. These results point toward the promise of using real-time feedback to increase the effectiveness of strengthening exercises in adolescents with PFP.
Despite a similar number of repetitions performed in both groups, the adolescents who received feedback received an exercise dose almost twice as high as the control group due to a higher mean contraction time. Longer contraction time per repetition has previously been found to increase myofibrillar protein synthesis leading to larger increases in strength (9). Two previous studies in adolescents with PFP reported an increase in hip and knee strength after an exercise intervention. The reported increase in strength of these studies is similar to the strength increase of the control group in the current study, whereas the feedback group had an increase more than twice as high (5,30).
Although not powered for the patient-reported outcomes, both KPS and GROC tended to improve more in the feedback group, which suggests that feedback may also improve patient-reported outcomes compared with no feedback. Previous trials on similar populations have found similar results and report that higher exercises doses may improve patient-reported outcome, but this is the first trial using objective measures of compliance (4,6). Collectively, this demonstrates the importance of adequate contraction time and compliance in interventions for adolescents with PFP.
Pain and the association with exercises
Pain during exercises is widely recognized as a contributor to poor compliance (31–33); however, the change in pain from before to after exercise during the first three training sessions was not associated with total exercise dose received during the intervention. This suggests that the first impression of the exercises in terms of pain might not play an important role in explaining poor compliance. Figure 3 shows a trend toward pain declining before exercise as a function of the number of training sessions performed. Increases in pain postexercise appeared to be the same throughout the intervention. This may be of importance when educating the adolescent on what to expect during rehabilitation.
Feedback on contraction time increases compliance
The exercises were primarily performed with a contraction time shorter than prescribed in both the feedback group (mean contraction time = 6.5 s) and the control group (mean contraction time = 3.9 s). This is in accordance with the results of Rathleff et al. (5) where adolescents performed exercises with a mean contraction time of 5.3 s. The control group in this trial performed 4% of the exercises with the prescribed contraction time ±1 s, which is similar to the 5% demonstrated by the adolescents in the aforementioned study. By contrast, the feedback group performed 49% of the repetitions with the prescribed contraction time.
Compliance to the total contraction time throughout the intervention of the control group of this trial was similar to compliance to the total contraction time throughout the intervention in the study by Rathleff et al. (5), whereas the feedback group reached a compliance more than double. Although the importance of contraction time was highlighted more than 10 yr ago (8) and has been a matter of interest in both healthy (9,22) and patient populations (34–36), the extent of low compliance to contraction time has only recently been investigated (5). This is not restricted to adolescents with PFP. A study of healthy individuals performing a shoulder abduction exercise found that only 14 of 29 participants performed the exercise with the predefined contraction time ±8% after a 2-wk intervention. These participants did not receive feedback (21). This warrants future research of using real-time feedback to improve exercise compliance.
Strengths and limitations
First, BandCizer™ is a valid and objective tool for measuring compliance to exercises (13,14). Second, the intervention resembled current recommended physiotherapy practice by combining home-based and supervised training sessions, which increases the external validity (4). Third, the adolescents were blinded toward the trial hypothesis and primary outcome, which negated a higher awareness of contraction time than that in normal clinical practice.
The primary investigator was not blinded toward group allocation and could have introduced bias; however, all exercise instructions were standardized and identical between groups to account for this. Data on pulling force from BandCizer™ could have shed light on potential group differences in loading; however, this could not be collected. Another limitation was that the home-based exercises could have been performed with less range of motion or with another body part, which BandCizer™ would not have been able to determine. However, this would have applied identically to both groups.
Real-time feedback on contraction time from BandCizer™ and an iPad improves compliance to the prescribed contraction time in adolescents with PFP. Although the number of repetitions performed across both groups was similar, contraction time closer to the prescribed lead to a higher exercise dose and larger improvements in strength. Whether feedback on contraction time also leads to superior outcomes regarding pain and function—as indicated by the secondary outcomes in this trial—remains to be investigated in a future trial.
All authors declare they have no conflicts of interest. No funding was received for this trial. The results of this trial do not constitute endorsement by the American College of Sports Medicine and are presented clearly, honestly, and without fabrication, falsification, or inappropriate data manipulation.
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Keywords:© 2018 American College of Sports Medicine
CONTRACTION TIME; REAL-TIME FEEDBACK; BandCizer™; HIP AND KNEE EXERCISES