Anterior knee pain (AKP) is a common musculoskeletal problem in individuals that one-fourth of people experience at least once in their lives [1]. Patients complain of peripatellar or retropatellar pain during daily (e.g., squatting, kneeling, and stair walking) [2] and athletic (e.g., running, jumping, landing) movements [3]. To control pain, therapeutic modalities such as high-frequency transcutaneous electrical nerve stimulation (TENS) and focal joint cooling are commonly used. Due to anaesthetic effect, the use of these modalities allows voluntary contraction and active movements— called cryokinetics [4].

Conservative therapeutic exercises for AKP include the restoration of quadriceps function [2] and strength development of the lower extremity [5] . Quadriceps inhibition is thought to be the result of reduced motor signals due to abnormal afferent sensory input stemming from the painful area (e.g., knee joint) [6]. Since the 2000s, a disinhibitory effect (e.g., reversal of neural inhibition in quadriceps) on TENS and focal knee joint cooling has been reported in controlled laboratory studies using an experimental injury model [7], as well as in clinical studies with knee pathologies [8, 9]. Theoretically, disinhibition provides a clinical situation where patients would be able to recruit more active motor units [10]. Therefore, reversing neural inhibition (by using modalities) prior to performing voluntary exercises is suggested as a course of action [11].

Strength development in the quadriceps and lower-extremity is typically guided for pain reduction and functional improvement in rehabilitation of AKP [11]. To accomplish this, a fixed external weight should be incorporated throughout the available range of motion (ROM) [10]. When performing resistance exercises for the isolated contraction of the quadriceps [12] or co-contraction with other muscles [13], the use of the daily adjustable progressive repetitive exercise (DAPRE) technique [14] is recommended. To maintain or improve the cardiorespiratory system to transfer the necessary oxygen supply to the working tissues, whole-body exercise is performed. This type of exercise is referred to as “aerobic exercise” and is typically performed as a warm-up activity. Considering the restrictions in the hip joint ROM [15, 16], flexibility exercises focusing on the hip flexors and extensors are also recommended.

While well-designed programmes and appropriate progressions in the interventions described above are keys to a successful rehabilitation outcome, no information regarding the degree of change or progression during multiple sessions of rehabilitation. Pain reduction and strength development are expected, it is unclear when and how such progressions occur. Clinicians and patients would benefit from referencing and applying such information when establishing and implementing each rehabilitation component. Therefore, the purpose of this study was to report changes in pain perception and the TENS pulse amplitude, and progressions in aerobic and resistance exercise at each session during a ten-session rehabilitation programme for patients with AKP. As secondary outcomes, hip joint flexibility (hamstring and iliopsoas muscle lengths) on each side and functional outcomes (Kujala AKP scale) [17] were assessed.

We sought to report changes and progressions of changes in pain perception and the TENS pulse amplitude, and progressions in aerobic, flexibility, resistance exercise, and functional outcome measure during and after a ten-session rehabilitation programme for patients with AKP. Our data on pain perception showed that a single treatment was not effective to reduce the pain perception <Table 2>, but overall pain (collapsed on pre- and post-treatment in each session) was reduced by 49% at the 4^th session <Figure 3A>. This indicates that multiple sessions are necessary to achieve a significant reduction of chronic AKP (pain duration: > 46 months), which is consistent with what previous studies have suggested [21, 22]. It should be noted that observed pain reduction in our study is the result of a combined effect on the rehabilitation programme. For example, the capacity of aerobic and resistance exercise increased at the 4^th session <Tables 2 & 3>, which could certainly have contributed to pain reduction. While a pain perception greater than 3 cm on a 10 cm in VAS has been used as an inclusion criterion for clinical study of AKP [1, 23], the observed pain level at the 4^th session from the baseline (3.4 to 1.7 cm, d=0.89) is clinically meaningful. Considering the data of increased capacities to perform aerobic and resistance exercises during the 2^nd and 3^rd sessions <Table 2 and Figure 3>, joint stability and movement efficiency in the lower extremity during daily activities could also be improved, which made patients relatively less conscious of their AKP. While the improved exercising capacities along with the pain reduction could have possibly led to a functional improvement, the relative effectiveness of the interventions (e.g., therapeutic modalities vs. rehabilitation exercises) is unclear. We think that the use of therapeutic modalities not only reduced pain perception but also facilitated voluntary exercises at each rehabilitation session, which could have improved lower extremity function.

There is a fading of electrical (tingling) sensation during and after receiving multiple treatments of TENS [24]. This response is attributed to accommodation (an increased threshold in depolorisation), adaptation (reduced subjective sensation), and habituation (reduced reaction of the central nervous system to action potential) [25]. There have been a couple of studies [26, 27] concerning the necessity of adjustment in TENS during a single-session intervention. In an earlier study [26], an adjusted amplitude resulted in an increased pain threshold as compared with fixed TENS. A recent study [27] did not report the superiority of adjusting a pulse amplitude over the fixed amplitude in terms of pain reduction. While the hypoalgesic effect of adjusting the amplitude is still under debate, we opted to increase it as tolerated every session because the adjusting amplitude method is usually practiced; there is insufficient evidence on the use of fixed amplitude. We observed a 20% increase in the pulse amplitude of TENS at the 4th session but no further increase thereafter. Previous studies similar to ours specified the adjustable range—up to 60 [28] or 80 mA [29]—for home training sessions but the amount of amplitude change was not reported. Therefore, our study is the first attempt to report the amount of adjustment in pulse amplitude of TENS application over multiple sessions in managing clinical knee pain. It should be noted that our observation was the change in pulse amplitude with a simultaneous application of knee joint cooling. This may explain why the highest amplitude was recorded as 31 mA in our data, while the adjustable range (60 and 80 mA) in previous studies [21, 28] were relatively larger.

Based on our progression criterion (simply calculated as > 70% of the age-based estimation on maximal heart rate), our participants showed an improvement on treadmill jogging speed at the 3^rd, 5^th, and 9^th sessions (a total of 28% improvement) <Table 2>. Despite its popularity in rehabilitation, progression guidelines of aerobic exercise have not been suggested. Therefore, we were interested to report how patients with AKP responded to the exercise intensity determined by a simple progression criterion without using additional devices. As no data are available on the progression of aerobic exercise over multiple rehabilitation sessions, our data would be a possible future reference for clinicians and patients in this regard.

A progression in working weight during resistance exercises was observed at every session, indicating that a single session of our interventions could result in significant strength development AKP <Table 3, Figures 3B and C>. Our rehabilitation programme was designed to accomplish the previously suggested “open, then exploit” strategy [11]. The time (approximately 10-min) between the modality use and resistance exercises was a sufficient duration for the disinhibitory effect [8]. Since neural activation was not directly measured, it is unclear as to whether quadriceps disinhibition actually happened at the time of resistance exercises. However, the observed increase in working weight at every session, especially the first five sessions (less than two-weeks of duration) may be evidence of the process of reversing neural inhibition. This assumption is reinforced by the content of previous research: knee extension exercises alone did not show an immediate increase in quadriceps strength and activation [30]. Given the occurrence of the aforementioned disinhibitory strategy, we believe that the facilitation of movements during exercises by numbing the painful area using knee joint cooling [4] contributed to some extent.

There was an overall (regardless of side) 5.1° increase in hip flexion test results <Figure 2D>. The normative value in this test position in a similar age group was known as 74° [31] that our patients’ hamstring lengths were within normal range. While we believe that stretching exercises directly influenced hip flexibility, the observed outcomes should be interpreted as result of combined effects with other therapeutic exercises. A previously reported critical criterion for iliopsoas tightness was –9.7° [29], indicating that our patients’ iliopsoas were much tighter and the flexibility exercises did not change this. Unlike hamstring flexibility, the testing (modified Thomas test) and training positions <Figure 2D> for iliopsoas were different than exercising while applying the modified Thomas test and could have yielded a different result. Additionally, a greater stretching load may have been applied to the knee extensors than the iliopsoas by the half-kneeling exercise. We gave correct instructions and continuous feedback on this stretching position, but one-onone supervision was limited due to helping other patients with modality applications. Although the exact causality has not been determined, it is believed that the level of function and the severity of AKP is associated with hip joint flexibility [15, 19]. Therefore, stretching exercises targeting hip joint flexibility should be included in the exercise prescription for AKP patients. The Kujala AKP scale is a valid and reliable tool by which to assess the functional level for young adults with AKP. Previously, 20-sessions of exercise [32] and 60-sessions of exercise in conjunction with neuromuscular electrical stimulation [22] improved the Kujala AKP scale by 17% (60 to 73/100) and 18% (73 to 89/100), respectively. We also observed a 17% of improvement (73 to 89 /100) after ten-sessions of rehabilitation in our patients may suggest that our rehabilitation programme (in terms of components and the order) could be an optimal means for pain reduction and functional improvements.

Assumptions should be noted that (1) physiological and biomechanical adaptations of the rehabilitation programme were not different across patients and (2) the psychological factors such as cognitive appraisal, emotional changes, motivation, and adherence were also similar. Lastly, the design of this study is limited to determine the superiority or interrelationship of therapeutic interventions (e.g., therapeutic modalities, and aerobic, anaerobic, and flexibility exercise) in reducing pain perception and improving functional outcomes. We acknowledge that the observed changes and progressions in each rehabilitation component were somewhat affected by each other. For example, increased flexibility could have been involved with pain reduction, or vice versa. Future research should attempt to examine an isolated effect of each rehabilitative intervention.

The current study reported changes in pain perception and the TENS pulse amplitude, and progressions in aerobic and resistance exercise over a series of ten-session rehabilitation programme for patients with AKP. The observed changes and progressions could be used as general guidelines for the establishment and implementation of a short-term consecutive rehabilitation.