Following previous publications, our experiment assumed that the intensity of acquired vibroacoustic emission waves is closely associated with the kinetic friction and represents friction-induced vibrations generated by relative sliding and rolling of adjacent articular surfaces. From biomechanical and tribological points of view the ability of the articular surfaces to move smoothly against each other with low frictional noise indicates optimal arthrokinematic motion related with low kinetic friction, and as a result contributes to slow wear of cartilage [20]. The presented findings of this study confirm that during a knee flexion-extension movement with the load of the body weight, the PFJ arthrokinematic motion possesses low-vibration characteristics, despite relatively high values of articular surface load, typical for this analyzed condition. It has been experimentally proven via cadaveric and mathematical models that the contact forces of the PFJ articular surfaces while performing a squat may even reach 3-3.5 kN and the peak contact pressure equals ~8-9 MPa at 90° of knee flexion, which corresponds to multifold values of human body weight [30, 31, 40]. However, due to the friction-reducing properties of synovial joints, the coefficient of friction, which is defined as the ratio between strength required to generate gliding motility and pressure force of the body towards a particular surface, is still very low in the mentioned condition.
It fluctuates around 0.03μ, which is only slightly higher than coefficient of friction values typical for unloaded PFJ motion [41-43]. In our study this phenomenon is observed as relatively smooth and flat VAG signals, representing low vibroacoustic emission. Both variability (VMS) and amplitude (R4) of representative signals are characterized by low values. Especially the low values of the R4 parameter that indicate a lack of repetitive large peaks, whose presence often corresponds with macroscopic joint cartilage abnormalities or significant maltracking of articular surfaces [18, 37]. In turn, low spectral activity analyzed by summing spectral power of the VAG signal in two bands: 50–250 Hz (P1) and 250–450 Hz (P2) seems to be related with intact integrity of the superficial layer of hyaline cartilage and an efficient lubrication mechanism [36, 44].
Nonetheless, in the present study we predominantly focused on the impact of the external load on the quality of PFJ arthrokinematic motion. As expected, the PFJ frictional noise raised near-linearly with increasing levels of external load. The application of 10 kg external load resulted in an average 3-fold higher variability of recorded signal (VMS) and an average ~1.5-fold increase of amplitude and spectral activity (R4, P1 and P2). Furthermore, the squats performed with the 20 kg load were characterized by further, analogous increase of the VAG signal variability/amplitude and higher, 2.5-fold increase of spectral power in band 50-450 Hz, when compared with the 10 kg load. Similar results have been presented by Andersen et al. [19], who also found divergences between different loads for all VAG parameters. However, this previous study was based on an open kinetic chain analysis and the applied loads (up to 5 kg) were lower than the loads analyzed in the current study, which are more typical for different activities of daily living and sports. Moreover, although the absolute values between studies differentiate as a result of disparate parameters’ settings being used, similar tendencies have been observed when referring to the increase of vibroacoustic emission.
We hypothesize that the above-mentioned load-associated with the growth of vibroacoustic emission is largely driven by the increase of contact stress within the PFJ. Research has previously reported, that squatting with an external load results in significantly greater patellofemoral joint reaction force and stress compared to an unloaded squat, and that the increase is proportional to the applied load [40]. Authors showed that a squat performed under a 35% of body weight external load yielded a 44% increase in PFJ stress across all knee flexion angles, with a peak patellofemoral contact stress of 13.06 MPa at 90° of the knee flexion, whereas when without load, the peak patellofemoral joint contact stress equaled 9.06 MPa. Such elevated values of contact stress may cause very high frictional shear stress during gliding motion of the patella, which consequently dramatically increases kinetic friction and related frictional noise [43]. It should be also noted, that the observed increase of VAG signal parameters may be the result of load-related decline in lubrication efficiency. Although many different lubrication mechanisms have been proposed, it is becoming increasingly apparent that mainly a fluid film lubrication is responsible for the low friction in synovial joints, especially at high loads [45, 46]. Fluid film lubrication involves a thin synovia film that provides separation of the joint surfaces. Synovia being squeezed out of cartilage into the joint space as loading increases creates a thicker layer of film. Therefore, a decreased coefficient of friction with increased contact stress has been reported in many studies [47]. This observation may explain why the results reported by Ladly et al. [48] indicated that a 3 pound (1.4 kg) external loading of the patellofemoral joint has an insignificant effect on VAG signal power. Nonetheless, it is generally accepted that when the contact stress reaches a medium level (4-8 MPa), the velocity of fluid exuded from the cartilage reaches its limit and there is a concomitant increase in the coefficient of friction (frictional noise) thereby, increasing contact stresses [43].
Since all participants used in this study were subjected to the same level of load, we were also able to assess intra-group relationships between the changes in characteristics of VAG signal courses under the influence of applied load and the anthropological data of the analyzed cohort. We expected some dependencies, especially between the body height and the VAG signal values, because contact stress (considered as the main determinant of observed changes) is a parameter that reflects distribution of load and strongly depends on the area of contact between articular surfaces. However, there were no statistically significant correlations. Nevertheless, weak but statistically significant correlations were noted for all the considered parameters for trials L0 and L10 as well as trials L10 and L20. This shows that subjects who had a low level of vibroacoustic emission during the bodyweight squats also obtained a low level during the loaded conditions. Thereafter, we discovered weak and moderate positive dependencies in values of all VAG parameters between the left and right lower limbs, but only for the loaded trials. Hence, there is an assumption that occurrence of the mentioned dependencies may indicate the presence of ontogenetic conditionings, associated with the friction-reducing properties of synovial joint environment and different levels of load tolerance. It is, however, a complex phenomenon that requires further extensive research.
In the light of findings reported in the current investigation, there are several limitations that should be noted. Consideration should be given that the conducted analyses were limited to asymptomatic young and agile subjects, who were able to perform the repeated squats with large loads. So, although the present study provides crucial insights into the PFJ function during squatting with and without loads, caution should be exercised in extrapolating these data to older patient populations or those suffering from anterior knee pain syndrome. Moreover, the inclusion/exclusion criteria were based on a medical interview, but not confirmed by advanced imaging procedures. Therefore, we cannot exclude that knees of some subjects revealed minor asymptomatic cartilage changes which could have affected our results. Another potential limitation is the fact that the VAG is a method of indirect arthokinematic analysis focused on acquisition of the vibroacoustic signal using skin-mounted accelerometers. Therefore, it should be clearly emphasized that the presented method allows for observation of only the effect of the kinetic friction phenomenon in the form of frictional noise which holds a multicomponent character. However, the mechanical properties of synovial joints related to the friction and wear cannot be directly measured in vivo. Thus, it is generally accepted that the VAG method, despite some limitations, may provide sufficient, clinically meaningful information about the function and biotribological changes within articular environment.
Accordingly, the presented results are clinically applicable, because weight-bearing exercises, such as the squat, are frequently used during rehabilitation and are specific to many activities of daily living and sport activities. The ability to understand how friction-related aspects of PFJ arthrokinematics vary among weight-bearing exercises with different loads will allow clinicians and trainers to prescribe safer and more effective rehabilitation treatments for patients and athletes during training. This is a particularly significant issue because excessive joint friction has been linked to articular cartilage degradation and may contribute to premature wear and knee pathologies (e.g. chondromalacia or osteoarthritis).