Towards optimisation of patellar tendon loading in musculoskeletal rehabilitation

Mechanotherapy is an important concept for physiotherapy in general and for the rehabilitation of patellar tendinopathy specifically. During a mechanotherapeutic rehabilitation program, the load on the injured structure is gradually increased according to the principles of supraphysiological overload and supercompensation until habitual load levels can be tolerated again. At present, progressive patellar tendon loading is monitored through external load parameters such as the mass of an external weight during strength training. However, the same external load can lead to different internal loads on specific structures. Internal patellar tendon load progressions during the rehabilitation of patellar tendinopathy are currently monitored through a combination of visual inspection, clinical reasoning and pain indications. Rehabilitation programs based on objectively quantified patellar tendon load progressions would offer an important improvement from a mechanotherapeutic perspective. Unfortunately, the objective quantification of patellar tendon forces requires expensive, specialised and time-consuming techniques such as advanced musculoskeletal modelling. This makes the objective quantification of patellar tendon forces in a clinical practice setting currently impossible.
The overall objective of this doctoral thesis was to improve the objective quantification of patellar tendon force progressions in a clinical practice setting. Since only three studies have objectively quantified patellar tendon forces through advanced musculoskeletal modelling so far, the first specific objective of this thesis was to further objectively quantify patellar tendon forces during frequently used rehabilitation exercises for patellar tendinopathy. Studies 1-3 objectively quantified patellar tendon forces during running, lunges and squats respectively. More specifically, the (interacting) effects of changes in multiple external load parameters were investigated. During running (study 1), the effectiveness of step frequency alterations at different running speeds was investigated. The results of this study showed that step frequency alterations are equally effective at low and high running speeds for the peak patellar tendon force, less effective at high running speeds for the patellar tendon force impulse per kilometre and not effective for the peak rate of patellar tendon force development. During forward lunges (study 2), the effects of an external weight’s mass and carrying position on the peak patellar tendon force were investigated. Here, the results showed that the external weight’s mass and carrying position both influence the peak patellar tendon force. Increases of peak patellar tendon force with increasing mass were similar for all positions. Finally, the effects of an external weight’s mass and the use of a decline board on the peak patellar tendon force were investigated during heavy load single-leg squats (study 3). Interestingly, only the use of a decline board increased the peak patellar tendon force. Increasing the mass of the external weight from 70% to 90% of the one repetition maximum did not result in a progressively higher peak patellar tendon force.
Since studies 1-3 have been conducted with healthy participants, the question can be asked whether patients with patellar tendinopathy may have different patellar tendon force progressions between exercises. The exercise order to achieve a gradual patellar tendon force progression may well be different in patients with patellar tendinopathy, for example because of pain-avoidance strategies. The objective of study 4 was to assess to which extent the peak patellar tendon force progressions for a large number of rehabilitation exercises are equivalent in patients with patellar tendinopathy and healthy individuals. Seven patients with patellar tendinopathy and seven healthy individuals performed 28 rehabilitation exercises. The patients with patellar tendinopathy executed the rehabilitation exercises with mild pain scores, while the healthy individuals performed the exercises pain-free. Nevertheless, the peak patellar tendon force progressions in both groups showed near-perfect equivalence. It can therefore be concluded that objectively quantified peak patellar tendon force progression principles for healthy individuals can be transferred to patients with patellar tendinopathy.
This doctoral thesis had the overall objective to improve the objective quantification of patellar tendon force progressions in a clinical practice setting. Within this context, it was important to establish the accuracy level of patellar tendon force estimations made by physiotherapists based on their visual inspection and clinical reasoning skills. In study 5 a video-based survey was sent to 50 actively practicing physiotherapists and 50 final year master students. These physiotherapists estimated the peak patellar tendon force on a 100-point numeric rating scale for one individual performing 19 rehabilitation exercises. The results were compared against objectively quantified peak patellar tendon forces based on advanced musculoskeletal modelling. The results showed that the overall ability to estimate peak patellar tendon forces was good to excellent for the entire group and all subgroups. However, with six overestimated and four underestimated exercises, load estimations can still be improved at the level of individual exercises.
In study 6 an initial step was then taken towards measurement tools that can help objectively quantify patellar tendon forces in a clinical practice setting. This was done by identifying potential kinematic and kinetic proxy signals for peak patellar tendon forces over multiple rehabilitation exercises. Based on basic stepwise linear regression and principal component regression, it was demonstrated that there is a potential for peak patellar tendon force estimations from sagittal plane kinematics and kinetics and even from kinematics alone. This is good news for physiotherapists since the collection of sagittal plane kinematic data would only require one video-camera.
In conclusion, this doctoral thesis has taken some important steps towards improved progressive patellar tendon loading programs. The thesis has taken initial steps towards the development of clinical measurement tools for the objective quantification of patellar tendon forces. While these clinical measurement tools are further developed, this thesis has provided evidence that patients with patellar tendinopathy can already now be confident in the progressive exercise programs created by physiotherapists. Especially now that physiotherapists have access to patellar tendon force progressions – objectively quantified through advanced musculoskeletal modelling – over several exercise variations. Physiotherapists can use these objectively quantified patellar tendon force progressions to improve their patellar tendinopathy rehabilitation programs even further.