Patellar tendon load in different types of eccentric squats

Abstract

Background

Differences in mechanical loading of the patellar tendon have been suggested as a reason for varying effects in rehabilitation of patellar tendinopathy using different eccentric squat exercises and devices. The aim was to characterize the magnitude and pattern of mechanical load at the knee and on the patellar tendon during four types of eccentric squat.

Methods

Subjects performed squats with a submaximal free weight and with maximal effort in a device for eccentric overloading (Bromsman^®), on a decline board and horizontal surface. Kinematics was recorded with a motion-capture system, reaction forces with force plates, and electromyography from three leg muscles with surface electrodes. Inverse dynamics was used to calculate knee joint kinetics.

Findings

Eccentric work, mean and peak patellar tendon force, and angle at peak force were greater (25–30%) for squats on decline board compared to horizontal surface with free weight, but not in Bromsman. Higher knee load forces (60–80%), but not work, were observed with Bromsman than free weight. Angular excursions at the knee and ankle were larger with decline board, particularly with free weight, and smaller in Bromsman than with free weight. Mean electromyography was greater on a decline board for gastrocnemius (13%) and vastus medialis (6%) with free weight, but in Bromsman only for gastrocnemius (7%).

Interpretation

The results demonstrated clear differences in the biomechanical loading on the knee during different squat exercises. Quantification of such differences provides information that could be used to explain differences in rehabilitation effects as well as in designing more optimal rehabilitation exercises for patellar tendinopathy.

Introduction

Mechanical loading is essential for the health and performance of tendons (e.g., Tipton et al., 1986, Wang, 2006). Although the optimal magnitude and characteristics of the mechanical loading still are largely unknown, eccentric loading seems to be an important ingredient (Rees et al., 2006, Stanish et al., 1986). The descending part of a squat is an often-used exercise to achieve eccentric loading of the knee extensor muscles, at least the single-joint ones, and concomitant force-induced elongation of the patellar tendon. Being a multi-joint movement, a squat can be carried out with varying coordination and in various external devices. Clinically, superior rehabilitation results on patellar tendinopathy have been reported for training with eccentric squats on a decline board (25° declined surface) as compared to a horizontal surface (Jonsson and Alfredson, 2005, Purdam et al., 2004, Young et al., 2005); cf., however, Visnes et al. (2005), who did not observe any significant differences. It was speculated that higher loads and greater muscle-tendon strain using the decline board could be one reason for the alleged superiority. In line with this hypothesis was the finding of Kongsgaard et al. (2006) of greater patellar tendon strain on a decline board based on ultrasonographic measurements in one static loaded squat position. Recently a new device, Bromsman^® (LARN, Lidingö, Sweden), has been introduced (Frohm et al., 2005) allowing maximal voluntary eccentric squat exercise against a supra-maximal load lowered under controlled and variable conditions. Preliminary results from training in this device have been promising (Frohm et al., unpublished).

The main purpose of this study was to use inverse dynamics analysis to compare the mechanical loading of the knee during four different types of eccentric squat exercises, including decline board and Bromsman, both with respect to magnitude and characteristics of the mechanical load at the knee and on the patellar tendon.

The null hypothesis was that there are no differences in patellar tendon loading between the different squat exercises.