Vastus lateralis and vastus medialis produce distinct mediolateral forces on the patella but similar forces on the tibia in the rat

Abstract

Improper activation of the quadriceps muscles vastus medialis (VM) and vastus lateralis (VL) has been implicated in the development of patellofemoral pain (PFP). This explanation of PFP assumes that VM and VL produce opposing mediolateral forces on the patella. Although studies have provided evidence for opposing actions of VM and VL on the patella, other studies have suggested that their actions might be similar. In this study, we took advantage of the experimental accessibility of the rat to directly measure the forces on the patella produced by VM and VL. We found that VM and VL produce opposing mediolateral forces on the patella when the patella was lifted away from the femur. These distinct mediolateral forces were not transmitted to the tibia, however: forces measured at the distal tibia were very similar for VM and VL. Further, when the patella was placed within the trochlear groove, the forces on the patella produced by VM and VL were very similar to one another. These results suggest that mediolateral forces produced by VM and VL are balanced by reaction forces from the trochlear groove and so are not transmitted to the tibia. These results provide a rich characterization of the mechanical actions of VM and VL and have implications about the potential role of these muscles in PFP and their neural control during behavior.

Coordination amongst quadriceps muscles suggests neural regulation of internal joint stresses, not simplification of task performance

“…Although previous studies have demonstrated a strong correlation between the activity of VM and VL [10,[29][30][31], the functional role of this correlation has not been clear. Since VM and VL have the same contribution to task variables and produce opposing mediolateral patellar forces [11], their strong correlation is consistent with both simplification of task performance [16,19] and regulation of internal joint stresses [29,32,33]. Similarly, the observation that RF could be controlled more independently than the vasti [31,34] is also consistent with both interpretations, as RF has a different task action from VM and VL [35,36] and produces minimal mediolateral patellar force (Fig.…”

“…Our ability to disambiguate between interpretations of muscle co-variation relied on the conceptual simplicity of the knee biomechanics in the rat. The rat knee joint provides a clear separation between the action of quadriceps muscles on task performance and internal joint stresses, allowing us to distinguish between the different interpretations of muscle co-variation [11,22]. In other joints and animals such a separation may not be as clear, and the actions of muscles on these different aspects of motor control may need to be estimated using detailed biomechanical models [52,53].…”

“…This strategy might simplify task performance by reducing the number of variables that need to be specified for the production of behavior. For example, the quadriceps muscles vastus lateralis (VL) and vastus medialis (VM) have very similar contributions to task performance, with each muscle producing a similar extension torque at the knee [9][10][11]. Consistent with the proposal that co-variation patterns reflect muscles' contribution to task performance [7,[12][13][14][15], the activations of VM and VL are strongly correlated [15][16][17][18], suggesting that the CNS might control VM and VL as a single functional unit to simplify the achievement of task goals [19].…”

“…Alternatively, co-variation of muscle activations might reflect aspects of motor control other than achieving task goals. Although VM and VL have similar contributions to task performance, they produce opposing mediolateral forces on the patella [11,20,21]. The strong correlation between VM and VL might therefore reflect minimization of net mediolateral patellar forces to prevent aberrant patellofemoral loading [17,22].…”

Adaptation after vastus lateralis denervation in rats demonstrates neural regulation of joint stresses and strains

“…In the experiments reported here, we overcome these challenges by taking advantage of the unique properties of the rat knee joint ( Figure 1a ). Activation of the quadriceps muscles vastus lateralis (VL) and vastus medialis (VM) in the rat produces very similar forces at the distal tibia and so will produce similar joint torques ( Sandercock et al, 2018 ). These muscles therefore have redundant contributions to task performance.…”