Consistent findings demonstrate the influence that the knee joint exerts angle on the knee extensors’ maximal torque production capacity. Lower maximal voluntary isometric contraction (MVIC) torque was, for instance, developed at a moderate flexed knee position (20–35° knee flexion, 0° = full extension) compared to more flexed positions (55–75° knee flexion; Babault et al., 2003; Cavalcante et al., 2021). The knee extensors encompass the three mono-articular vastii muscles [vastus lateralis (VL), vastus intermedius (VI), and vastus medialis (VM)] and the bi-articular rectus femoris (RF) muscle who share a common termination on the quadriceps tendon. Because the RF muscle originates from the acetabulum, a change in hip joint position could therefore also influence the knee joint’s torque due to the bi-articular RF muscle’s contribution (Herzog and ter Keurs, 1988). This model was further validated by the greater MVIC torque developed in a seated compared to a supine position (i.e., flexed vs. extended hip position; Maffiuletti and Lepers, 2003; Rochette et al., 2003; Ema et al., 2017), while others did not find a difference between these two positions (Bampouras et al., 2017; Cavalcante et al., 2021). Contrary to the general agreement about the influence of knee joint angle on knee extensor’s MVIC, the effect of hip joint angle remains discussed.

Deficits in MVIC torque could result from an alteration in contractile properties of the knee extensor muscles. Previous studies tested the influence of hip or knee joint angle configuration on the excitation-contraction coupling process of the knee extensor using percutaneous electrical stimulation. Similar decrements in MVIC and twitch torque amplitude (Babault et al., 2003) or maximal electrically induced contraction torque (Cavalcante et al., 2021) occurred in moderate knee flexed positions (20°–35° knee flexion) compared to more flexed positions (55–75°). Additionally, Cavalcante et al. (2021) reported that hip position does not influence maximal electrically induced contraction torque irrespective of knee angle so does for the MVIC. These findings led these authors to suggest that the lower MVIC torque would likely result from a mechanical disadvantage impairing knee extensors’ contractile properties (Babault et al., 2003; Cavalcante et al., 2021). However, this hypothesis is questioned by the greater electrically evoked responses elicited in the knee extensors in a supine compared to a seated position despite a lower (Maffiuletti and Lepers, 2003) or a similar (Bampouras et al., 2017) MVIC torque, respectively.

Using ultrasonography, Cavalcante et al. (2021) measured shorter fascicle length and greater pennation angle of the vastii and RF muscles for a knee extended position than a more flexed position, irrespective of hip angle. On the contrary, an increase in hip extension decreased pennation angle and increased the fascicle length of the RF muscle only at a 60° knee extension angle. At the same time, no change occurred at 20° of knee extension (Cavalcante et al., 2021). The VL and RF muscle fibers intermingled on the rectus-vastus aponeurosis attached to the quadriceps tendon (Glenn and Samojla, 2002). Hence, one could suggest that for any knee position placing the RF muscle above its slack length (Xu et al., 2018), an increase in hip extension improves force production capacity of both the RF and the vastii muscles due to the presence of the rectus-vastus aponeurosis and the vastus aponeurosis gathering muscle fibers from all the vastii muscles (Glenn and Samojla, 2002). An apparent mismatch appeared, therefore, between the impairment in maximal voluntary force production capacity that could occur in hip extended position, while intrinsic contractile properties of the knee extensors would be improved.

The maximal force production capacity of the knee extensors can also depend upon neural components driving muscle recruitment. Parallel reduction in voluntary activation and MVIC was reported for the knee extensors when increasing knee extension (Kubo et al., 2004; Doguet et al., 2017b). However, others reported no change in voluntary activation level despite a reduction in MVIC (Becker and Awiszus, 2001; Babault et al., 2003). Less is known about the influence of hip angle position, since to the best of our knowledge, only Maffiuletti and Lepers documented lower voluntary activation level of the knee extensors in a supine than a seated position (Maffiuletti and Lepers, 2003). The scarce and contradictory findings about the impairment of the neural drive to the muscle as a function of the knee or hip joint angle required further investigation to decipher their respective influence.

Using the twitch interpolated technique or the surface EMG analysis of a single muscle of the knee extensors remains limited to infer changes in the voluntary drive during voluntary contraction. Single-pulse transcranial magnetic stimulation could represent an alternative technique to quantify the corticospinal pathway’s excitability and infer the impact of hip or knee angle configuration on the muscles’ neural drive (Weavil and Amann, 2018). A reduction in corticospinal excitability of the VL muscle occurred, for instance, during MVIC of the knee extensors performed at 100° compared to 75° of knee flexion (Doguet et al., 2017a). It has been suggested that joint position may influence corticospinal excitability due to increased Ia afferent discharge affecting spinal excitability at long muscle length (Doguet et al., 2017a) or impairment of neuromechanical properties requiring different neural control strategies (Forman et al., 2016). However, in the absence of muscle length measurement in the study conducted by Doguet et al. (2017a), the extent to which knee angle affects corticospinal excitability through a change in muscle length remains speculative. Furthermore, the influence of hip angulation on central factors driving muscle recruitment during low-intensity voluntary contraction remains to be precise.

In this context, the present study aimed to examine the influence of hip and knee angle on the force production capacity of the knee extensors and determine the neuromuscular and mechanical consequences affecting the VL mono- and RF bi-articular synergistic muscles. We first evaluated maximal torque production capacity and muscle architecture with different knee and hip angle configurations. Secondly, we examined whether knee or hip joint angle influences neural drive of the VL and RF muscles during sub-maximal contractions. We hypothesized that (i) maximal force production capacity of the knee extensors would increase in knee flexed position while the increase in hip flexion would only have a minor effect and (ii) flexed knee angle or hip extended angle would alter recruitment strategies of synergist and antagonists muscles during sub-maximal contraction.

This study sought to examine the influence of hip and knee angle position on maximal force production capacity and their consequence on muscle architecture and neural drive of knee extensors. The main findings validated our first hypothesis, whereby knee flexion increased maximal force production capacity of the knee extensors highlighted by greater voluntary and electrically evoked contraction torques, while the hip position does not influence maximal force production capacity. However, our findings invalidate our second hypothesis about alteration in the neural drive of the knee extensors in knee extended or hip flexed position since no change in muscle activity or corticospinal excitability occurred between the different positions. The knee flexed positions also resulted in significant changes in muscle architecture of the VL and RF muscles, with greater fascicule length and lower pennation angle. These elements suggest a non-negligible influence of the knee extensors’ contractile properties on the torque production capacity of the knee extensors rather than neural components.

The greater MVIC torque of the knee extensors recorded in knee flexed (i.e., 110° flexion) compared to knee extended position (i.e., 20° flexion) highlights the prominent effect that knee angle position exerts on MVIC of the knee extensors (Babault et al., 2003; Cavalcante et al., 2021). The absence of difference between MVIC achieved in a 0° or 60° hip flexion position irrespective of knee angle position confirmed that hip angle position does not influence knee extensors maximal force production capacity (Bampouras et al., 2017; Cavalcante et al., 2021). However, these findings contradict the reduction in MVIC reported in hip extended position than hip flexed position (Maffiuletti and Lepers, 2003; Rochette et al., 2003; Ema et al., 2017). The constant EMG RMS_MVIC/M ratio of the VL and RF muscles between positions suggests that reduction in MVIC does not result from altered muscle recruitment (Becker and Awiszus, 2001; Babault et al., 2003). A difference in maximal MEP amplitude was only reported for the VL muscle with greater amplitude in position K₁₁₀H₆₀ than K₂₀H₆₀. This increase in corticospinal activity could be explained by a greater muscle activity transcribed by the higher RMS_MEP/RMS_MVIC ratio observed both for the VL and RF muscle, suggesting that neural drive during voluntary contraction of low intensity adapts likely to overcome a deficit in mechanical disadvantages (Babault et al., 2003). Neither the intensity required to evoke maximal twitch torque amplitude nor the amplitude of the maximal M-wave was different between positions. However, the evoked twitch torque amplitude was impaired similarly to MVIC by the knee angle position (i.e., greater at 110° than 20° knee flexion) while unaffected by the hip angle position. Taken together, these findings suggest that reduction in MVIC occurring in the knee extended positions resulted from impairments in the excitation-contraction coupling process. Strengthen by the constant MVIC/Pt ratio between positions (see Figure 3C), our findings suggest that contractile properties were a determinant factor in force production capacity of the knee extensors in the investigated positions (Folland et al., 2014). Because of the change in knee position angle, impairments in contractile properties may partly result from a difference in muscle length and architecture conditioning their force production capacity (Gordon et al., 1966; Eng et al., 2018).

The use of ultrasonography in the present study was intended to characterize the influence of hip and knee angle positions on muscular architecture and infer their impact on contractile properties of the knee extensors. The architectural properties of the VL and RF muscles (i.e., fascicle length and pennation angle) reported in the present study are consistent with other measurements made in previous studies (Blazevich et al., 2003; Cavalcante et al., 2021). Despite a direct link between the RF and the VL muscle ensured by the presence of the rectus-vastus aponeurosis (Glenn and Samojla, 2002), current findings demonstrated that VL muscle length varied only as a function of knee angle, being significantly greater in knee flexed position than knee extended position irrespectively of hip angle position. Significant differences in RF muscle length occurred when hip and knee angles varied in the opposite direction; significantly shorter or longer muscle length was measured in positions K₂₀H₆₀ or K₁₁₀H₀ than all other positions, respectively. Changes in fascicle length followed a similar trend for the two investigated muscles, being greater in positions K₁₁₀H₀ and K₁₁₀H₆₀ than in positions K₂₀H₀ and K₂₀H₆₀. Therefore, these findings suggest that only knee angle position affected the fascicle length of the VL and RF muscle while hip angle position did not. One should note that the higher MVIC observed in the present study was developed in positions that significantly increase the fascicle length of the VL and RF muscles. In accordance with the sliding filament theory (Gordon et al., 1966), the present findings suggest that the greater MVIC developed in the more flexed knee positions would transcribe a more favorable fascicle length, increasing the number of actin-myosin bridges that overlap during contraction.

Another interesting finding of the present study concerns changes in pennation angle across the different positions. Indeed, our results showed lower Pα of the VL muscle in position K₁₁₀H₀ than K₂₀H₀, while no difference occurred between position K₂₀H₆₀ and K₁₁₀H₆₀. Additionally, lower Pα of the VL muscle was observed in position K₁₁₀H₀ than K₁₁₀H₆₀, while no difference occurred between position K₂₀H₀ and K₂₀H₆₀. Taken together, these findings showed that a knee flexed position (i.e., 110°) decreases Pα of the VL muscle compared to a knee extended position (i.e., 20°) only when the hip joint was in an extended position (i.e., 0°), but not when placed in a flexed position (i.e., 60°). Alternatively, the hip extension can also decrease Pα of the VL muscle when the knee joint angle is flexed but not when positioned in an extended position. Present findings also demonstrated lower Pα of the RF muscle in position K₁₁₀H₀ than position K₂₀H₀, while no difference was observed between positions K₁₁₀H₀ and K₁₁₀H₆₀ or positions K₂₀H₀ and K₂₀H₆₀. These findings also showed that Pα of the RF muscle was decreased only in response to an increase in knee flexion when the hip is extended, but not when the hip is in a flexed position, and that change in the hip angle position has no influence on Pα of the RF muscle irrespectively of the knee position. These findings demonstrate thus that an increase in hip extension can decrease Pα of the VL muscle only when the knee was flexed at 110°. In contrast, a decrease in Pα of the RF muscle occurred when VL muscle length increased and the hip joint was extended, while no change in Pα occurred for the RF muscle when VL muscle remained constant. Altogether, these findings demonstrate that the VL and RF muscles can mutually influence on Pα of their synergist counterpart, particularly when the hip was extended or the knee was flexed. One could suggest that this mutual influence that each muscle exerted on its synergist counterpart can partly be explained by the strong bonds that their respective muscle fibers form when attaching together into the rectus-vastus aponeurosis (Glenn and Samojla, 2002). Therefore, these findings agreed with the conclusion drawn by Glenn and Samojla (2002), whereby the VL and RF muscle would improve force production when they function similarly to a bipennate muscle thanks to their structural link provided by the rectus-vastus aponeurosis. Furthermore, it appeared that the influence that the VL and RF muscles exert on Pα of their counterpart would make it possible only for knee joint angle placing these muscles beyond their slack angle (Xu et al., 2018). However, it remains to determine whether this finding also holds for the hip joint angle by determining its slack angle.

The present results suggest that reduction in MVIC, reported for knee extended positions, was mainly due to mechanical disadvantage, particularly short fascicle length leading to an inadequate actin-myosin overlap. In accordance with a recent study (Cavalcante et al., 2021), the different hip angulations tested in the present study did not impair the force production capacity of the knee extensors. This discrepancy with precedent findings (Maffiuletti and Lepers, 2003; Rochette et al., 2003; Ema et al., 2017) might be explained by the different range of simultaneous change in hip and knee angle used in the present study compared to previous ones. Our findings also showed that the hip angle position could influence the change in pennation angle of the VL muscle, which suggests that the RF muscle state can exert a non-negligible influence on VL architecture. Future studies should be conducted to determine more precisely the mutual influence that the VL and the RF muscle can exert on each other through the rectus-vastus aponeurosis and the behavior of the two other heads of the knee extensors (vastus medialis and vastus intermedius).

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

The studies involving human participants were reviewed and approved by the French Ethics Committee (ClinicalTrials.gov Identifier: NCT03334526). The patients/participants provided their written informed consent to participate in this study.

YG, RL, AM, and CP conceived and designed the work. All authors acquired, analyzed or interpreted the data of the work, and drafted and revised critically the work. All authors approved the final version of the manuscript. All authors agree to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved. All persons designated as authors qualify for authorship, and all those who qualify for authorship are listed.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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