Hip- Versus Knee-Dominant Task Categorization Oversimplifies Multijoint Dynamics

TO THE EDITOR: Beardsley and Contreras (1) evaluated certain biomechanical data to propose the importance of hip versus knee extensor training. Although their proposal is interesting and training the hip extensors has merit, we have concerns regarding the characterization of tasks as hip- versus knee-dominant from these data. In particular, salient features—such as temporal sequencing—are ignored (3); and joint moments, which we discuss here, are misinterpreted. The authors describe joint moment as “the product of muscular force and moment arm length” (p. 49, Ref. (1)), which is incorrect in specific regard to the research they cite. The studies referenced calculated net joint moment (NJM), which involves reaction forces and rigid-body modeling (6), or NJM-based parameters. As individual effects of muscles cannot be rendered without musculoskeletal modelling, NJM represents the sum moment of all muscles, including agonists and antagonists, acting at a joint (6). NJM describes the minimum muscular effort; this assumes no antagonist co-contraction (4,6). The actual agonist muscular effort is underestimated if antagonist co-contraction is present (4,6). Knee NJM (NJMK) is the sum of quadriceps (MQ), hamstrings (MH), and gastrocnemius (MG) moments acting at the knee (NJMK = MQ − MH − MG). Positive and negative NJMK indicate extensor and flexor moments, respectively. For NJMK to remain constant when hamstrings moment increases, quadriceps moment must also increase. This relation is significant in regard to Beardsley and Contreras's (1) proposal that hip extensor effort increases more than knee extensor effort with increasing movement demands. As the hamstrings contribute both hip extensor and knee flexor moments, an increased hip extensor NJM necessitates an increased quadriceps moment. Consequently, quadriceps effort is underestimated when accompanied by hamstrings co-contraction, a point that Bryanton et al. (2) discuss. The hamstrings contribute approximately 50% to maximum hip extensor strength (8). Because hip extensor strength in athletes is reportedly 2.5–4.5 N·m/kg body mass (7), hip extensor NJM exceeding 1.25–2.25 N·m/kg requires hamstrings activation. In vertical jumping, peak hip extensor NJM ranges from 2.5 to 3.5 N·m/kg body mass (3). In running, peak stance phase hip extensor NJM is 2.0–4.1 N·m/kg body mass (5). The hamstrings must contribute to hip extensor NJM during jumping and running, thus they will co-contract with the quadriceps at the knee. The authors (1) overvalue the role of hip extensors and undervalue the role of knee extensors in athletic movements, as greater hip extensor NJM must be accompanied by greater quadriceps efforts. In fact, increasing hip extensor effort without concomitantly increased quadriceps effort would unbalance the dynamic conditions and reduce knee extensor NJM (4). Therefore, strong quadriceps are required to achieve a large hip extensor NJM. The categorization of multijoint movement as hip- or knee-dominant oversimplifies the complex interactions between muscles, particularly when multijoint muscles are involved (3). Biomechanics provides insight into muscle involvement in exercises and functional tasks. However, appropriate interpretation of data requires correct understanding of relevant constructs, including knowledge of assumptions and limitations. Strength and conditioning professionals are negatively impacted by misconstrued biomechanical constructs, as they may incorporate faulty interpretations into practice.