## 1 Introduction  
Knowledge of individual muscle power profiles may augment our understanding of their functional tasks and how alterations in power production after impairment may affect movement [1]. Muscle forces may be associated with concentric (shortening) contractions, eccentric (lengthening) contractions, or isometric contractions. Muscle power is the mathematical product of that muscle force and that muscle's contraction velocity, where positive power is associated with power that generates movement (positive power) or restrains movement (negative power). There is a lack of consensus regarding the role of individual knee muscles during level, self-selected speed gait in neurologically intact adults [2, 3]. The differences in the interpretation of muscle responsibilities in gait are important, as rehabilitation protocols are developed, in part, based on these assumptions. Our understanding of the factors that contribute to normal walking would be advanced if each muscle's power--and, as such, their unique role(s)--could be determined in vivo. Muscle power is an important quantity in the analysis of human movement. Its relationship to mechanical work has been examined in walking, running, and other locomotor activities [4, 5, 6, 7, 8]. Joint work, which is computed as the integral of the net power at a joint (via standard inverse dynamics techniques), may represent mechanical work by the muscle-tendon unit (MTU) most accurately as compared to internal or external work estimates. Muscle work and storage of elastic energy have been analyzed in walking, mostly using muscle-activated forward dynamics simulations. Perhaps the most comprehensive collection of work estimating muscle forces via an EMG-driven model was by [PERSON] and colleagues [9; 10; 11; 12; 13; 14]. This early work showed realistic estimates of in vivo muscle forces during adult human walking, while primarily limited to calf (triceps surae) muscles. [PERSON] and colleagues [15] examined muscle activity and coordination in the terminal swing phase of adult walking. They note that knee motions in early swing were influenced by muscles that cross the knee and by moment at adjacent joints (ankle and hip). They demonstrated that the hamstrings decelerated the forward motion of the swing-limb shank, without influencing rotational motion about the knee (simultaneously accelerating the pelvis forward and the hip into extension). However, that work stopped short of estimating forces and power in individual muscles (the model included some \"lumping\" of synergistic muscles), and the EMG profiles used as model input were of longer duration than is typically found in clinical trials. The use of surface EMG may have been an additional confound (related to intramuscular crosstalk). [PERSON] et al. [16], using a forward-dynamics approach, examined muscle forces and power at the ankle, knee, and hip via a sagittal plane musculoskeletal bipedal model. Surface electromyograms (EMGs) were used to determine periods of muscle force. An outstanding feature of this work was its inclusion of passive moments in the model. The model was limited to muscles primarily in the (examined) sagittal plane and EMG relative intensity scaled to peak gait activity. This non-physiological step likely led to the overestimation of muscles that have relatively little activation in gait (e.g., hamstrings) and underestimation of muscles that have contraction intensity close to maximum contraction (e.g., soleus). Despite these limitations, the aforementioned studies have advanced our knowledge of the role of selected lower extremity muscles in gait. The power produced by synergistic muscles (or joint power) may be used to establish the scope of muscle groups to generate or restrain movement [17; 18; 19; 20; 21; 22; 23; 24; 25] and seems to be a valid indicator of a person's ability to control their limbs [26; 27; 28]. Concentric power typically produces motion, while negative power implies motion restraint (isometric contractions have zero power due to a contraction velocity of zero).