Contributions of the individual ankle plantar flexors to support, forward progression and swing initiation during walking
Introduction
Studies have identified strong correlations between the net ankle moment and power produced by the ankle plantar flexors and gait performance in several patient populations (Mueller et al., 1995; Nadeau et al., 1999; Olney et al., 1990; Olney et al., 1994; Winter et al., 1990). Nevertheless, the functional role of the ankle muscles during gait (normal and pathological) has remained controversial. Previous experimental (e.g. Winter, 1991; Perry, 1992) and theoretical (e.g. Kepple et al., 1997; Riley and Kerrigan, 1999) studies have been limited to assessing the functional role of the ankle plantar flexors as a single unit because the analyses were based on the net ankle joint moment derived from inverse dynamics. Biomechanical analyses based on net ankle (and knee) joint moments cannot elucidate the potentially different mechanical contributions of individual uniarticular and biarticular plantar flexor muscles to the overall gait performance (e.g. support and forward progression). We believe that the lack of consensus regarding the functional roles of the plantar flexor muscles results in part because of the difficulty in rigorously quantifying a muscle's contribution to the individual body segment energetics (i.e. acceleration and power) and in part because individual muscles within the plantar flexor group likely contribute to the body segment energetics differently.
The three main theories advanced in the literature have been that the ankle plantar flexor group: (1) provide a controlled roll-off (e.g. Sutherland et al., 1980; Perry, 1992), (2) actively provide forward progression or push-off (e.g. Winter, 1983; Kepple et al., 1997) and (3) accelerate the leg into swing (e.g. Hof et al., 1993; Meinders et al., 1998). These theories are not likely to be mutually exclusive, as the plantar flexor group may contribute to each of the proposed functions, either by individual plantar flexor muscles performing the different functions or by synergistic actions between them.
The controlled roll-off theory describes forward progression during single-leg stance as a controlled fall (Perry, 1992). Thus, the proposed primary action of the ankle plantar flexors during the controlled roll-off is to decelerate tibia rotation and prevent knee flexion as the body rotates over the stance leg. Forward progression is then the result of a passive mechanism as the body moves forward as a result of momentum and inertia. Supporting evidence for the controlled roll-off theory is found in a pair of clinical studies using tibial-nerve blocks to temporarily paralyze the plantar flexors (Simon et al., 1978; Sutherland et al., 1980). Both studies found that in the absence of normal plantar flexor activity, walking velocity increased, leading them to conclude that the plantar flexors restrain forward momentum rather than propel the body forward. However, during both studies, walking mechanics (e.g. step length, step time, joint angles) were altered by the nerve blocks, making comparisons with unaltered plantar flexor function difficult.
The active push-off theory hypothesizes that the energy generated by the plantar flexor group is transferred to the trunk to provide support and forward progression. Winter (1983) examined the power output of the net ankle and knee joint moments during normal gait and found that the ankle moment was the primary source of positive work, and that plantar flexor activity coincided with the second peak of the vertical ground reaction force. He concluded that an active plantar flexor push-off, rather than a passive roll-off, provides forward progression. Supporting evidence was provided in a recent theoretical study that showed the plantar flexor moment was the primary contributor to the accelerations of the head–arms–trunk segment in both the horizontal (considered analogous to forward progression) and vertical (considered analogous to support) directions during the second-half of the single-leg stance phase (Kepple et al. 1997).
The final theory suggests that the primary function of the ankle plantar flexors is to accelerate the leg into swing, and forward progression is provided later in the swing phase as energy from the swing leg is transferred to the trunk (Hof et al., 1993, Meinders et al., 1998). Meinders et al. (1998) performed inverse dynamics and mechanical energy analyses to show that, although the net ankle moment generated the majority of the mechanical work during the push-off phase, only a small portion of this mechanical energy was transmitted to the trunk segment. Instead, their data showed that the mechanical work generated by the net ankle moment was stored in the swing leg as kinetic and potential energy. Similarly, Hof et al. (1993) examined correlations between changes in body segment mechanical energy and work of the triceps surae group determined from electromyogram to force processing and concluded that the primary function of the ankle plantar flexors is to provide the energy necessary for swing leg initiation.
The three different theories for the role of the plantar flexors in gait may not, however, be mutually exclusive since the uniarticular plantar flexors (e.g., soleus) and biarticular plantar flexors (e.g., gastrocnemii) individually or working in synergy may contribute to each of the proposed theories above. However, net joint torque-based analyses, as used in the studies proposing these theories, cannot differentiate between the contributions of the uniarticular and biarticular plantar flexors to task performance and, therefore, cannot identify their functional roles.
To date, no study has quantified the contributions of individual plantar flexor muscles to the acceleration of (and power delivery to) the individual body segments during walking, which are crucial to understanding the distinct roles of the uniarticular and biarticular plantar flexors. Previous studies have suggested functional roles for individual muscles based on correlational-type analyses (e.g., correlation of EMG activity with kinematics and kinetics, Pedotti, 1977; Winter, 1991; Perry, 1992). However, a muscle force causes significant reaction forces throughout the body, which are either ignored in such analyses or, at best, recognized but provide no solution for calculating them. Similarly, solving the force-sharing problem alone (e.g. Anderson and Pandy, 2001), like inverse dynamics-based analyses, does not provide insight into causal relationships between muscle activity and task performance. But, acceleration and power analyses (Fregly and Zajac, 1996) of forward dynamics simulations of walking, that are driven by individual muscles, can identify how each muscle contributes to the acceleration and power of the leg segments and the trunk to affirm or refute the above three theories.
Therefore, the objective of this study was to use a forward dynamics based analysis to identify how the individual uniarticular and biarticular plantar flexors contribute to support, forward progression and swing initiation. We considered a muscle to contribute to forward progression if it accelerated the trunk forward (Kepple et al., 1997), support if it accelerated the trunk vertically (Kepple et al., 1997), and swing initiation if it contributed positive power directly to the leg segments in pre-swing (Hof et al., 1993). By definition, the controlled roll-off theory implies that muscles do not contribute directly to forward progression.
Section snippets
Methods
A forward dynamics simulation of walking driven by individual muscle actuators was developed. This consisted of modeling the musculoskeletal system, muscle force generation and ground contact forces, identifying appropriate initial conditions (positions and orientations of the body segments at heel-strike) and finding the muscle excitations that replicate walking kinematics and kinetics.
Results
A simulation was generated such that the simulated kinematics closely matched the group-averaged kinematics (Fig. 1), and the simulated kinetics (joint torques and powers; ground reaction forces) were near ±2 SD of the experimental data (Fig. 2). Since the muscle excitation timing was constrained in the optimization, the muscle timing compared well with published EMG data (Fig. 3). Therefore, the timing of muscle force development throughout the gait cycle can be expected to represent normal
Discussion
The objective of the present study was to quantify the contributions of the individual uniarticular and biarticular ankle plantar flexors to walking tasks of support, forward progression and swing initiation. Analyses performed on walking simulation data generated from a forward dynamics approach identified how the individual plantar flexors accelerate the trunk and leg and contribute to the power flow throughout the musculoskeletal system. Thus, how each muscle contributes to the three task
Acknowledgements
The authors are grateful to Dr. Tom Andriacchi and Ajit Chaudhari for providing the experimental data. This work was supported by NIH grant NS17662 and the Rehabilitation R&D Service of the Department of Veterans Affairs (VA).
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