1996 Project Reports | Home | Contents | Previous | Next |
Christine Raasch, PhD; Felix E. Zajac, PhD; Lena H. Ting, MS; Steven A. Kautz, PhD; David Brown, PhD; H. F. Machiel Van der Loos, PhD; Christine A. Dairaghi, BS
Objective - Understanding the organization of muscle coordination during movements helps us to better understand how the nervous system controls motor task execution. Such knowledge can lead to more successful rehabilitation strategies for those who may have neurological damage that impairs movement, such as persons with hemiplegia following a stroke. We study pedaling because of the importance of lower limb coordination in walking. Pedaling can be considered a stepping stone to walking. The patterns of alternating flexion and extension of the leg in pedaling are similar to those in walking, yet the added tasks of maintaining balance and weightbearing can be minimized, or even eliminated completely, if desired. Compared with walking, pedaling is more amenable to laboratory studies, because kinesiological and biomechanical variables can be more easily measured. Pedaling lends itself to theoretical inquiry because the coordination involved is only that of the leg musculature in a seated position. Pedaling, therefore, affords us the ability to investigate each of the major components of muscle coordination seen in walking. Our goal is to discover the rules of how the nervous system processes sensory information to excite muscles (the neuromotor control rules), and rules of how the musculoskeletal system transforms the muscle excitation pattern into movement of the body segments (the musculoskeletal rules).
Approach - The neuromotor control and musculoskeletal rules are to be formalized through the development of a computer-based sensorimotor control model. Past work by us has emphasized the creation of a computer simulation of pedaling, a technique well known to contain the ingredients needed to elucidate the musculoskeletal rules. First, a computer model of the musculoskeletal system of the leg was constructed. This model consists of all the muscles and bones in the leg. To study pedaling, the muscles and bones were arranged into nine muscle sets (defined in Fig. 1) and three body segments (the thigh, the shank, and the foot). Second, a model of a stationary ergometer and the load it applies through the crank and pedals to the feet was computer-implemented (Fig. 1, inset). Third, these two models were integrated, creating a computer model of pedaling. Fourth, by assuming the nervous system can excite muscles independently, if necessary, a computer simulation was derived for maximum-speed startup pedaling. The simulation was found to agree with kinesiological and biomechanical measurements of subjects performing the same task. Thus, the musculoskeletal rules for pedaling could now be studied.
 Figure 1. In pedaling, the 32 muscles in the leg can be represented by these 9 muscle sets: GMAX (gluteus maximus and adductor magnus); HAM (medial hamstrings and biceps femoris long head); RF: (rectus femoris); VAS (three heads of vastus); GAS (gastrocnemius); SOL (soleus and other uniarticular plantarflexors); TA (tibialis anterior); BFsh (biceps femoris short head); IL (iliacus and psoas). |
Results - Progress analysis of this maximum-speed startup simulation, which replicates real subjects pedaling at maximum effort, showed how muscles must be coordinated to generate the force and the energy required to pedal. All muscles have a specific phase in the pedaling cycle where they must be excited in order to produce force and energy. Some muscles dominate the production of energy required to pedal. Some muscles deliver energy directly to the crank. Other muscles deliver energy to the limb segments, where it is stored as kinetic energy, and still other muscles transfer this stored energy to the crank. Some muscles participate in more than one of these energetic mechanisms. Thus, the musculoskeletal rules for how muscles transform excitation signals (neural output) into movement of the crank and body segments were found.
Computer simulations of pedaling, assuming neuromotor control rules less complex than those associated with independent control of each muscle, revealed that maximum-speed startup pedaling can be replicated with a simple neuromotor strategy. The strategy consists of co-exciting muscles, with the muscles in each leg arranged into two pairs of reciprocating muscle groups with their excitations linked to specific crank phases or, equivalently, to specific positions and/or motions of the leg (Fig. 2a). The uniarticular extensor and flexor muscles (the Ext-Flex pair; E-F pair) are excited during downstroke (leg extension) and upstroke (leg flexion), respectively. The ankle muscles and the biarticular thigh muscles (the TA/RF-TS/HAM pair) are excited near the top of the crank cycle (when the leg is most flexed and moving anteriorly, or forwards) and near the bottom of the cycle (when the leg is most extended and moving posteriorly, or backwards), respectively. Each muscle group also alternates with the comparable muscle group in the other leg.
 Figure 2. Coordination of the 9 muscle sets, relative to the position of the crank in the cycle, to pedal smoothly forward (A) or backwards (B). Other simulations showed that this neuromotor control strategy of exciting two pairs of reciprocating muscle groups in each leg in alternation with the muscle groups in the other leg can accommodate steady pedaling at a variety of cadences and workloads by assuming the control strategy has the additional capability of adjusting one other control parameter, the average amount of excitation to the muscles. Furthermore, by assuming the nervous system can also adjust the relative excitations among the groups, very smooth or very efficient pedaling can be accommodated. |
Simulations show, however, that co-excitation of the posterior biarticular thigh muscles (HAM) with the ankle extensor muscles (TS), and the anterior biarticular thigh muscle (RF) with the ankle flexor muscles (TA), should reverse in backward pedaling (i.e., HAM co-excited with TA, and RF with TS), especially if smooth backward pedaling is desired (Fig. 2b). Thus, the neuromotor control rules to accommodate forwards and backwards pedaling must increase somewhat in complexity. Experiments are underway to see if HAM and RF do, indeed, reverse their phasing with pedaling direction. Other experiments, which depend on the ability to have one leg pedal as if it were engaged in a two-legged pedaling task while the other leg performs another task, will be performed to discover the rules for interleg coordination.
Republished from the 1996 Rehabilitation R&D Center Progress Report. For current information about this project, contact: Christine Raasch.
