Running mileage, movement mileage, and fitness in male U.S. Navy recruits.

PURPOSE: The purpose of this study was to determine whether there is a relationship between overall fitness improvement and varying amounts of running and movement mileage. METHODS: Subjects were male U.S. Navy recruits (N = 1703, 25 divisions), ages 17-35 yr (mean age = 20.1 +/- 2.9 yr), who attended boot camp from April 1996 through August 1996. During the first week of training, recruits performed a 1.5-mile run to determine baseline fitness levels. The results from the initial run were compared with a final 1.5-mile run conducted 6 wk later. RESULTS: Based on an age-adjusted fitness scale for a 1.5-mile run time, about one third of the recruits began recruit training in "Excellent-Superior" condition (N = 558), one third began in "Good" condition (N = 582), and one third began in "Poor-Fair" condition (N = 563). Running mileage among divisions ranged from 11.5 to 43.5 miles for the entire 7-wk training period (mean = 22.7 +/- 7.2 miles; 8-22 run days, mean = 13 +/- 4 d). In addition to running, the divisions accumulated many movement miles (110-202 miles; mean = 145 +/- 26 miles) while marching in formation. Recruits who began training in Poor-Fair condition improved the most with an average decrease in run time of 1:55 +/- 1:06 min (15.6% improvement). The Good group improved by 47 +/- 37 s (7.3% improvement), and the Excellent-Superior group improved by 17 +/- 32 s (2.9% improvement). CONCLUSION: The magnitude of fitness improvement, as measured by run time improvement, was directly related to baseline fitness level but not related to movement mileage or high-intensity run mileage accrued during training.

Organization of recurrent inhibition and facilitation in motoneuron pools innervating dorsiflexors of the cat hindlimb.

The incidence of recurrent inhibition and facilitation in motor nuclei innervating the dorsiflexors of the ankle and digits was examined in spinalized, decerebrate cats. Motoneurons innervating the anterior and posterior portions of the tibialis anterior (TAa and TAp, respectively) received strong recurrent inhibition following stimulation of either of the homonymous muscle nerves. Both motoneuron species received substantial recurrent inhibition from the semitendinosus (St), but stimulation of the nerve to the extensor digitorum longus (EDL), an ankle flexor synergist, evoked smaller recurrent IPSPs. TA motoneurons received mainly facilitation from hindlimb extensors of the hip and ankle. Motoneurons of the EDL and extensor digitorum brevis (EDB), synergists which share mechanical action at the metatarsophalangeal joint and the digits, received little recurrent inhibition in response to stimulation of the nerve to either muscle. Overall, stimulation of heteronymous flexor nerves (including TAa, TAp, and St) failed to evoke responses in most of the EDB and EDL neurons tested (50-83%), and the amplitude of recurrent inhibitory responses was small. Recurrent facilitation from the extensors was more common in these motor nuclei. Most responses recorded in EDB motoneurons following either flexor or extensor nerve stimulation were recurrent facilitations. The sensitivity of this facilitation in EDB motoneurons to injection of polarizing current and its central latency indicate that it is mediated by a disinhibitory, trisynaptic pathway. Stimulation of the nerve to EDB produced recurrent IPSPs in some flexor motoneurons, but these potentials were infrequent and their amplitude was usually small. Based on a comparison of the distribution of recurrent inhibition to published reports of the activities of TAa, TAp, EDL, and EDB during different forms of locomotion, we conclude that recurrent inhibition is large for motor nuclei that exhibit stereotypical activity, while motor nuclei that are activated independently receive and produce little recurrent inhibition. Despite the absence of recurrent inhibition in some motor nuclei, recurrent circuits may still participate in their control through disinhibitory, facilitatory mechanisms.

Correlations between neurograms and locomotor drive potentials in motoneurons during fictive locomotion: implications for the organization of locomotor commands.

The patterns of correlation found between motoneuron pools during fictive locomotion are the same whether the coherence functions used to detect the correlations are determined using pairs of rectified ENGs or motoneuron LDPs and rectified ENGs. This finding suggests that the higher frequencies in rectified ENGs (and, perhaps, EMGs) contain information about the synaptic input to motoneurons. Nevertheless, differences between the coherence functions of rectified ENG pairs and those of LDPs and rectified ENGs suggests that this information is distorted by harmonics introduced by rectification. The activities of many motoneuron pools are correlated during the flexor or extensor phase of fictive locomotion, indicating that they receive common synaptic input from branched presynaptic axons or from pools of interneurons whose activities are synchronized. Similar findings were reported by Bayev (1978), based on temporal correlations. Our results indicate that the investigated motor nuclei, which innervate muscles with actions at the hip, knee and ankle, are subject to a set of common locomotor commands. These commands are also received by inhibitory interneurons that project to the motor nuclei of antagonists, as indicated by the correlations between the hyperpolarizing phase of LDPs and activity in the rectified ENGs of antagonists. This last result is consistent with a modular organization for the spinal locomotor generator, in which one set of interneurons drives a motor pool and the inhibitory interneurons that project to the motor pool's antagonist (Jordan, 1991). However, these results also suggest that the spinal modules for locomotion may not be separable into independent unit-burst generators that produce commands for control of each joint as Grillner (1981) has suggested. Our results are more consistent with a model in which a generator distributes flexor and extensor commands to many motor pools (like the half-center model) with as yet unidentified spinal mechanisms that determine differences in the initiation and termination of activity of individual motor nuclei. Alternatively, the correlations between motor pools that we have observed could be explained by spinal mechanisms that synchronize the activity of unit-burst type generators. Despite the distribution of common locomotor commands to many functionally diverse motor nuclei, the spinal locomotor pattern generator is differentiated to the extent that some motor nuclei, like EDL and FDL, receive separate locomotor commands. This conclusion is consistent with other observations. EDL and FDL display distinctive, individualized patterns of locomotor activity that may vary in a facultative manner or in different forms of locomotion (O'Donovan et al., 1980; Trank et al., 1996). A recent study has shown that during fictive locomotion EDL and FDL motoneurons receive input from different sets of last-order interneurons than those which project to other motor pools (Degtyarenko et al., 1998). These results suggest that spinal locomotor generators are differentiated for the individualized control of some digit muscles, like FDL and EDL.

Forms of forward quadrupedal locomotion. II. A comparison of posture, hindlimb kinematics, and motor patterns for upslope and level walking.

To gain insight into the neural mechanisms controlling different forms of quadrupedal walking of normal cats, data on postural orientation, hindlimb kinematics, and motor patterns of selected hindlimb muscles were assessed for four grades of upslope walking, from 25 to 100% (45 degrees incline), and compared with similar data for level treadmill walking (0.6 m/s). Kinematic data for the hip, knee, ankle, and metatarsophalangeal joints were obtained from digitizing ciné film that was synchronized with electromyographic (EMG) records from 13 different hindlimb muscles. Cycle periods, the structure of the step cycle, and paw-contact sequences were similar at all grades and typical of lateral-sequence walking. Also, a few half-bound and transverse gallop steps were assessed from trials at the 100% grade; these steps had shorter cycle periods than the walking steps and less of the cycle (68 vs. 56%) was devoted to stance. Each cat assumed a crouched posture at the steeper grades of upslope walking and stride length decreased, whereas the overall position of the stride shifted caudally with respect to the hip joint. At the steeper grades, the range and duration of swing-related flexion increased at all joints, the stance-phase yield was absent at the knee and ankle joints, and the range of stance-phase extension at knee and ankle joints increased. Patterns of muscle activity for upslope and level walking were similar with some notable exceptions. At the steeper grades, the EMG activity of muscles with swing-related activity, such as the digit flexor muscle, the flexor digitorum longus (FDL), and the knee flexor muscle, the semitendinosus (ST), was prolonged and continued well into midswing. The EMG activity of stance-related muscles also increased in amplitude with grade, and three muscles not active during the stance phase of level walking had stance activity that increased in amplitude and duration at the steepest grades; these muscles were the ST, FDL, and extensor digitorum brevis. Overall the changes in posture, hindlimb kinematics, and the activity patterns of hindlimb muscles during upslope walking reflected the need to continually move the body mass forward and upward during stance and to ensure that the paw cleared the inclined slope during swing. The implications of these changes for the neural control of walking and expected changes in hindlimb kinetics for slope walking are discussed.

Forms of forward quadrupedal locomotion. III. A comparison of posture, hindlimb kinematics, and motor patterns for downslope and level walking.

To gain further insight into the neural mechanisms for different forms of quadrupedal walking, data on postural orientation, hindlimb kinematics, and motor patterns were assessed for four grades of downslope walking, from 25% (14 degrees slope) to 100% (45 degrees), and compared with data from level and downslope walking at five grades (5-25%) on the treadmill (0.6 m/s). Kinematic data were obtained by digitizing ciné film, and electromyograms (EMGs) synchronized with kinematic records were taken from 13 different hindlimb muscles. At grades from 25 to 75%, cycle periods were similar, but at the steepest grade the cycle was shorter because of a reduced stance phase. Paw-contact sequences at all grades were consistent with lateral-sequence walking, but pace walking often occurred at the steepest grades. The cats crouched at the steeper grades, and crouching was associated with changes in fore- and hindlimb orientation that were consistent with increasing braking forces and decreasing propulsive forces during stance. The average ranges of motion at the hindlimb joints, except at the hip, were often different at the two steepest slopes. During swing, the range of knee- and ankle-joint flexion decreased, and the range and duration of extension increased at the ankle joint to lower the paw downward for contact. During stance the range of flexion during yield increased at the ankle joint, and the range of extension decreased at the knee and metatarsophalangeal joints. Downslope walking was also associated with EMG changes for several muscles. The hip extensors were not active during stance; instead, hip flexors were active, presumably to slow the rate of hip extension. Although ankle extensors were active during stance, their burst durations were truncated and centered around paw contact. Ankle flexors were active after midstance at the steeper slopes before the need to initiate swing, whereas flexor and extensor digit muscles were coactive throughout stance. Overall the changes in posture, hindlimb kinematics, and activity patterns of hindlimb muscles during stance reflected a need to counteract external forces that would accelerate angular displacements at some joints. Implications of these changes are discussed by using current models for the neural control of walking.

Forms of forward quadrupedal locomotion. I. A comparison of posture, hindlimb kinematics, and motor patterns for normal and crouched walking.

1. Posture, hindlimb kinematics, and activity patterns of selected hindlimb muscles were compared for normal and crouched treadmill walking (0.5-0.6 m/s) for eight cats. To elicit crouched walking in which the trunk and head were lowered, cats were encouraged to walk under a light-weight Plexiglas ceiling suspended 17-20 cm above the treadmill belt. Kinematic data were obtained from high-speed ciné film, and electromyograms (EMGs)-synchronized with the kinematic records-were taken from 11 hindlimb muscles. 2. The postures for the two forms of walking were distinctly different. During crouched walking, each cat lowered its entire body keeping its trunk horizontal to the treadmill belt. Also the head was lowered, with the top of the head in line with the dorsal surface of the trunk. Hip height, used as a measure for hindlimb crouch, was reduced by 30%, from an average height of 23 cm to an average height of 16 cm above the belt during the entire step cycle. 3. Average cycle periods (766 +/- 30 ms, mean +/- SD) and percentage of time devoted to swing (30%) and stance (70%) were similar for normal and crouched walking. The profiles of the hindlimb kinematics were also similar for the hip, knee, ankle, and metatarsophalangeal (MTP) joints during the step cycle, but the timing of some of the motion reversal, as well as the ranges of motion during various phases, were different at some joints for the two forms of walking. 4. During the swing phase, the transition between the flexion and extension (F-E1 reversal) occurred later in the normalized swing phase at the hip, knee, and ankle joints, and the range of flexion was increased at each joint. With greater flexion at these joints, the anatomic axis of the hindlimb (measured from hip joint to toe) was decreased and the hind paw advanced in the narrow space between the abdomen and treadmill belt. At contact, the position of the paw was less anterior to the perpendicular reference line (hip joint marker to belt) and all joints were more flexed for crouched than normal walking. 5. Throughout the stance phase, the knee and ankle joints remained significantly more flexed by 41-45 deg during crouched than normal walking. Although the hip and MTP joints started in a more flexed position at paw contact, both joints extended more during stance for crouched than normal walking, and at the time of peak extension (just before paw lift-off), the degree of extension at the hip and MTP joints was similar for both forms of walking. 6. Muscle patterns for crouched and normal walking were similar with some exceptions. The burst durations for three primary flexor muscles, the semitendinosus (knee flexor), extensor digitorum longus (EDL, ankle flexor), and flexor digitorum longus (digit flexor) were longer for crouched than normal walking, and this was consistent with the increased range and duration of flexion during the swing phase of crouched walking. Also, two muscles that normally showed mainly swing-related activity during normal walking, the EDL and the extensor digitorum brevis, had distinct stance-related bursts that occurred after midstance during crouched walking. 7. Crouched walking requires a postural change that typically occurs when cats stalk prey and when cats walk up and down sleep slopes. Postural set during walking appears to be determined by brain stem and diencephalic centers, and the postural orientation of the cat may require adjustments in the motor program provided by spinal centers for the cat to walk. The role of posture and locomotion and the adjustments in hindlimb kinematics and EMG activity patterns have been studied for forward and backward walking in the cat and now for crouched walking on the treadmill. These data will assist us in understanding the role of posture, especially crouched posture, during other walking behaviors.

Adaptive control for backward quadrupedal walking VI. metatarsophalangeal joint dynamics and motor patterns of digit muscles.

1. We compared the dynamics of the metatarsophalangeal (MTP) joint of the cat's hind paw and the motor patterns of two short and four long muscles of the digits for two walking forms, forward (FWD) and backward (BWD). Kinematic (angular displacements) data digitized from high-speed ciné film and electromyographic (EMG) data were synchronized and assessed for bouts of treadmill walking. Kinetic data (joint forces) were calculated from kinematic and anthropometric data with the use of inverse-dynamic calculations in which the MTP joint net torque was divided into gravitational, motion-dependent, ground contact (absent for swing), and muscle torque components. Swing-phase kinetics were calculated from treadmill steps and stance-phase kinetics from overground steps in which one hind paw contacted a miniature force platform embedded in the walkway. 2. The plantar angle at the intersection of the metatarsal and phalangeal segmental lines was used to measure MTP angular displacements. During swing for both walking forms, the MTP joint flexed (F) and then extended (E); however, the F-E transition occurred at the onset of FWD swing and at the end of BWD swing. For FWD walking, the MTP joint extended at a constant velocity during most of stance as the cat's weight rotated forward over the paw. During the unweighting phase at the end of stance, the MTP joint flexed rapidly before paw lift off. For BWD walking, the MTP joint extended briefly at stance onset (similar to a yield) and then flexed at a constant velocity as the cat's weight rotated backward over the paw. At the end of stance, the MTP joint extended and then flexed slightly as the paw was unweighted before paw lift off. 3. For both forms of walking, three of the six muscles tested were recruited just before paw contact and remained active for most (75-80%) of stance for both walking forms: plantaris (PLT), flexor hallucis longus (FHL), and flexor digitorum brevis (FDB). Their recruitment contributed to the flexor muscle torque at the MTP joint during most of FWD and BWD stance and was responsible for the absorption of mechanical power at the MTP joint for FWD stance and generation of mechanical power at the MTP joint during BWD stance. Also, the FHL and PLT, along with the soleus (SOL; also recorded in this study), contributed to an extensor muscle torque (described in paper IV of this series) and the generation of mechanical power at the ankle joint during stance of FWD and BWD walking. 4. The timing of activity for three muscles recruited during FWD swing was distinct for the two walking forms. The hallmark burst of the flexor digitorum longus (FDL)--a single burst, brief in duration and high in amplitude--occurred at the end of FWD swing (as the toes flexed rapidly) but shifted to the onset of BWD stance (as the claws protruded and toes extended) during paw weighting. The extensor digitorum longus (EDL) was recruited after paw off and was active for most of FWD swing; its activity contributed to an extensor muscle torque at the MTP joint and a flexor muscle torque at the ankle joint. For BWD walking, EDL recruitment shifted to an earlier phase in the step cycle and coincided with toe extension, which occurred at the end of stance before paw lift off. This pre-lift off activity continued into the first part of swing and contributed to an extensor muscle torque at the MTP joint and a flexor muscle torque at the ankle.(ABSTRACT TRUNCATED AT 250 WORDS)