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Hindlimb muscle function in relation to speed and gait: in vivo patterns of strain and activation in a hip and knee extensor of the rat (Rattus norvegicus)

Gary B. Gillis^* and Andrew A. Biewener

Department of Organismic and Evolutionary Biology, Harvard University, Concord Field Station, Old Causeway Road, Bedford, MA 01730, USA

View larger version (14K): [in a new window]   Fig.1. Effects of speed on cycle (A), stance (B) and swing (C) duration during rat locomotion. Cycle duration decreases with increases in locomotor speed (A). This decrease mirrors the decrease in stance phase duration with increasing speed (B). In contrast, swing phase duration remains relatively constant over a large speed range (C). Vertical lines generally separate data into walking, trotting and galloping trials. However, several slow-speed galloping trials are found between 60 and 75cms-1. Each symbol represents the average values from locomotor sequences at various speeds and gaits. Different symbols represent different individuals.

View larger version (39K): [in a new window]   Fig.2. Two-dimensional angular excursions of the hip and knee joints during walking (A,E), trotting (B,F) and galloping (leading limb, C,G; trailing limb, D,H). These plots reveal the general kinematic patterns observed at the hip and knee during rat locomotion and some of the variation related to changes in gait. Data are from four individuals, three strides per individual. Dark background shading represents the stance phase, and light background shading represents the swing phase. Values are means ± S.D. In all gaits, the hip mainly extends during stance and flexes during swing. The knee typically exhibits flexion followed by extension during both the stance and swing phases. The timing and extent of these angular excursions differ among gaits.

View larger version (21K): [in a new window]   Fig.3. Average patterns of electromyographic activity in the biceps femoris (BF) and vastus lateralis (VL) during walking (mean speed 36cms-1), trotting (mean speed 64cms-1), galloping trailing limb (mean speed 96cms-1) and galloping leading limb (mean speed 83cms-1). Horizontal bars represent average temporal periods of EMG activity. For each muscle, bar thickness denotes the average relative intensity of activity. Black vertical lines represent the onset of the stance phase (foot down) and the onset of the swing phase (foot up). Values are means ± S.D. (N=4 individuals per muscle).

View larger version (29K): [in a new window]   Fig.4. The relative timing of EMG onset and offset (A,E), absolute EMG burst duration (B,F), relative burst duration (C,G) and relative EMG intensity (D,H) for the biceps femoris (left column) and vastus lateralis (right column). Data are shown for walking (W), trotting (T) and galloping [trailing limb G(T), leading limb G(L)]. Mean locomotor speeds for each gait are the same as for Fig.3. The dashed horizontal lines in A and E represent the onset of the stance phase (time of foot down). Each symbol represents the average values of all analyzed trials from a particular individual for that particular gait. Different symbols stand for different individuals (N=4 for each muscle). Values are means ± S.D.

View larger version (34K): [in a new window]   Fig.5. (A) Electromyographic trace from the vastus lateralis of the trailing limb from a rat during level galloping. (B) An expanded view of two cycles of EMG activity showing an initial small burst of vastus activity late in the swing phase followed by a larger burst of activity commencing near the onset of the stance phase (demarcated by the vertical line separating the light and dark shading). The initial bursts of activity late in the swing phase were always of much lower intensity than the stance-related bursts and have been discussed in some detail previously (de Leon et al., 1994).

View larger version (49K): [in a new window]   Fig.6. In vivo patterns of strain and activation in the biceps femoris (A) and vastus lateralis (B) during two strides of walking, trotting and galloping (trailing limb and leading limb). Dark shading represents the stance phase, and light shading represents the swing phase. Slightly different scales are used between muscles for both strain and voltage. Note that total shortening strain levels in the biceps femoris are reduced during galloping relative to other gaits. In addition, the vastus lateralis during most gaits is active (as prescribed by EMG activity) during lengthening, but exhibits very little shortening during this time. However, in the leading limb during galloping, vastus fascicles exhibit more shortening when active.

View larger version (33K): [in a new window]   Fig.7. Strain levels in the biceps femoris and vastus lateralis during different portions of the stance and swing phases. Generalized fascicle strain patterns (based on average strain levels during trotting) for the biceps and vastus are shown in A and C, respectively, as are the five subdivisions of the step cycle for the two muscles (four stance and one swing). Average strain levels for walking, trotting, galloping trailing limb (TL) and galloping leading limb (LL) are shown for each of the phases in the biceps (B) and vastus (D) (N=4 individuals per muscle). Methods used for obtaining these fascicle strain levels are described under ‘Data analysis’ in the Materials and methods section. Negative values for muscle strain indicate fascicle shortening, whereas positive values reflect fascicle lengthening. If both shortening and lengthening occur during one of the intervals, both positive and negative columns are present for that interval. Error bars are standard deviations. Dark gray shading represents strain occurring during the stance phase, and light gray shading represents strain occurring during the swing phase.

View larger version (15K): [in a new window]   Fig.8. Average biceps shortening velocity (A) and vastus lengthening velocity (B) as a function of locomotor speed. Biceps shortening velocities are calculated by dividing the total distance shortened by the biceps by the duration of shortening. Only shortening velocities during walking and trotting are shown because, during these gaits, shortening velocity remained relatively constant over most of the shortening period (unlike during galloping, for which shortening velocities varied substantially). Negative values are used to indicate shortening velocities. Shortening velocities increase with speed and are highest during fast trotting. Average vastus lengthening velocities are calculated by dividing the total distance lengthened during the first half of the stance phase (the ‘yield’ phase) by the duration of this lengthening phase. Positive values are used to indicate lengthening velocities. Lengthening velocities increase with speed and are, therefore, greatest during fast galloping. Different symbols represent the average values from locomotor sequences at various speeds and gaits. Different symbol types represent different individuals. L, resting fascicle length.