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The energetics of rat papillary muscles undergoing realistic strain patterns

L. J. Mellors^* and C. J. Barclay

Department of Physiology, PO Box 13F, Monash University, Victoria 3800, Australia

View larger version (18K): [in a new window]   Fig. 1. Examples of muscle length changes (Length) during realistic contraction protocols at 2 Hz. A, B and C show the length change protocols for experiment 1 and D for experiment 2. In A, the duration of initial isometric contraction was 10, 15 or 20 % of the total inter-stimulus interval (0.5 s) (shown for 10 % Lmax shortening only), where Lmax is the length at which active force production is maximal. In B, the isometric phase was substituted by a 2 or 4 % Lmax length increase, accounting for 15 % of the cycle duration. At each isometric duration, the muscle was shortened by 5, 10 or 15 % Lmax (shown for an isometric duration of 15 % of the cycle only) (C). The duration of the lengthening phase was kept at 50 % of the cycle duration in A, B and C. In D, isometric phase duration was 20 % of the cycle duration, while shortening was for 20, 30 or 40 % of the inter-stimulus interval. The duration of the lengthening phase was adjusted accordingly. Shortening amplitude was 10 % Lmax in B and D. The stimulus pulse was applied at time zero.

View larger version (12K): [in a new window]   Fig. 2. Recordings of force output (lower trace) and muscle length change (Length) (upper trace) during an isotonic contraction illustrating the method used to determine the stiffness of the series elastic component. The example shown is from a muscle shortening against a load of 0.6P0, where P0 is the maximum isometric twitch force. The dashed vertical line indicates the time at which shortening started. The solid straight lines were fitted to the muscle length change trace (upper trace) at the start of shortening and to the force trace (lower trace) immediately before shortening started. The slopes of these lines give the initial velocity of shortening (dL/dt) and rate of force development just prior to shortening (dP/dt), respectively. The ratio of dP/dt to dL/dt gives the stiffness of the elastic component in series with the contractile element at that load. The sampling rate for these experiments was 1000 Hz.

View larger version (16K): [in a new window]   Fig. 3. Examples of recordings made during a realistic contraction protocol. The recordings shown are (from top to bottom): change in muscle length (Length), force output, change in muscle temperature (Temp) and cumulative work (solid line), heat (dashed line) and enthalpy (dotted line) outputs. The enthalpy output is the sum of the work and heat outputs. The muscle returned to its pre-stimulation temperature within 1 min of completion of the contractions, indicating that all recovery metabolism was complete. Muscle mass, 3.58 mg; length, 4.5 mm, cross-sectional area, 0.75 mm2.

View larger version (12K): [in a new window]   Fig. 4. Examples of work loops. (A) The duration of the initial isometric contraction was 10 % of the cycle duration, and the amplitude of shortening was 5 % Lmax (solid line), 10 % Lmax (dashed line) or 15 % Lmax (dotted line), where Lmax is the length at which active force production is maximal. The duration of the lengthening phase was constant. Therefore, the velocity of shortening increased as the amplitude of shortening increased. (B) The amplitude of shortening was 10 % Lmax and the duration of the lengthening phase was 50 % of the cycle duration. The duration of the initial isometric phase was 10 % (solid line), 15 % (dashed line) or 20 % (dotted line) of the cycle duration. Time progressed around the loops in an anticlockwise direction, indicating that net work was performed. Length, change in muscle length.

View larger version (13K): [in a new window]   Fig. 5. Effects of varying the shortening amplitude (%Lmax, where Lmax is the length at which active force production is maximal) and the duration of isometric contraction (percentage cycle duration) on mean work output per cycle (A), enthalpy output per cycle (B) and net mechanical efficiency, Net (C). The duration of isometric contraction was 10 % (black columns), 15 % (white columns) or 20 % (grey columns) of the cycle duration. Letters indicate statistically significant differences between variables attributable to the duration of isometric contraction. Asterisks indicate statistical significance, at a particular isometric duration, attributable to shortening amplitude. Values are means + S.E.M., N=7.

View larger version (16K): [in a new window]   Fig. 6. Effects of incorporating an initial stretch in place of the initial isometric period on mean work output per cycle (A), enthalpy output per cycle (B) and net mechanical efficiency, Net (C) (N=6). The duration of the initial stretch or isometric period was 75 ms or 15 % of the cycle duration, and shortening was 10 % Lmax, where Lmax is the length at which active force production is maximal. Letters indicate statistically significant differences. Values are means + S.E.M.

View larger version (16K): [in a new window]   Fig. 7. Effects of varying the shortening duration of contractions on mean work output per cycle (A), enthalpy output per cycle (B) and net mechanical efficiency, Net (C) (N=7). Isometric phase duration was 100 ms. Letters indicate statistically significant differences. Values are means + S.E.M.

View larger version (12K): [in a new window]   Fig. 8. Effects of varying contraction frequency on mean work output per cycle (A), enthalpy output per cycle (B) and net mechanical efficiency, Net (C) (N=6). The duration of the initial stretch or isometric period was 75 ms or 15 % of the cycle duration, and shortening was 10 % Lmax, where Lmax is the length at which active force production is maximal. Letters indicate statistically significant differences. Values are means ± S.E.M.

View larger version (9K): [in a new window]   Fig. 9. Example of the time course of enthalpy output from a muscle performing 40 (circles) and 60 (diamonds) realistic contractions at 2 Hz. Isometric contraction was 15 % of the cycle duration, while shortening and lengthening phases accounted for 35 % and 50 % of the cycle duration respectively. Data points represent the mean enthalpy output per cycle averaged over five successive cycles. The horizontal position of each data point corresponds to the middle of the cycles over which the mean values were calculated.

View larger version (11K): [in a new window]   Fig. 10. Examples of muscle length changes (Length) (A,B) and work loops (C) during realistic contractions with (dotted lines) or without (solid lines) an isometric relaxation period. The length changes shown in A are from a muscle shortening by 10 % Lmax, where Lmax is the length at which active force production is maximal, or incorporating an isometric relaxation period such that the absolute amplitude of shortening was less than 10 % Lmax. The length changes pictured in B are similar to those in A, except that the maximum amplitude of shortening was 15 % Lmax. The work loops in C correspond to the length changes in B. The numbers beside the traces correspond to the contraction types listed in Table 2.

View larger version (10K): [in a new window]   Fig. 11. Example of the relationship between afterload and the stiffness of the series elastic component (SEC). The range of loads for this preparation (length 6.1 mm) corresponded to 0.2–0.8P0, where P0 is the maximum isometric twitch force. The line was fitted through the data using the method of least squares (r2=0.984). The method used to calculate stiffness is described in Materials and methods and illustrated in Fig. 2.

View larger version (15K): [in a new window]   Fig. 12. Examples of the analysis of the time course of contractile component length changes. Length changes of the whole muscle (solid line) and the calculated changes in length of the contractile component (dashed line) and the series elastic component (dotted line) are shown in A. The length changes applied to the whole muscle at shortening amplitudes of 5 (top traces), 10 (middle traces) and 15 % Lmax (bottom traces), where Lmax is the length at which active force production is maximal, are shown in B, while C shows the calculated length changes of the contractile component only, at the same shortening amplitudes as in B. All recordings were made on the same muscle (Lmax 4.8 mm).