To isolate any difference muscular contraction history may have on concentric work output, 40 trained male subjects performed three separate isokinetic concentric squats that involved differing contraction histories, 1) a concentric-only (CO) squat, 2) a concentric squat preceded by an isometric preload (IS), and 3) a stretch-shorten cycle (SSC) squat. Over the first 300 ms of the concentric movement, work output for both the SSC and IS conditions was significantly greater (154.8 +/- 39.8 and 147.9 +/- 34.7 J, respectively; P < 0.001) compared with the CO squat (129.7 +/- 34.4 J). In addition, work output after the SSC test over the first 300 ms was also significantly larger than that for the corresponding period after the IS protocol (P < 0.05). There was no difference in normalized, integrated electromyogram among any of the conditions. It was concluded that concentric performance enhancement derived from a preceding stretch of the muscle-tendon complex was largely due to the attainment of a higher active muscle state before the start of the concentric movement. However, it was also hypothesized that contractile element potentiation was a significant contributor to stretch-induced muscular performance under these conditions.

In recent years, it has been recognised that improvements to classic models of muscle mechanical behaviour are often necessary for properly modelling coordinated multi-joint actions. In this respect, the purpose of the present study was to improve on modelling stretch-induced force enhancement and shortening-induced force depression of muscle contraction. For this purpose, two models were used: a modified Hill model and a model based loosely on mechano-chemistry of the cross-bridge cycle (exponential decay model). The models were compared with a classic Hill modeland experimental data. Parameter values were based, as much as possible, on experimental findings in the literature, and tested with new experiments on the gastrocnemius of the rat. Both models describe many features of slow-ramp movements well during short contractions (300-500 ms), but long-duration behaviour is described only partly. The exponential decay model does not incorporate a force-velocity curve. Therefore, its good performance indicates that the status ofthe classic force-velocity characteristic may have to be reconsidered. Like movement-induced force depression and enhancement, it seems a particular manifestation of time-dependent force behaviour of muscle, rather than a fundamental property of muscle (like the length-tension curve). It is argued that a combination of the exponential decay model (or other models based on the mechano-chemistry of contraction) and structurally based models may be fruitful in explaining this time-dependent contraction behaviour. Furthermore, not in the least because of its relative simplicity, the exponential decay model may prove more suitable for modelling multi-joint movements than the Hill model.

Background For understanding the relationship between skeletal muscle architecture and muscle function in vivo, the development of accurate geometric models relating muscle length to joint angles is crucial. Therefore, a geometric model of the calf of mammals was developed to predict the length of the gastrocnemius muscle‐tendon complex from knee and ankle angles. Methods The model requires three skeletal length measurements (radius of femoral condyle, ankle lever, and tibia length) to predict muscle‐tendon length. The model was tested on the hopping mouse (Notomys alexis) by comparing polynomial fittings with geometrical fits of muscle length‐joint angle measurements (i.e., the equation of the geometric model was used for least square fitting of the data). The model was applied to the hopping mouse and the rat to study (in vivo) joint‐angle‐muscle length‐force relationships. Results It appeared that small and, on average, statistically nonsignificant length adjustments of the skeletal lengths were needed for the geometrical fit. Combinations of joint angles that normally occur during locomotion yielded muscle lengths close to optimum (i.e., with the highest isometric force potential). Conclusions By relying on the geometry of the animal's leg, the calculated moment arms of the model appeared more reliable than those calculated from the polynomial fit. It was concluded that the architecture regarding length‐force properties of the gastrocnemius muscle in both hopping mouse and rat is well adapted for the locomotion patterns. Anat. Rec. 247:1–8 © 1997 Wiley‐Liss, Inc.

In rat gastrocnemius medialis, the magnitude of the effects of these distributions is similar to pennation effects. The current results help to explain the relative wide working range of skeletal muscles in human movement and the differences in specific muscle tension as affected by muscle type, species, and age.