The central hypothesis, on which all of our experiments are based, proposes that during each eccentric contraction there is an uneven distribution of the length change, some sarcomeres in muscle fibres taking up most of the stretch, others lengthening very little (Morgan et al. 2000). It means that some sarcomeres are extended to beyond myofilament overlap. On relaxation, the filaments in these sarcomeres may not re-interdigitate properly and become disrupted (Wood et al. 1993; Talbot & Morgan, 1996). During repeated eccentric contractions, the disruptions spread until a point is reached where there is some membrane damage (Morgan & Allen, 1999). That, in turn, leads to disturbance of calcium homeostasis, uncontrolled calcium movement and development of a contracture. It is proposed that such contractures are responsible for the rise in passive tension after eccentric exercise. The presence of disrupted sarcomeres is signalled by a shift in the muscle's length-tension curve in the direction of longer muscle lengths (Jones et al. 1997). The drop in tension, apart from the effects of fatigue, indicates that some muscle fibres have become damaged to the point where, in response to stimulation, they no longer develop any tension. The muscle swelling and DOMS during the days after the exercise we attribute to the inflammatory process triggered by the damaged fibres (Smith, 1991).
The main result of the human experiments was a significant increase in passive torque immediately after the eccentric exercise (Fig. 3). The increase was large, 40 % above control values, and was sustained, remaining over 20 % for 4 days. In two earlier studies we had reported similar increases (Jones et al. 1997; Whitehead et al. 1998), but there passive torque did not become significant until 24 h post-exercise. We attribute this difference to the method of measurement.
In the earlier work passive torque was measured by moving the ankle joint in 10 deg steps over the range 50-90 deg starting at 50 deg, that is, starting with the foot maximally dorsiflexed. Subsequently we realised that, particularly for the lower values, passive torque could be influenced by muscle history effects (Proske et al. 1993). Recent experiments (N. P. Whitehead, D. L. Morgan, J. E. Gregory & U. Proske, unpublished observations) support that view. We therefore changed the protocol, starting always from the most plantarflexed position. In addition, values for passive torque from different subjects were compared at the optimum angle for active torque generation. This meant that for subjects with different angle-torque relations, the passive torque was always measured at an angle representing a similar degree of myofilament overlap in muscle fibres. The modified protocol exposed a large and immediate increase in passive torque, post-exercise.
Importantly, muscle swelling in these subjects did not increase significantly until 24 h after the exercise (Fig. 4). It meant that the mechanism for the immediate increase in passive torque did not involve muscle swelling. However, our results do not exclude the possibility of swelling contributing to the maintained elevation of passive torque, 24 h post-exercise onwards.
Howell et al. (1986) suggested that delayed increases in muscle stiffness at or near maximum length were the result of volume changes exerting strain on perimysial and epimysial connective tissue elements. A quantitative biomechanical model supported such a view (Purslow, 1989). However, like ourselves, Howell et al. (1993) also saw stiffness changes at intermediate angles and immediately post-exercise. They measured stiffness in elbow flexors from the slope of the relation between elbow angle and passive torque. They did not see any increase in baseline level of tension and to account for the observed pattern of stiffness changes they postulated that stretch of the injured muscle fibres activated them.
In our experiments passive torque was maintained above control levels for the period of measurement at each angle, so, unlike Howell et al. (1993), we did observe an increase in static torque. We did not make stiffness measurements for triceps surae because there is no linear portion in the passive torque-angle curve as there is for elbow flexors (Fig. 1). Given that we saw an immediate increase in passive torque post-exercise and no additional component when swelling had become significant (Fig. 3), we conclude that swelling is likely to play only a minor role in the observed rise in passive torque, post-exercise. A similar conclusion was arrived at by Chleboun et al. (1998).
The central aim of the animal experiments was to try to obtain further evidence in support of our hypothesis that the rise in passive tension immediately after a series of eccentric contractions was due to an injury contracture in some muscle fibres. Towards that end, we examined muscle properties only immediately post-exercise. We did not measure swelling. However, we noted that the optimum length for a contraction had shifted in the direction of longer muscle lengths, and that peak isometric force had fallen significantly.
There was also a large increase in passive tension, 97 % above the pre-exercise value at the optimum length (Fig. 5). This was two and a half times as great as the passive tension increase seen in human subjects (Fig. 3). The reasons for the difference are most probably the fact that the eccentric exercise in humans involved submaximal voluntary contractions, while in the cat the muscle was stimulated synchronously at a high rate (80 pulses s−1). In addition, in the cat, the stretches were arranged to cover a length range which included a portion of the descending limb of the active length-tension curve, a region where muscle damage is more likely to occur. Preliminary measurements made by us suggest that treadmill walking in humans does not stretch triceps significantly onto its descending limb.
As discussed previously, our interpretation of a shift in the active length-tension curve is that it is a consequence of sarcomere disruption producing an increased compliance in series with the actively contracting sarcomeres (Morgan & Allen, 1999). We hypothesised that the membrane damage and loss of calcium homeostasis following sarcomere disruption led to development of a region of contracture in the damaged fibres and it was this which was responsible for the rise in passive tension. In support of that view, the percentage increase in passive tension approximately followed the active length-tension relation for the muscle (Fig. 5). In an attempt to obtain further support for this idea we imposed on the passive muscle a series of lengthening-shortening movements and measured work absorption. There were two results. The first was that after the eccentric contractions the area contained within the length-tension figure, representing the amount of work absorption, had increased significantly. Second, there was a drop in work absorption after the first in the series of five stretches, seen both before and after the contractions. However, the drop was larger after the eccentric contractions (Fig. 7). We attribute this fall in work absorption to muscle history effects. If there are injury contractures in some muscle fibres, the first stretch may also partially break these up and redistribute them.
Inspection of the tension changes during the first and second stretch cycles (Fig. 6, bottom panel) shows that the tension rise during stretch is biphasic. We attribute this to the presence of a short-range elastic component in the muscle due to the presence of stable cross-bridges between myofilaments in muscle fibres (Hill, 1968). Interestingly, after the eccentric contractions, the tension rise is less clearly bi-phasic, largely because after the initial rise, the subsequent increase is much steeper than before the contractions.
We suggest that when a passive muscle, containing regions of injury contracture, undergoes lengthening and shortening movements it is likely that there is some detachment and reattachment of the cycling cross-bridges in the contracting segments. We attribute the large increase in work absorption after the eccentric contractions, that is, the steep rise in tension during stretch, to the presence, in the electrically silent muscle, of some fibres in a state of contracture.
Notice that both before and after the exercise the tension rise at the start of the second stretch was delayed, that is, the muscle had to be stretched further before tension began to rise (Fig. 6). It suggests that the release phase of the first stretch introduced slack in the muscle, slack which had to be taken up by the second stretch before tension could rise (Proske & Morgan, 1999).
A second point is that the difference in length change required for tension to rise between the first and second stretches was less after the eccentric contractions. This could be interpreted as some of the slack being taken up by an injury contracture. Another difference was that before the eccentric contractions, tension in response to the first stretch rose almost immediately, while afterwards it was delayed (Fig. 6). The eccentric contractions had led to a shift of the active length-tension relation in the direction of longer muscle lengths because, we claim, there was an increase in series compliance as a result of disruption of some sarcomeres. This increase in compliance, we suggest, shows up as a delayed tension rise in response to a stretch. Thus there are two opposing effects after the contractions, a take-up of some slack by the contracture and an increase in compliance from the damage.
Support for the view that eccentric contractions lead to contracture in damaged muscle fibres comes from electron microscopic observations on the rat soleus muscle after the animals had undergone downhill running exercises (Ogilvie et al. 1988). Muscles from exercised animals showed Z-line dissolution, A-band disruption and fibre clotting. Similarly, Fridén & Lieber (1998) found in the extensor digitorium muscle of rabbits after eccentric contractions, cytoskeletal disruption, including loss of myofibrillar registry, that is, Z-disc streaming and A-band disorganisation as well as the presence in fibres of hypercontracted regions.
We have chosen to interpret these experiments in terms of a non-uniform distribution of the length change during the eccentric contractions leading to development of local areas of damage. The evidence we have provided in support of our view remains indirect. It will be important, in the future, to confirm by direct observation using techniques such as calcium fluorescence and confocal microscopy that such areas of damage contracture actually exist.
It is generally agreed that after eccentric contractions the muscle shows a small increase in resting calcium concentration (Balnave et al. 1997; Ingalls et al. 1998). It was shown by Balnave et al. that, within the resolving power of their system, the rise in resting calcium appeared to be distributed uniformly throughout the muscle fibre and that there were no localised regions with high concentrations. It may be that the calcium rise is sufficient to trigger a low level of activation of the contractile machinery, increasing passive tension to the levels observed in our experiments. However, it should be pointed out that there is no direct evidence for such a calcium-triggered contracture, plus that the time courses of the calcium increase and tension rise do not match (Ingalls et al. 1998).
From the point of view of our experiments it does not matter whether the extra tension arises from a generalised increase in resting calcium, or is the result of localised area of contracture. We prefer the latter interpretation because it is difficult to explain the accompanying shift in active length-tension relation in terms other than the presence of regions of disrupted sarcomeres.
The possibility cannot be excluded that at least some of the rise in passive tension observed after the eccentric contractions was the result of changes in connective tissue or other passive elements in the muscle (Howell et al. 1986; Jones et al. 1987). It is, however, not clear how these could lead to an immediate rise in passive tension. An indirect piece of additional evidence, supporting the idea that cross-bridges in some fibres are generating active tension, was obtained from observations which formed part of another project. A previous study had shown that in human subjects, after eccentric exercise of elbow flexor muscles, there was a disturbance of the sense of tension (Brockett et al. 1997). In an attempt to obtain a neural basis for this observation, we have been studying responses of tendon organs in the cat MG muscle after eccentric exercise (J. E. Gregory, N. P. Whitehead, C. Brockett, D. L. Morgan & U. Proske, unpublished observations). Over most of the range of lengths, responses of tendon organs in the passive muscle had their levels of activity elevated after the exercise. The finding suggested that the increase in passive tension was widespread throughout the muscle, and resided in elements in series with tendon organs. Tendon organs are known to have muscle fibres inserted directly into their capsule (Zelena, 1994).