Changes in passive tension of muscle in humans and animals after eccentric exercise

Abstract

1. This is a report of experiments on ankle extensor muscles of human subjects and a parallel series on the medial gastrocnemius of the anaesthetised cat, investigating the origin of the rise in passive tension after a period of eccentric exercise. 2. Subjects exercised their triceps surae of one leg eccentrically by walking backwards on an inclined, forward-moving treadmill. Concentric exercise required walking forwards on a backwards-moving treadmill. For all subjects the other leg acted as a control. 3. Immediately after both eccentric and concentric exercise there was a significant drop in peak active torque, but only after eccentric exercise was this accompanied by a shift in optimum angle for torque generation and a rise in passive torque. In the eccentrically exercised group some swelling and soreness developed but not until 24 h post-exercise. 4. In the animal experiments the contracting muscle was stretched by 6 mm at 50 mm s(-1) over a length range symmetrical about the optimum length for tension generation. Measurements of passive tension were made before and after the eccentric contractions, using small stretches to a range of muscle lengths, or with large stretches covering the full physiological range. 5. After 150 eccentric contractions, passive tension was significantly elevated over most of the range of lengths. Measurements of work absorption during stretch-release cycles showed significant increases after the contractions. 6. It is suggested that the rise in passive tension in both human and animal muscles after eccentric contractions is the result of development of injury contractures in damaged muscle fibres.

Figure 1. Methods of measurement of active and passive torque in human ankle extensor muscles

Upper panel, torque in triceps surae of one subject in response to double pulse stimulation of the tibial nerve at each of a number of ankle angles. Peak active torque at each angle was used to generate an active angle-torque curve. The level of passive torque at each angle was measured during the 1 s period before nerve stimulation. Lower panel, construction of passive angle-torque curves for the same subject, before (○) and 2 h after (•) a 1 h period of eccentric exercise of the muscle. Optimum angle for active torque generation before the exercise is indicated by the arrow.

Figure 5. Changes in passive tension in the medial gastrocnemius muscle of the cat

Upper panel, passive tension values shown as means (±s.e.m.) before (•) and after (○) 150 eccentric contractions (n = 5). Length is expressed relative to the optimum for tension generation (L_opt). Asterisks indicate the muscle lengths at which passive tension after the eccentric contractions was significantly above the control value (P < 0.05). Lower panel: •, plot of the passive tension measured at each muscle length after the eccentric contractions, expressed as a percentage increase above pre-exercise values. Values shown as means (±s.e.m.) from the 5 experiments. ○, active length-tension curve from one experiment, measured after the eccentric contractions.

Figure 6. Measurement of work absorption

The top panel shows the method of measurement. Upper trace, tension; lower trace, length. The muscle was stretched by 20 mm at 1 mm s⁻¹, up to maximum physiological length and then shortened at the same speed back to its original length. Five successive stretch-shortening sequences were applied. Middle panel, plots of muscle tension against length for the first stretch-shortening movement before (thin trace) and after 150 eccentric contractions (thick trace). The arrows indicate the lengthening phase of the movement. Bottom trace, an expanded view of the tension changes during lengthening in the region of record enclosed by the dashed line in the middle panel. Here tension changes during both the first and second stretches (numbered) have been shown. Thin traces, before the eccentric contractions; thickened traces, after the eccentric contractions.

Figure 2. Eccentric contractions of medial gastrocnemius in the anaesthetised cat

Upper panel, active length-tension curve for MG, measured before the eccentric contractions, using 80 pulses s⁻¹ stimulation of the nerve at each of a number of muscle lengths, expressing length relative to maximal physiological length (L_max). The line drawn through the points is a Gaussian curve fitted to values greater than 75 % of the peak tension. The optimum length is indicated by an arrow. Lower panel, sample records of length trace (second from bottom) and tension (upper three traces) during a series of eccentric contractions. The bottom trace shows the period of stimulation at 80 pulses s⁻¹. The top, continuous trace shows tension during the first eccentric contraction. The dotted trace shows the 10th and the dashed trace the 150th contraction. Notice the progressive drop in isometric tension at the start of each stretch. Length range was adjusted so that the 6 mm stretch at 50 mm s⁻¹ lay symmetrically about the optimum length for tension generation.

Figure 3. Changes in passive torque in human triceps surae after eccentric and concentric exercise

Upper panel, mean (±s.e.m.) change in passive torque for 7 subjects, measured in the triceps surae muscle of one leg, before and for 96 h after it had undergone 1 h of eccentric exercise (•, continuous line) while the muscle of the other leg, which had not undergone any exercise, acted as a control (○, dashed line). Points are expressed as a percentage change from pre-exercise values. At all times, post-exercise values for passive torque were significantly higher than for the control muscle (* P < 0.05). Lower panel, mean (±s.e.m.) change in passive torque, for 6 subjects, in the concentrically exercised muscle (•, continuous line) compared with the unexercised, control muscle of the other leg (○, dashed line). At no time was passive torque in the concentrically exercised muscle significantly different from that in the control muscle.

Figure 4. Muscle swelling after eccentric and concentric exercise

Muscle swelling was measured by immersing the leg, up to the knee, in a calibrated volume of water. Swelling was therefore measured for the whole lower leg, not just the triceps surae muscle. Upper panel, mean (±s.e.m.) swelling expressed as a percentage increase in lower leg volume for the eccentrically exercised leg (•, continuous line) compared with the unexercised, control leg (○, dashed line). The asterisks indicate where swelling in the exercised leg was significantly greater than in the control leg (P < 0.05). Lower panel, mean (±s.e.m.) swelling in the concentrically exercised leg (•, continuous line) compared with the other, unexercised control leg (○, dashed line). At no time after the concentric exercise was swelling in the exercised muscle greater than in the control muscle.

Figure 7. Changes in work absorption during stretch of the passive muscle before and after eccentric contractions

Work absorption was measured as the area contained within a length-tension figure, like those shown in the middle panel of Fig. 6, and expressed as a percentage of the work put into the muscle. Work absorption was measured for each of five successive lengthening-shortening cycles before (•, mean ±s.e.m.) and after 150 eccentric contractions (○, mean ±s.e.m.). After the contractions, all five measurements of work absorption were significantly above the values measured before the contractions (* P < 0.05).