Changes in muscle strength, vastus lateralis fibre characteristics and myosin heavy‐chain (MyoHC) gene expression were examined in 48 men and women following 3 weeks of knee immobilization and after 12 weeks of retraining with 1866 eccentric, concentric or mixed contractions. Immobilization reduced eccentric, concentric and isometric strength by 47 %. After 2 weeks of spontaneous recovery there still was an average strength deficit of 11 %. With eccentric and mixed compared with concentric retraining the rate of strength recovery was faster and the eccentric and isometric strength gains greater. Immobilization reduced type I, IIa and IIx muscle fibre areas by 13, 10 and 10 %, respectively and after 2 weeks of spontaneous recovery from immobilization these fibres were 5 % smaller than at baseline. Hypertrophy of type I, IIa and IIx fibres relative to baseline was 10, 16 and 16 % after eccentric and 11, 9 and 10 % after mixed training (all P < 0.05), exceeding the 4, 5 and 5 % gains after concentric training. Type IIa and IIx fibre enlargements were greatest after eccentric training. Total RNA/wet muscle weight and type I, IIa and IIx MyoHC mRNA levels did not change differently after immobilization and retraining. Immobilization downregulated the expression of type I MyoHC mRNA to 0.72‐fold of baseline and exercise training upregulated it to 0.95 of baseline. No changes occurred in type IIa MyoHC mRNA. Immobilization and exercise training upregulated type IIx MyoHC mRNA 2.9‐fold and 1.2‐fold, respectively. For the immobilization segment, type I, IIa and IIx fibre area and type I, IIa and IIx MyoHC mRNA correlated (r= 0.66, r= 0.07 and r=−0.71, respectively). The present data underscore the role muscle lengthening plays in human neuromuscular function and adaptation.

Cross-education enhances the performance of muscles not directly involved in the chronic conditioning of the muscles in a remote limb. Substantial crosseducation occurs after training with eccentric contractious or with contractions evoked by electromyostimulation (EMS). Since during EMS and eccentric contractions, skin and muscle afFerents are activated that have excitatory effects on contralateral homologous muscles, it was hypothesized that exercise training with stimulated vs. voluntary eccentric contractions would lead to greater crosseducation. Thirty-two women were randomly assigned to a voluntary (Vol), an EMS, or a remote EMS (rEMS) exercise group and performed 840 voluntary or stimulated eccentric contractions over 6 weeks. All subjects, including nonexercising controls (Con), were tested pre-and posttraining for maximal voluntary and stimulated isometric and eccentric quadriceps strength. Ipsilateral voluntary and stimulated forces increased in all groups. Changes in EMG activity paralleled those in voluntary force in each limb. No changes occurred in grip strength. The greater contra-and ipsilateral strength gains after EMS training were most likely related to an additive effect of EMS and muscle lengthening. 30422 grant. We thank Mr. J. Hill and Mrs. C. Webb for their help with data collection, and Drs. Paul DeVita and Robert Hickner for their constructive comments on the manuscript.

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Repeated isometric or shortening contractions of skeletal muscle cause muscle fatigue but several prior studies have reported an apparent absence of muscle fatigue when humans performed up to 70 lengthening contractions. We pursued the hypothesis that perhaps muscle excitability is a factor that aids force preservation with repeated eccentric actions. Soleus compound muscle action potential (M-wave) latency, peak-to-peak amplitude (PPA), duration, and area under the curve were examined in 12 subjects (mean age 24.3 y) over 4 testing days that included: no exercise, isometric exercise (neutral ankle angle), isokinetic (0.5 rad.s-1) concentric and eccentric exercise of the plantar flexors in the seated position on a Biodex dynamometer. Supramaximal shocks were delivered to the tibial nerve in the popliteal fossa at baseline (3 shocks, 1 min apart), during exercise (1 shock-after each of 5 bouts/10 contractions), and during 10-min recovery. From initial to final contractions, concentric, isometric, and eccentric fatigue was -32, -41 and + 2% (Condition by Trial Interaction, F2,22 = 25.1, p = 0.000). No changes occurred in latency or duration (p > 0.05), but PPA (Condition by Time interaction, F51,561 = 3.7, p = 0.000) increased during isometric and eccentric exercise and remained elevated during recovery. Area increased (F51,561 = 3.1, p = 0.000) significantly during all three exercise conditions and approximated baseline by minute 8 of recovery. It was concluded that although the potentiation of the action potential of individual muscle fibers seems to be the common mechanism underlying the increase in muscle excitability during plantar flexion exercise, it is possible that different factors could cause such a non-specific response.

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