Introduction 2003 1997 2000 2000 1997 2002 2003 2004 2002 2004 1998 1999 2006 2002 2001 1997 2001 2006 2007 2007 2006 2007 2006 2005 2000 1998 2000 2005 1995 2000 2001 1996 2006 Methods Subjects 2006 n n General design 2006 Exercise countermeasure 2006 2006 Experimental set-up 2006 Experimental procedures 1996 2006 2006 2002 2007 With the sEMG system properly positioned, each subject performed another MVC of 2–4 s. If the torque deviated more than 5% from the highest torque attained during the previous MVC-task (i.e., the highest value of the attempts before the application of electrical stimulation), another attempt was made. 2002 2004 Data analysis Fast voluntary isometric knee extensions 2004 2004 40 1 Fig. 1 thick line thin line arrow  diamond  shaded area  40 horizontal bar  40 −40–0   1 −40–0 40 −40–0 −40–0 40 2007 Electrically evoked contractions T T 10 T 150 T 10 T 150 2 1999 1997 Fig. 2 T 150 T 10 Os T m  300 150 Statistical analysis t  t  y t  P Results Fast voluntary isometric knee extensions 2006 40 −40–0 P −40–0 40 r 40 r P r P 3 −40–0 40 Fig. 3 40 vTTI 40 a RMS −40–0 b BR 40 −40–0 MVT  Intrinsic contractile properties 4 P P 4 300 P 300 P 4 150 P 4 150 T 10 T 150 P P 4 Fig. 4 FOA a TPT 300 b HRT 150 T 10 /T 150 BR  Discussion Contractile characteristics of electrically evoked knee extensions 1995 2000 2001 1996 1995 In the absence of the exercise countermeasure, the knee extensors exhibited characteristics of a faster muscle following 56 days of bed rest. The degree of fusion at 10 Hz stimulation was decreased, the time to reach peak torque at 300 Hz stimulation was reduced and relaxation after tetanic stimulation at 150 Hz tended to be faster. The possibility of an error seems unlikely, since we found complementary changes towards enhanced contractile speed at all frequencies studied. In addition, these changes were observed only for the inactive control group. 2005 2000 2005 1982 1987 2004 2003 1994 2001 2001 2000 2003 1999 2000 2004 2001 2004 1999 2006 2006 T 10 T 150 4 300 P 150 P 1999 Despite the observation that speed characteristics were unaltered for the exercise-trained group, the relative peak torque at low stimulation frequency also increased for this group. As the level of force fusion remained unaltered during bed rest for RVE, other factors are likely involved. Once possibility is that the peak torque during 10 Hz stimulation increased as a consequence of an enlarged twitch response. Although not directly measured in the present study, the torque developed during the first response of the 10 Hz tetanus increased during the course of the bed rest by about 26%, which was comparable to the elevation in 10 Hz peak torque (∼40%) for RVE. Interestingly, the relative torque production of the first response of the 10 Hz contraction also increased (by about 30%) for the inactive control group. However, the reduced fusion of successive individual twitches diminished the increase in peak torque at 10 Hz stimulation to about 15%. Despite the differences in contractile speed characteristics, the percent change in 10 Hz peak torque after 56 days of bed rest was not different between groups. At present, the selectivity of the exercise countermeasure paradigm to prevent changes in contractile speed properties, but not in the torque response at low frequencies of stimulation, remains difficult to explain. Indeed, more research is needed to determine the effectiveness of resistive vibration exercise as a countermeasure, since the individual merits of resistance training versus vibration training could not be quantified in the present study. Fast voluntary isometric knee extensions at maximal effort 1998 1999 2003 1998 3 2006 2007 2006 1998 2007 2007 2007 In conclusion, in the subjects who were confined to 8 weeks of bed rest without preventive measures the knee extensor muscle group acquired intrinsic contractile properties of a faster muscle. Resistive vibration exercise proved effective to counteract these changes at the muscle level. An unexpected finding of the present study was that neither group showed deterioration in the capacity to maximally activate the knee extensors at the very start of a voluntary contraction performed as fast and forcefully as possible. For the RVE group this might indicate an effective countermeasure design. However, considering that neural activation and voluntary muscle function were also maintained in the Ctrl group, it is also conceivable that the multiple retesting of the subjects resulted in or at least contributed to these preservations.