Limitations of serum values to estimate glomerular filtration rate during exercise.

OBJECTIVE: Glomerular filtration rate (GFR) is part of routine medical practice for clinical assessment of kidney function in health and disease conditions, and is determined by measuring the clearance of creatinine (Cl-Crn) or estimated (eGFR) from equations using serum creatinine (Crn) or cystatin C (Cyst C). Crn and Cyst C methods obviate the need for urine collection but their reliability under non-resting conditions is uncertain. This study compared GFR determined by Cl-Crn, Crn and Cyst C methods under the conditions of rest and after exercise. METHODS: Twelve young male subjects performed a 30 min treadmill exercise at 80% of the maximal oxygen capacity. Venous blood samples and urine collections were collected before and after exercise and after recovery period. GFR rates were calculated from serum Crn and Cyst C equations, and Cl-Crn measured from serum and urine Crn output. Albumin was also determined for all samples. RESULTS: Under resting conditions, eGFR from Crn and Cyst C did not differ from Cl-Crn (p=0.39). Immediately after exercise, GFR decreased significantly, regardless of the method, but more so for Cl-Crn (-30.0%; p<0.05) compared with Crn (-18.2%) and Cyst C (-19.8%). After the recovery period, GFR determined by Cl-Crn was returned to initial values whereas Crn and Cyst C remained reduced. Although eGFR methods accurately estimate GFR at rest, those methods underestimated the change in GFR after acute exercise. CONCLUSIONS: These results indicate that exercise-induced changes in GFR should be determined by Cl-Crn method.

Postactivation potentiation in a human muscle: effect on the load-velocity relation of tetanic and voluntary shortening contractions.

Recently it was demonstrated that postactivation potentiation (PAP), which refers to the enhancement of the muscle twitch torque as a result of a prior conditioning contraction, increased the maximal rate of torque development of tetanic and voluntary isometric contractions (3). In this study, we investigated the effects of PAP and its decay over time on the load-velocity relation. To that purpose, angular velocity of thumb adduction in response to a single electrical stimulus (twitch), a high-frequency train of 15 pulses at 250 Hz (HFT(250)), and during ballistic voluntary shortening contractions, performed against loads ranging from 10 to 50% of the maximum torque, were recorded before and after a conditioning 6-s maximal voluntary contraction (MVC). The results showed an increase of the peak angular velocity for the different loads tested after the conditioning MVC (P < 0.001), but the effect was greatest for the twitch ( approximately 182%) compared with the HFT(250) or voluntary contractions ( approximately 14% for both contraction types). The maximal potentiation occurred immediately following the conditioning MVC for the twitch, whereas it was reached 1 min later for the tetanic and ballistic voluntary contractions. At that time, the load-velocity relation was significantly shifted upward, and the maximal power of the muscle was increased ( approximately 13%; P < 0.001). Furthermore, the results also indicated that the effect of PAP on shortening contractions was not related to the modality of muscle activation. In conclusion, the findings suggest a functional significance of PAP in human movements by improving muscle performance of voluntary dynamic contractions.