Relationship between quadriceps muscle strength an dmuscle/cerebral oxygenation in healthy subjects

Introduction

The stimulus to contract a muscle is initiated in the cerebral motor cortex. However, maximal muscular force production is also dependent upon neuromuscular function including peripheral motor unit recruitment and fiber type composition.1 It has recently been suggested that cerebral activation may play an important role in limiting muscle performance. Experimental evidence to support this hypothesis is lacking. Near Infrared Spectroscopy (NIRS) is a non-invasive optical technique that has been used to evaluate cerebral and muscle oxygenation (Ox) during exercise. The feasibility of using NIRS to simultaneously measure cerebral and muscle oxygenation during exercise has been demonstrated.2 In cerebral tissue, increases in Ox are related to greater neuronal activation, whereas in muscle tissue decreases in muscle Ox reflect localized tissue metabolism and muscle activation.3 Therefore, the purpose of this study was to examine the relationship between maximal muscular force production and cerebral as well as muscle Ox changes during quadriceps muscle function in healthy subjects.

Methods

Written informed consent was obtained from nine healthy subjects free from cardiorespiratory, metabolic or orthopedic disorders. In session one, each subject completed an isotonic 1RM knee extension (KE) test through 90 degrees with the dominant leg using a KE machine (Righetto Fitness Equipment, Brazil). In session two, each subject completed the following protocol: 2 min of rest  the 1RM isotonic KE coupled with 15 sec isometric contraction (1RM-isom) in the extended position  3 min of recovery. NIRS (MicroRunman, Philadelphia, PA) was used to measure: (1) muscle Ox from the vastus lateralis of the exercising leg, (2) cerebral Ox from the contra-lateral frontal lobe. The following cerebral and muscle NIRS variables were calculated: (1) Delta Ox as the difference between the maximum or minimum value during the contraction and the resting value prior to KE, and (2) maximum or minimum Ox values during the recovery phase. Pearson correlations (r) were used to examine the relationship between the 1RM load (kg) and the pertinent cerebral and muscle NIRS variables.

Results

During the 1RM-isom (mean ± SD = 79.9 ± 31.8 kg), cerebral Ox increased whereas muscle oxygenation decreased in all the subjects. During the recovery phase, cerebral and muscle Ox demonstrated further increases for the first few seconds in some subjects but returned towards resting baseline for the remainder of the recovery period. Significant correlations were observed between the 1RM load and cerebral Delta Ox (r = .710) and maximum Ox (r = .765), but not muscle Ox variables.

Discussion / Conclusions

To the best of our knowledge, this is the first study that has documented simultaneously the changes in cerebral and muscle oxygenation during resistance exercise using NIRS. The changes observed are consistent with previous reports that have independently evaluated cerebral oxygenation during motor stimulation4 and muscle oxygenation5 during resistance exercise. These pilot results suggest that 1RM load during KE is more dependent on cerebral Delta and maximum Ox changes than in the muscle Ox variables. Further research is needed to confirm these results in a larger number of subjects and to verify these findings using different muscle groups.

References

[1]. Hoffman JR et al. (2003). Med Sci Sports Exer 35:1929-1934.
[2]. Nielsen HB. (1999). Am J Physiol 277:H1045-H1052.
[3]. Miura H. (2000). Int J Sports Med 21:180-184.
[4]. Obrig H et al. (1996). J Appl Physiol 81:1174-1183.
[5]. Azuma K et al. (2000). J Biomed Opt 5:97-101.
Acknowledgements: Funding from CAPES - Ministry of Education; Fundação Carlos Chagas Filho de Amparo à Pesquisa do Rio de Janeiro (FAPERJ); Righetto Fitness Equipment; and internal funds from Universidade Gama Filho, Rio de Janeiro, and IPCFEx, Brazilian Army, Brazil.