The aim of this study was to increase the understating of vitamin D and musculoskeletal health in men with CP. Our findings show that 1) the CP participants had lower KE strength, smaller VL ACSA, lower VJ and sprint performance compared to TDC, 2) bone health group comparisons were site specific to upper and lower limbs, with the CP group showing lower radius scores, but no difference in the tibia from TDC, 3) 25(OH)D levels were similar between CP and TDC groups, however almost all participants were below levels considered sufficient, and 4) there was a positive association between 25(OH)D and KE iMVC/LBM in the CP group, but no association in the TDC group.
Due to the previous associations between impaired walking gait pattern and plantar flexors (PF) weakness and size in adults with CP 39 most studies in CP measure PF strength and PF ACSA 12, 40, 41. However, the KE were deemed an important muscle group to investigate as they play a dominant role in sports performance 42, falls prevention 43 and are limited in their description within CP 44. The 40.5% lower KE iMVC in the present study is consistent with the 52% weaker PF 45, and the likely consequence of the PF being more directly impacted by the CP condition 12. In contrast to our more modest levels of KE weakness, the previous report of a 69% weaker KE in CP 44, likely reflects the sex difference in strength between their mixed sex CP participants, compared to their male only TDC. In the present study, men with CP had 23.7% smaller VL ACSA in men with CP, which is consistent with the 20% smaller PF ACSA and 27.9% smaller in VL ACSA in men with CP compared to TDC 40. The men with CP in this current study have a higher LBM% of 86.5% compared to other studies which used BIA in young ambulatory males with CP who had a LBM% of 74.8% 28. The high levels of PA in the men with CP in this current study is likely to be the prominent factor accounting to the 11.7% higher LBM%, as the men with CP in this study performed 3996 steps more/day on average when compared to other young ambulatory CP 46. Despite the higher levels of LBM in men with CP in this current study compared to other CP populations 28, LBM was still lower in men with CP when compared to TDC, which is consistent with other studies on body composition and muscle size 28. It should be noted, that it is very common for CP athletes to be recruited for musculoskeletal investigations 12, 44, 47, (see limitations below).
A lower knee extensor strength relative to LBM has a number of functional implications in both CP and other conditions of muscle weakness. It is likely that this is related to the increased impact of a maintained BM and a reduced muscle mass and quality on the affected limb, and suggests that the strength of this limb may influence how well those with CP can move their BM. Beyond our population of men with CP, a lower KEMVC/BM is an observation that has been made in a number of conditions such as: obese-aging men 48, postmenopausal women 49 and individuals with degenerative muscle impairments 50. Due to the non-progressive nature of CP on neuromuscular properties, strategies to train and improve the quality (contractile properties, size and strength) of the effected muscles, particularly the lower limbs need to be considered alongside body fat management to contribute to improved neuromuscular function.
Surprisingly, there was no difference in 10 m sprint time between groups in the current study. A combination of factors may account for lack of 10m-sprint difference. One could be attributed to 18/24 (75%) of the current CP group classifying as GMFCS I. In higher GMFC scoring adults with CP there is increased gait symmetry to TDC 51 which could contribute to effective ground reaction force vectors when sprinting 52. de Groot et al. 44 also found no difference peak power output in a CP group with a GMFCS of I when compared to TDC, yet peak power output was strongly correlated with sprint speed (R2 = 0.94). Therefore, due to the majority of men with CP being classed as GMFCS I in this current study, it is possible that improved 10 m sprint times in the CP group when compared to previous studies in CP 53 are due to the low number of GMFCS II classifications in the current study. It should also be considered that the majority of men with CP were hemiplegic in this study (Table 1), and running gait patterns are shown to have increased symmetry with increasing running speeds 54. Therefore, peak power output and increased sprinting gait symmetry may explain why there is no difference in 10m-sprint.
The bone health group differences showed site dependence, with the distal radius being lower in the CP group compared to TDC, and no difference in the distal tibia between groups. This is similar to bone health data presented previously from adolescents with CP who had radius and tibia T scores of -1.07 and − 0.38, respectively 55. In the current study, the T score of -1.32 suggested some bone loss (osteopenia) and a bone fracture risk factor of 2.3 fold greater when compared to normative values 56. In contrast to the radius, the distal tibia showed no difference between groups. Serum PTH levels supported this finding and were not different in CP and TDC. However, despite current literature showing that serum PTH is elevated with decreasing 25(OH)D 57, 58, it is of surprise to find that both participants in the CP and TDC group who are classed as deficient in 25(OH)D, have normal levels (< 65 pg·ml− 1) of serum PTH of 25.1 pg·ml− 1 and 31.8 pg·ml− 1 respectively. The normal PTH levels reflect the observations in this study of similar tibia bone health 58. A potential reason for the CP and TDC showing similar tibia but lower radius T and Z scores, could be their matched activity levels. In adolescents with CP, tibia density was previously reported to be higher with increasing ambulation levels 59. Indeed, football is specifically effective at preserving age related bone health in those without CP 60. It is therefore likely that our observations of preserved tibia bone health, may be a consequence of regular lower limb bone loading through activities such as football and associated exercise. For future research, it is important to acknowledge that the present study does not show that football alone improves bone health in men with CP and more work is required to show this association with the sport. It is more probable that the general activity levels of the CP group, which included large elements of football, matched the bone health of their tibia to TDC. Certainly though, clinicians and physical trainers should incorporate exercises that load the upper limbs to ensure bone turnover and development occurs in groups with CP.
There is a paucity of data on the vitamin D concentration in disabled populations, let alone athletes with disabilities, despite low vitamin D concentrations being documented in several nonathletic populations 14–16. Despite our findings that there are no differences in 25(OH)D between the men with CP and TDC, both groups were on average classified as deficient, with an insufficiency prevalence of ~ 90%. These low vitamin D levels relative to summer values appear to be the norm in the UK during the winter months 61. Yet, in the present study, Low vitamin D levels may exacerbate the condition specific weakness as 25(OH)D explained 26% of the variance in KE iMVC/LBM in the CP participants. In contrast we saw no associations between 25(OH)D and other outcomes, a likely consequence of the very low levels of 25(OH)D. This was particularly noticeable in the lack of association between 25(OH)D and either dietary vitamin D or TSE in both participant groups, likely as all three could be considered very low. The participants all had low dietary vitamin D intake (185.5 ± 155 IU/d) not coming close to meeting the Institute of Medicine’s recommendations of 400-600IU/d in adults 19–50 years old 3. Furthermore, dietary vitamin D intake accounts for < 20% of circulating vitamin D, our observations of no association between dietary vitamin D intake and 25(OH)D is of no surprise 1. Similarly, that there was no association between TSE and 25(OH)D in this current study, is consistent with negligible UV b radiation from sun exposure is during the latter winter months in the UK (i.e., February-March). Thus, even with the highest TSE scores, 25(OH)D would likely not have been high 61. Despite the seasonal contribution to negligible 25(OH)D variance between groups, the role of 25(OH)D to KE MVC/BM is consistent with well-established role of 25(OH)D on skeletal muscle myogenesis, cell proliferation, differentiation, regulation of protein synthesis and mitochondrial metabolism 62. With this knowledge (and the prevalent insufficiency in all participants), interventions such as vitamin D supplementation should be sought by both men with CP and TDC to correct for low vitamin D. In future, it is important that seasonal variations in vitamin D are measured to identify if increased UV b radiation from sun exposure improves 25(OH)D to adequate levels and if musculoskeletal health outcome measures are impacted in men with CP and other para-athletes.
Strengths and Limitations
The participants in the present study represent a highly functional proportion of men with CP (GMFCS I-II). Our participant groups are however similar to the only other description of KE iMVC and sprint outcomes 44, and are consistent with populations of CP athletes investigated by others 47. Comparisons with population based studies suggest that the proportion of GMFCS I: GMFCSII participants is around n = 3:2 63. In contrast, the strength comparisons made in the present study, 44 and 47 are made in ambulatory participants with CP at a ratio of GMFCS I:GMFCS II, n = 3:1, with more severe impairments also included in 47. The high levels of PA in these active CP participants will however mean that the findings may not be generalisable to other more impaired populations with CP. Although a broader population would of course be relevant for generalisation, our population of lower GMFCS impairment likely reduces the contribution of PA variance to the group differences and suggest that by regularly undertaking football and other forms of PA, there is some benefit to the musculoskeletal health of men with CP, particularly in the bone health of the lower limbs. As addressed throughout these limitations, and consistent with the caveat of all studies, the outcomes reflect the population under investigation. To this end it is important to note we include no data from women. Although not adverse to presenting sex disaggregated musculoskeletal outcomes such as, KE strength and tendon stiffness 64, 65, our recruitment of CP participants was driven by the prevailing opportunities for those with CP, and noticeably a much more limited development pathway for women’s para-sport. In previous studies including active CP participants, where women are presented, it is as a minority (men: women, 2:1 and 4:1, respectively, 44, 66. Given the known sex differences in bone measures, and the greater risk for osteoporosis 67, future research should strive to uncover the particular risks and implications of low vitamin D in women with CP.