this specification includes references to “ one embodiment ” or “ an embodiment .” the appearances of the phrases “ in one embodiment ” or “ in an embodiment ” do not necessarily refer to the same embodiment . particular features , structures , or characteristics may be combined in any suitable manner consistent with this disclosure . turning to fig1 , an illustration of one embodiment of training apparatus 10 is shown . as depicted , training apparatus 10 includes core 100 and outer portion 200 . training apparatus 10 may be configured roll along the ground , in response to force exerted by user 500 , by rotating about a center axis . by virtue of the shape of training apparatus 10 , user 500 may be required to exert force with both a horizontal and upward component in order to effectively move training apparatus 10 . such actions by user 500 may provide beneficial strength and technique training for general fitness , and / or for various sports , including for example football , rugby , wrestling , hockey , basketball , lacrosse , and baseball . for example , force exerted by user 500 from a stance that includes a lowered center of gravity , with knees bent , and having hips lowered and in proper alignment with the back and with the head , while the legs drive and the upper body pushes in an aligned upward and forward direction will result in the optimal efficacy in moving training apparatus 10 . in contrast , pushing in a purely horizontal direction or a downward and forward direction from a more upright stance will produce much inferior results in attempting to move training apparatus 10 . physical training using training apparatus 10 in accordance with the above - described optimal technique corresponds to the desired techniques for blocking and tackling in football . best practices for performance of various tasks in other sports corresponds similarly . for example , the effectiveness of delivering or resisting a block in football or a check in hockey may be increased by ensuring that the body is in an alignment having a lowered center of gravity such that force may be exerted through the legs , hips , torso , shoulders , and arms in an upward and forward direction . thus , physical training using training apparatus 10 may efficiently simulate efforts required in various sports such as football , and therefore may efficiently provide gains in strength that are particularly useful for enhancing performance in those various sports . furthermore , repetitive training using training apparatus 10 may reinforce proper body alignment to user 500 , thereby training user 500 ( e . g ., through increased muscle memory ) to achieve improved technique for those various sports . continuing with fig1 , outer portion 200 may provide a compliant portion that serves to receive force from user 500 , and also provide increased rolling resistance to training apparatus 10 . for example , due to the weight of the depicted embodiment of training apparatus 10 , outer portion 200 deforms at area 210 . for a given weight of training apparatus 10 , the rolling resistance may be increased by using a outer portion 200 that is more compliant ( e . g ., less rigid ). the compliance of outer portion 200 also serves as a cushion to receive impacts that may be delivered via the hands ( or other parts ) of user 500 , thereby reducing shock to the user &# 39 ; s body . various embodiments of outer portion 200 may include one or more of various compliant materials . for example , outer portion 200 may include a foam ( e . g ., a viscoelastic foam ), and may be surrounded by a protective cover or coating ( e . g ., nylon , rubber ) to protect the foam from environmental factors ( e . g ., rain , sunlight ) and impact - related damage ( e . g . shearing forces that may tend to rip or delaminate outer portion 200 ). in some embodiments , outer portion 200 may include a combination of different materials ( e . g ., multiple layers of different foams or other materials ). in some embodiments , outer portion 200 may include a protective cover or lining and a contained amount of a compliant filling material . for example , particular embodiments of outer portion 200 may include a user configurable amount of water ( or other liquid ) that facilitates adjustment of the compliance / stiffness of outer portion 200 . thus , in such embodiments the rolling resistance of training apparatus 10 may be adjusted by varying the amount of filling material in outer portion 200 . some of these embodiments , or other embodiments , may include outer portion 200 containing a variable amounts of a gel , and / or a gas ( e . g ., air , nitrogen ). in some cases , outer portion 200 may include , for example , sand , gravel , cement , or other solid materials . adjustment of the amount of filling material within outer portion 200 may also serve as a height adjustment for training apparatus 10 , whereby the height can be increased by adding an additional amount of one or more filling materials to outer portion 200 . thus , one means of adjusting the height of training apparatus 10 to accommodate users of varying heights ( e . g ., in use by children and adults ) is provided by some embodiments of outer portion 200 . turning now to fig2 , an example of core 100 that may be used in particular embodiments of training apparatus 10 is shown . various embodiments of core 100 may be constructed from any suitable material or materials , including for example plastics , metals , and composite materials . core 100 may in some cases be primarily of one - piece construction , or in other cases may be a composite of multiple components . as depicted in fig2 , core 100 includes outer surface 160 extending between end surface 150 a and end surface 150 b that are each normal to center axis 910 . embodiments of core 100 may be configured to contain a ballast material to provide training apparatus 10 with additional weight . in some embodiments , the amount of ballast material is user configurable so that the weight of training apparatus 10 may be adjusted ( e . g ., thereby adjusting the rolling resistance and / or the inertia of training apparatus 10 ). in such a way , the force required to rotate and move training apparatus 10 may be adjusted . fig2 depicts core 100 that may be well suited for use with a configurable amount of water ( or other fluid ) as ballast . fill port 110 may facilitate filling of core 100 with , for example , water from a hose . some embodiments may include fluids having higher density as ballast . in some embodiments , a combination of a fluid and a solid ( e . g ., water and sand , water and gravel ) may be used as ballast . drain port 120 may facilitate draining of the ballast , and vent port 130 may be useful in aiding in the filling and / or draining of the ballast . various embodiments may include many different combinations of one or more ports , such as fill port 110 , drain port 120 , vent port 130 , or additional ports . the ports may be of various different suitable configurations , included threaded caps , quarter - turn caps , caps retained using fasteners , valves , etc . fig3 depicts an embodiment of training apparatus 10 that includes outer portion 200 and core 100 . outer portion 200 may be fixed to core 100 at outer surface 160 ( see fig2 ) using various methods . for example , permanent fastening such as bonding , riveting , etc . may be used in some instances . in other cases , removable fastening such as straps , snaps , hook and loop fasteners , and other removable fastening techniques may be used . fig3 depicts use of core 100 having fill port 110 that is well suited for filling or removing ballast that includes solid material ( e . g ., sand , gravel , cement , metal bearings ). turning now to fig4 - 6 , views of various embodiments of training apparatus 10 are presented to further highlight aspects of the present disclosure . fig4 depicts an end view of an embodiment of training apparatus 10 that includes core 100 and outer portion 200 . as noted above , different overall heights of training apparatus 10 may be desirable , depending on the user ( e . g ., training of youth or adults ). in some embodiments , core diameter 920 of core 100 may be about 40 inches , and length 940 may be about 48 inches . in some of these embodiments , outer portion thickness 930 of outer portion 200 may be about 6 inches , resulting in training apparatus 10 having an overall height of about 52 inches . other embodiments of training apparatus 10 may have a greater overall height due to a larger core diameter 920 and / or greater outer portion thickness 930 , or a smaller overall height due to a smaller core diameter 920 and / or smaller outer portion thickness 930 . as noted above , in some embodiments outer portion thickness 930 may be adjustable , thereby resulting in an adjustable overall height of training apparatus 10 . various embodiments of the present disclosure may have length 940 that is different than the above - discussed example . in some embodiments , length 940 may be much greater to accommodate two or more users simultaneously training with the same apparatus ( e . g ., pushing the apparatus in tandem ). fig5 depicts an embodiment in which core 100 is not cylindrical , but is instead polygonal in cross section . the depicted embodiment includes and 18 - sided core 100 having surfaces 180 of equal size . other embodiments may include polygonal core 100 having 12 , 16 , 20 , or any other number of sides , which may be of equal size or of differing size . fig6 presents a section view of particular embodiments of training apparatus 10 having either cylindrical or polygonal core 100 and outer portion 200 located at outer surface 160 of core 100 . ballast area 170 may contain a configurable amount of ballast . referring to fig7 and 8 , some embodiments of training apparatus 10 may include adapter body 300 as well as core 100 and outer portion 200 . adapter body 300 may include annular shaped end surfaces 350 a and 350 b , with outer surface 360 and inner surface 362 extending between end surface 350 a and end surface 350 b . outer surface 360 may intersect end surface 350 b at edge 390 b , and inner surface 362 may intersect end surface 350 b at edge 390 a . various adapter body 300 of differing size may be used in some cases to configure training apparatus 10 with differing overall heights . for example , several adapter body 300 may be available for use with a particular core 100 , with each adapter body 300 having inner surface 362 configured to fit over outer surface 160 of core 100 . the various ones of the several adapter body 300 may have various different adapter thickness 950 . because the overall height of the embodiments training apparatus 10 depicted in fig7 and 8 is the sum of core diameter 920 , adapter thickness 950 , and outer portion thickness 930 , the various different adapter thickness 950 may be used to provide different overall heights for training apparatus 10 . in some embodiments , adapter body 300 may be configured to contain a fixed or adjustable amount of ballast at ballast area 370 ( e . g ., in a similar manner as ballast area 170 of core 100 ). various embodiments of training apparatus 10 may include ballast area 370 and ballast area 170 that contain ballast of the same materials ( e . g ., both containing sand ; water , concrete , gravel , metal bearings ). some embodiments may include ballast area 370 containing a particular material as ballast , and ballast area 170 containing a different material as ballast . adapter body 300 may be fixed to core 100 , and outer portion 200 may be fixed to outer surface 360 of adapter body 300 . although the embodiments above have been described in considerable detail , numerous variations and modifications will become apparent to those skilled in the art once the above disclosure is fully appreciated . it is intended that the following claims be interpreted to embrace all such variations and modifications .